Every tank question eventually becomes a time question: how long the pool needs to fill before the weekend, how long a rain barrel keeps dripping after the rain stops, how many times per hour the filter sees the whole aquarium. This page answers all three at once — constant-flow filling and draining, gravity draining through an outlet, and turnover-rate math — in imperial and metric units, live on every keystroke.
Real-time results Imperial & metric Free, no sign-up
The Calculator: Constant Flow · Gravity · Turnover
Presets:
Tank volume
Optional — set 10% → 90% for a partial window. The time uses only the water between the two levels: t = ΔV / Q.
Time to fill
05:33:20hh:mm:ss
5.56 h · 0.18 full passes per hour at this rate
Effective volume: 15,000 gal (56,781 L)
Whole-tank volume: 15,000 gal
Fills from 0% to 100% — a rise of 100 points (15,000 gal). Plan a chemistry check and a re-test window once the level holds.
Preliminary estimates for planning and education. Pool, spa, and aquarium practices vary by manufacturer and local codes — verify before final decisions.
Match Your Water Body
The Scenario Cards: Pick Your Water Body
Fill-and-drain math looks identical for every container, but the target numbers are not. Find your scenario below, load its typical parameters into the calculator, and check the two pitfalls and the red line that most often trip people who own exactly that water body.
Popular
🏊
Swimming pool
▸Typical turnover: 4–6 h per full pass (typical industry practice, PHTA-aligned guidance)
▸Worked reference: 15,000 gal at 45 GPM → one turnover in ≈ 5 h 33 min (0.18 turnovers/h)
Reading the pump's nameplate GPM as the filter-loop flow — filter loading and pipe friction derate the real figure, often by 20–30%.
Treating a top-up as 'done' — water added after backwashing or evaporation needs its own chemistry-balancing window before anyone swims.
Red line: Never exceed the filter's maximum-flow rating to shorten turnover — over-driven flow channels water past the media and can channel a sand bed.
♨️
Hot tub / spa
▸Typical turnover: 30–60 min per pass (typical industry practice)
▸Typical volume: 300–500 gal on a dedicated 240 V circulation pump
Counting evaporation top-ups as turnover — an inch of weekly top-up replaces lost water but adds nothing to the filtration pass count.
Quoting jet-pump flow (high, unrestricted) as filter-loop flow; jets recirculate through the seat loop, not through the cartridge.
Red line: Keep the cartridge in the loop: bypassing the media to chase a faster turnover leaves the heater cycling on unfiltered, scale-forming water.
▸In-tank circulation: a separate 20–40×/h metric driven by wavemakers and gyres
Adding wavemaker GPH into the filtration turnover — the two flows do different jobs and are sized by different rules.
Sizing the return pump by its zero-head rating; every metre of vertical lift and every elbow cuts real delivery below the box figure.
Red line: Bare-bottom and dense live-rock layouts produce wildly different flow fields — verify against an in-tank flow-speed target (2–4 in/s, typical), not pump GPH alone.
▸Rated filter GPH is an open-water figure — real flow drops as media loads up
Choosing a canister by tank volume alone; stocking density and feeding rate set the bioload, and bioload sets the turnover target.
Cleaning all media in the same week — stripping the biofilter resets the nitrogen cycle even though the 'turnover' number never changed.
Red line: Gentle swimmers (bettas, guppies, dwarf shrimp) need calmer water: divert the return against the glass rather than oversizing the filter.
🌿
Pond / koi
▸Typical full-pool turnover: 1–2 h (typical hobbyist practice)
▸Biofilter and UV clarifier each add their own flow limits on top of turnover
Sizing one GPH number for pump, filter, and UV — a UV unit flowed above its maximum rating loses the contact time its algae kill depends on.
Overestimating pond volume: shelves, plant pockets, and irregular walls put the real volume 10–20% below the rectangle estimate.
Red line: Check the UV clarifier's maximum-flow plate before buying the pump — exceeding it voids the clarifier's rating, whatever the turnover math says.
🛢️
Rain barrel & storage tank
▸Gravity mode applies: instantaneous flow ∝ √h — 0.9 m of head through a 25 mm outlet starts near 1.28 L/s
▸A 0.6 m barrel at 0.9 m head drains to empty in ≈ 6.6 min through a 25 mm outlet (worked example below)
Assuming a constant drain rate — flow decays with the square root of depth, so the second half of the water takes far longer than the first.
Choosing the outlet size after the barrel: a 13 mm outlet takes almost four times as long as a 25 mm one (drain time scales with 1/d²).
Red line: A fully-opened bottom valve is not a Torricelli orifice — as outlet area approaches surface area the model distorts and the tank glugs instead of draining cleanly.
All turnover and flow-speed figures above are typical recommended ranges — confirm with manufacturer guidance and local codes.
Mode 1
Constant-Flow Fill and Drain: t = V / Q
t = V / Q · time = volume ÷ flow rate
The constant-flow mode is division, and everything about it is only as good as the two numbers you feed the division. Volume is the honest one — the geometry entry computes it from your container’s real dimensions, which is why a “55-gallon” drum modeled as a cylinder can come out slightly different from its label. Flow is the liar: brochure ratings describe a pump pushing through nothing, and hose ratings assume a spigot at full pressure. The number that makes fill times real is the flow your system actually delivers once filter loading, fittings, vertical lift, and friction take their share.
The start and end level fields exist because most real jobs are partial. Topping a pool back from 85% to 100% after evaporation moves only 15% of the volume, so the wait is proportionally short; drawing a dosing tank down from 80% to 20% moves 60 points of level. The calculator converts the two levels into an effective volume ΔV and divides that — same formula, smaller V.
💡 Tip — measure the flow you actually have. Before trusting any fill-time result, time how long your real hose, spigot, or pump takes to fill a bucket of known size, and enter that measured rate. A 5-gallon bucket filled in 45 seconds is about 6.7 GPM — and that figure beats any nameplate.
Worked example A — the 15,000 gallon pool
A 15,000 gal residential pool fills at a true 45 GPM. Volume over flow: 15,000 ÷ 45 = 333 minutes — that is 5 h 33 min 20 s, or 5.56 h in decimal hours, or 0.18 turnovers per hour. Load the preset (15,000 gal pool @ 45 GPM) and the constant-flow tab reproduces every one of those figures on the same keystroke. The same inputs flipped to Drain answer the pump-down question: an equal-rate sump clears the pool in the identical 5 h 33 min — which is why drain jobs live and die by pump flow, not by pool size alone.
If the flow figure itself is the unknown — you know pipe size and velocity, or a pressure drop, but not Q — work one step upstream. The flow rate calculator on the home page derives Q from geometry and velocity, and the calculators hub collects every dedicated tool in one place. This tank fill time calculator is the consumer end of that pipeline: it takes the flow number those tools produce and returns the answer you actually plan around — time. More worked examples sit in the home page’s worked-examples section.
🌊Filling is division — the hard part is the honest Q
Filling is division — the hard part is the honest QPhoto: Unsplash
Mode 2
Gravity Drain: Torricelli and the Square-Root Slowdown
Q = Cd · Ao · √(2g·h) · instantaneous outlet flow
t = (At / (Cd·Ao)) · √(2/g) · (√h1 − √h2) · drain time
When there is no pump — a rain barrel with an open spigot, a tank draining to a lower channel, a pool emptying through a bottom drain — the water’s own weight is the pump, and the physics changes shape. Torricelli’s result says the outlet velocity equals that of a free fall from the water surface: v = √(2g·h), where h is the depth above the outlet. Integration turns that velocity into the drain-time formula above: surface area At divided by the effective orifice Cd·Ao, scaled by the square roots of the starting and target depths. Every number in the gravity tab comes from those two expressions with g = 9.80665 m/s².
The discharge coefficient Cd absorbs the messy reality of a real hole — vena contracta, edge friction — and 0.62 is the standard value for a sharp-edged orifice, which is why the calculator ships with it. Short tubes behave closer to 0.8; well-rounded entrances push past 0.95. The constant matters linearly in time, so guessing it wrong shifts every estimate by the same proportion.
Worked example B — the rain barrel
Take the preset drum: 0.6 m diameter (surface area At = 0.2827 m²), a 25 mm outlet (Ao = 4.909 × 10⁻⁴ m²), Cd = 0.62, draining from 0.9 m to empty. The formula gives t = (0.2827 / (0.62 × 4.909 × 10⁻⁴)) × √(2/9.80665) × √0.9 ≈ 398 s ≈ 6 min 38 s ≈ 6.6 min. The instantaneous starting flow is Q0 = 0.62 × 4.909 × 10⁻⁴ × √(2 × 9.80665 × 0.9) ≈ 1.28 L/s — about 76.7 L/min or 20.3 GPM — falling continuously to zero as the last film of water reaches the outlet. The gravity tab reproduces all of it, including the final instantaneous flow at any target depth you name.
The intuition trap: half the water, 71% of the time
Gravity draining looks like the most straightforward scenario on this page — no pump curve, no pressure regulator, just water finding the floor. That is precisely why its blind spot survives: the constant-flow intuition says the second half of the water should take as long as the first. It never does. Because flow scales with √h, the time to move the upper half — depth h1 down to h1/2 — is
So the brisk first half of the volume leaves in about 29% of the total drain time, and the sluggish second half owns the remaining 71%. That is the mechanism behind every rain barrel that “gushed and then took all afternoon,” and it is why the decay bars in the calculator show 100% → 70.7% → 50% relative flow at full, half, and quarter depth. The slowdown is not a fault — it is the square-root law doing exactly what physics asked of it.
⚠️ Warning — what Torricelli quietly assumes. The integration treats the outlet as a small sharp-edged hole in a thin wall, the tank as freely vented (no vacuum lock), and Cd as constant. It also ignores friction in any hose attached to the outlet — a long small-bore hose throttles the flow well below the orifice prediction. When the outlet area rivals the surface area, the model distorts and the page flags it in amber.
Two sibling problems live nearby. When the head is held constant by a pressurized line rather than a falling surface — an orifice plate, a valve under steady ΔP — the flow stops decaying and the orifice flow calculator is the right tool. And once the barrel empties into a swale or channel rather than free air, the receiving side becomes its own problem for the open-channel flow calculator.
🕳️Every outlet is a square-root law wearing a valve
Every outlet is a square-root law wearing a valvePhoto: Unsplash
Mode 3
Turnover Rate: Pools, Spas, Aquariums, Ponds
cycles/h = Q / V · turnover time = V / Q
Turnover is the reciprocal lens on the same division: instead of asking how long a single fill takes, it asks how many times per hour the loop re-processes the whole volume. A filtration loop moving 2,700 GPH through a 15,000 gal pool delivers 2,700 ÷ 15,000 = 0.18 turnovers per hour — one full pass every 5.56 h. The tab runs both directions, because real jobs start from either end: an owner knows the pump and wants the pass rate, or a keeper knows the target and wants the flow to buy.
Worked example C — the 120 gallon reef
Hobbyist practice targets 3–5 turnovers per hour through the filtration loop of a reef display. For a 120 gal tank at 5×/h: 120 × 5 = 600 GPH — which is 37.9 L/min, or exactly 10 GPM — giving one turnover every 12 minutes. Load the preset (120 gal reef tank · 5× turnover) and the turnover tab shows the same 600 GPH, its LPH twin (2,271 LPH), and the 12-minute pass. Drop the target to 3×/h and the requirement falls to 360 GPH — the difference between a mid-sized return pump and a large one, both in purchase price and in the heat the pump dumps into the water.
⚠️ Mismatch alert — the nameplate is not the system. Treating the pump’s nameplate flow as the system’s actual flow is the single most common turnover error. Filter media loads, pipe friction spends pressure, vertical lift eats head, and every elbow taxes the curve — real loops commonly deliver 20–30% below the box figure. Measure the loop, or derate the nameplate deliberately, before promising any turnover number.
❌ Don’t — one turnover number for a saltwater tank. Reef systems carry two flow metrics that must never be blended: filtration/sump turnover (3–5×/h through media, typical hobbyist practice) and in-tank circulation (20–40×/h from wavemakers, serving coral tissue flow). Adding wavemaker GPH into the filtration figure — or buying return pumps sized for circulation — produces a tank that is simultaneously over-plumbed and under-filtered.
Typical turnover windows differ by water body — pools and spas follow industry practice, aquariums and ponds follow hobbyist practice — and the reference table below collects them with their nature labelled. None of them are standards: they are starting points to confirm against manufacturer manuals and local codes.
🐠Turnover: how many times per hour the loop sees everything
Turnover: how many times per hour the loop sees everythingPhoto: Unsplash
Numbers on Hand
Reference Tables
Four working tables: nominal container volumes so the volume input starts honest, typical turnover windows so the target is grounded, a Torricelli flow-versus-head table computed from the on-page formula, and a turnover-to-GPH conversion for the common tank sizes.
Typical turnover windows
All values are typical recommended ranges drawn from industry and hobbyist practice — confirm with manufacturer guidance and local codes.
Water body
Typical turnover
Notes (source nature)
Swimming pool
4–6 h per turnover
Typical industry practice (PHTA-aligned); higher bather load argues for the short end
Hot tub / spa
30–60 min per turnover
Typical industry practice; small volume and heavy bather load
Reef aquarium — filtration / sump
3–5×/h
Typical hobbyist practice; in-tank circulation is a separate 20–40×/h metric
Freshwater community tank
3–5×/h
Typical hobbyist practice through filter media only
Gravity-drain scenario — use the Torricelli tab instead
Common container volumes (nominal)
Manufacturer nominal ratings unless noted. Pool figures are computed on this page from L × W × uniform depth × 7.48052 gal/ft³. Aquariums hold 10–15% less than nominal once substrate, rock, and decor displace water.
Nominal rating; real water volume typically 10–15% less after substrate & rock
Aquarium 40 breeder
40 gal
151.4 L
Nominal rating; same 10–15% deduction applies
Aquarium 55
55 gal
208.2 L
Nominal rating; same 10–15% deduction applies
Aquarium 75
75 gal
283.9 L
Nominal rating; same 10–15% deduction applies
Aquarium 125
125 gal
473.2 L
Nominal rating; same 10–15% deduction applies
Torricelli flow vs water head (25 mm outlet, Cd = 0.62)
Instantaneous flows computed on this page from Q = Cd·Ao·√(2g·h). Remaining drain time assumes the worked-example barrel: 0.6 m diameter surface (At = 0.2827 m²) draining to empty — scale by your own At.
Head h (m)
Instantaneous flow (L/s)
Flow (L/min)
Flow (GPM)
Remaining time to empty (on-page formula)
0.25
0.67
40.4
10.7
≈ 3 min 30 s (3.5 min)
0.50
0.95
57.2
15.1
≈ 4 min 57 s (4.9 min)
0.90
1.28
76.7
20.3
≈ 6 min 38 s (6.6 min)
1.50
1.65
99.0
26.2
≈ 8 min 34 s (8.6 min)
Turnover target → required flow
Computed on this page as GPH = volume × turnovers per hour. Read the row for your tank, the column for your target.
Tank volume (nominal)
3×/h → GPH
5×/h → GPH
10×/h → GPH
55 gal
165 GPH
275 GPH
550 GPH
120 gal
360 GPH
600 GPH
1,200 GPH
180 gal
540 GPH
900 GPH
1,800 GPH
💡 Tip — nominal is a label, not a volume. When the table and the tape measure disagree, believe the tape: enter real inside dimensions (or a weighed fill) in the constant-flow tab, and subtract the displacement of substrate, rock, and equipment before quoting any turnover.
Scenario Extension
Two-Tank Transfer: When the Scenario Grows a Second Container
Gravity transfer between two vented tanks is the single-tank problem wearing a second container: the driving head is now the level difference Δh between the two surfaces, and it shrinks as water moves. The connecting opening obeys the same square-root law, Q = Cd·Ao·√(2g·Δh), and integration produces a result that maps perfectly onto the single-tank formula — replace the surface area with an effective area and the depth with the starting difference:
The final state is equilibrium — both surfaces at the same level — reached asymptotically as Δh decays to zero. One consequence surprises people: with two equal tanks, Aeff is half of either tank’s area, so equalization takes half the time a single tank needs to drain through the same opening from the same starting difference. Each gallon transferred lowers Δh twice as fast because one tank rises while the other falls.
💡 Tip — sanity-check with the mapping. Two 0.6 m barrels with a 0.3 m level difference equalize in ≈ 115 s through a 25 mm outlet (Cd = 0.62), while a single barrel draining 0.3 m deep through the same outlet needs ≈ 230 s. The Aeff mapping reproduces both — a fast mental audit before trusting any spreadsheet.
Where the approximation bends
01The connecting line must be short and large-bore — long or thin lines add friction that throttles the flow below the orifice prediction.
02Both tanks need free venting; a sealed low tank pulls a vacuum and stalls the transfer regardless of Δh.
03The formula stops applying if one tank’s surface falls to its own outlet level first — from there the problem is a single-tank drain with a partial head, not equalization.
04Any pump in the loop converts the job to driven flow: use the constant-flow tab with the pump’s real delivery instead of Torricelli.
Red Flags
Common Mistakes and Red Flags
Six failures that account for most bad fill, drain, and turnover estimates — each one a quiet substitution of an easier number for the true one.
Nameplate as system flow. Pump boxes quote unrestricted test-stand flow; filter loops deliver 20–30% less once media, friction, and lift take their share. Measure or derate before dividing.
Constant-flow intuition on gravity drains. A barrel that empties its first half in 29% of the total time is not malfunctioning — flow follows √h. Budget the second half honestly.
Blended flow metrics in saltwater tanks. Filtration turnover (3–5×/h) and in-tank circulation (20–40×/h) are different numbers for different jobs — never add wavemakers into the filter figure.
Nominal volume as real volume. Labels, substrate, rock, and irregular walls: aquariums commonly hold 10–15% less than their rating, and ponds often 10–20% less than the rectangle estimate.
Top-ups mistaken for turnover. Evaporation replacement restores level, not pass count — spa and pool hygiene depends on the loop, not on the refill.
Unit slips at the worst moment. Mixing imperial dims with a metric outlet, or quoting a US gallon as an imperial one, moves results by 3.8–20% silently. Pick one unit set per calculation and let the selects carry it.
Extreme inputs and how this page handles them
The calculator is built to fail loudly and locally: any empty, zero, or negative input raises a rose error card in the affected tab while the other tabs keep computing, and Reset restores the worked-example defaults in one click. Beyond those obvious cases, four physical boundaries get explicit treatment:
01Outlet area rivals surface area (Ao ≥ At): the Torricelli integration assumes a small orifice; when the opening is the cross-section, the model distorts and the page raises an amber alarm — real tanks glug and surge in that regime.
02Target above starting depth (h2 > h1): flagged as a reversal — through the outlet the tank would be filling, not draining; the page asks you to swap the values rather than returning a negative time.
03Discharge coefficient outside 0–1: Cd is bounded physically; values at or beyond the bounds return an error pointing at the sharp-edged default of 0.62.
04Huge values: results keep full precision with thousands grouping (a 100,000 m³ reservoir at 5,000 GPM reads 52.9 h, not a rounded surprise), and identical inputs always reproduce identical outputs — no hidden caps or saturation.
❌ Don’t — treat a model warning as decoration. The amber and rose cards exist because the math has edges. If the page flags an orifice-to-surface conflict or a reversed depth, the physically honest next step is a different tool or a different measurement — not a bigger Cd to force a tidy answer.
Provenance
Data Sources & References
Constant-flow relation t = V/Q — general physics (volume ÷ flow); derived, not sourced. Partial-fill levels use the same relation on ΔV.
Gravity drain — Torricelli’s law (a Bernoulli consequence) integrated for a falling head; classical fluid mechanics. Computed on this page with g = 9.80665 m/s².
Discharge coefficient Cd ≈ 0.62 — standard hydraulics reference value for sharp-edged orifices (rounded entrances 0.95+, short tubes ≈ 0.8). Adjustable in the tool.
Turnover windows — typical industry practice for pools and spas (PHTA-aligned guidance) and typical hobbyist practice for aquariums and ponds. Ranges, not standards; confirmed against manufacturer manuals and local codes is the user’s step.
Container volumes — manufacturer nominal ratings (drums, IBC totes, standard aquarium sizes); pool figures computed on-page from dimensions × 7.48052 gal/ft³ under a uniform-depth assumption (NIST conversion constant).
Unit conversions — 1 US gallon = 3.785411784 L; 1 ft³ = 7.48052 gal; exact SI-traceable factors applied throughout the tool and tables.
📚Every reference number traces to a formula or a label
Every reference number traces to a formula or a labelPhoto: Unsplash
Frequently Asked Questions
How long does it take to fill a pool?▼
Divide the pool volume by the true fill rate — not the hydrant's or hose's best-case rating. A garden spigot with a 5/8 in hose usually delivers 8–12 GPM on household pressure, so a 15,000 gal pool refills in roughly 21–31 hours, while a dedicated 45 GPM fill line does the same job in about 5½ hours. Measure your actual rate once with the bucket-timing method, enter it in the constant-flow tab, and let the start/end level fields handle a partial refill — topping up from 85% to 100%, for example, takes a fraction of the full-volume time.
How do I calculate tank drain flow rate?▼
For a gravity outlet, flow at any instant is Q = Cd·A·√(2g·h), where h is the water depth above the outlet, A is the outlet area, and Cd ≈ 0.62 for a sharp-edged opening. It is not a constant: at 0.9 m of head a 25 mm outlet passes about 1.28 L/s, but at 0.25 m of head only 0.67 L/s. If a pump or a mains line empties the tank instead, the flow is simply whatever that line delivers — use the constant-flow tab and divide volume by rate. The gravity tab computes the whole curve for you, including the final flow as the tank empties.
What GPH flow rate does my aquarium need?▼
For the filtration loop, hobbyist practice targets 3–5 turnovers per hour: multiply display volume by that band. A 120 gal reef therefore wants 360–600 GPH through filter and sump; a 55 gal community tank wants 165–275 GPH. Keep that number separate from circulation — wavemakers moving 20–40×/h inside the display serve coral flow needs and never pass through filter media. When in doubt, start at the low end for fish-only tanks, sit mid-band for mixed reefs, and check manufacturer guidance for delicate species.
Why does a gravity drain slow down over time?▼
Because the driving head itself is falling. Torricelli's result makes outlet velocity proportional to √h, so when the depth halves, the outlet velocity drops to about 70.7% — and the first half of the water leaves in only 29% of the total drain time, leaving 71% of the time for the second half. Pressure-assisted drains (a pump or mains feed) do not behave this way; only head-driven ones do. That mismatch is why a rain barrel that gushes when full can take frustratingly long to finish its last few inches.
How do I calculate flow rate between two tanks?▼
The driver is the level difference Δh between the two surfaces, and the connecting opening obeys the same square-root law: Q = Cd·A·√(2g·Δh). Because each level moves as water transfers, Δh shrinks continuously; for two vented tanks of surface areas A₁ and A₂ linked by an orifice, equalization time works out to the single-tank Torricelli formula with the surface area replaced by A₁A₂/(A₁+A₂) and the depth replaced by the starting difference. Two equal tanks therefore level out in half the time a single tank needs to drain through the same opening. The two-tank section below walks through the approximation and its limits.
What is pool turnover rate?▼
Turnover rate is how long the filtration loop takes to pass one entire pool volume — not how long the water 'sits'. A 15,000 gal pool moving 2,700 GPH through the filter turns over in about 5 hours 33 minutes, or 0.18 turnovers per hour. Industry practice for residential pools is a 4–6 hour turnover (typical figures — confirm against your local code), and the calculation should use the real filter-loop flow, which always sits somewhat below the pump's nameplate. The turnover tab converts between volume, GPH, turnover time, and cycles per hour in both directions.
The guide library explains why a discharge coefficient moves, how velocity bands shape every loop, and where the square-root law comes from — the reading behind every number this calculator prints.
Flow Calculator (Flow-Calculator.com) · Last updated: November 13, 2026 · Results are planning estimates — verify against manufacturer guidance and local codes.
