Flow Rate Calculator

Volumetric flow rate is cross-sectional area times mean velocity: Q = A × v. For a circular pipe, A = π × d²/4, so Q = π × d²/4 × v. Multiply the volumetric flow by the fluid density to get the mass flow rate, ṁ = ρ × Q. Enter any two of area (or diameter), velocity, and flow in the calculator and the third follows instantly.

Volumetric flow rate (m³/s, L/min, GPM, CFM) measures how much volume passes a cross-section per unit time; mass flow rate (kg/s, lb/h) measures how much mass. They are linked by density: ṁ = ρ × Q. Volume flow is what a bucket and stopwatch measure; mass flow is what fans, burners, and process balances are rated in.

For gases, volume depends on pressure and temperature. The calculator applies the ideal-gas law at your operating pressure and temperature: SCFM = ACFM × (P_op / 14.696 psia) × (68 °F / T_op), and Nm³/h uses 1 atm and 0 °C as the reference. Pick a gas preset (air, natural gas, nitrogen…), set operating pressure and temperature, and both figures appear automatically.

Domestic water supply is typically designed at 1.0–2.5 m/s (3–8 ft/s). Below about 0.6 m/s sediment can settle; above about 2.5–3 m/s you invite water hammer, noise, and erosion at fittings. Fixture branches run quieter at 0.6–1.5 m/s. Select an application preset in the calculator to check your velocity against its published range.

Reynolds number Re = ρ·v·D/μ compares inertial to viscous forces and tells you the flow character. Re below 2300 is laminar (smooth layers), above 4000 is turbulent (well mixed). The regime decides which friction law applies: laminar friction factor f = 64/Re, turbulent flow uses the Colebrook equation — which is exactly what the calculator runs.

The calculator uses the Darcy–Weisbach equation: ΔP = f × (L/D) × ρv²/2, with the friction factor f from the Colebrook–White equation using your pipe material's roughness (PVC 0.0015 mm, commercial steel 0.045 mm, cast iron 0.26 mm, and so on). Results are shown as head loss and as pressure drop over the full length and per 100 m.

Yes — that is what the standard pipe size presets are for. Choose Steel NPS Schedule 40 or 80, PVC Schedule 40 or 80, Copper Type K/L/M, PEX, or hose, then pick the nominal size: the actual internal diameter is filled in automatically from ASME B36.10M / ASTM D1785 / ASTM B88 data, so you never have to look it up.

Yes. Switch the cross-section shape to rectangular duct (HVAC, plenums) or half-round channel (gravity sewers running half-full). Non-circular sections are handled through the hydraulic diameter Dₕ = 4A/P, which the calculator reports alongside the flow area — the same basis HVAC and drainage design uses.

Yes. Select the water preset and the results panel reports the same flow as litres per minute, GPM, L/s, m³/h, and m³/s side by side — no conversion needed. For reference, 1 GPM = 3.785 L/min, so a shower head at 9 L/min reads about 2.4 GPM. Fixture branches designed at 0.6–1.5 m/s show their delivered GPM directly in the same panel.

Diameter gives the area, A = πd²/4; pressure is what drives the velocity through it. With only pressure and diameter known, the missing piece is velocity: enter a trial velocity in the friction-loss panel and read the Darcy–Weisbach head it consumes over your pipe length — the velocity whose loss matches the available pressure is the one to use, then Q = A × v. As a sanity check, a 15 mm pipe at 1.5 m/s carries about 16 L/min.

It runs entirely online in your browser — phone, tablet, or desktop — with nothing to download or install. Because every calculation happens locally on the device, the loaded page keeps working like a native flow calculator app even on a weak connection, and you can save it to your home screen to open full-screen like an installed app.

Yes — it is built on the standard fluid mechanics toolkit: Reynolds number and flow regime, hydraulic diameter for non-circular sections, the Colebrook–White friction factor, Darcy–Weisbach head loss, and ideal-gas standard-flow conversion (SCFM, Nm³/h). Every formula is printed on the page, so results can be verified by hand for coursework or design reviews.

Time how long a fully-open source takes to fill a bucket of known size: GPM = gallons ÷ (seconds ÷ 60). A 5-gallon bucket filled in 45 s is 6.7 GPM (≈ 25 L/min). With a water meter instead: make sure nothing else is drawing, run exactly one minute, and read the register — each cubic foot is 7.48 gallons, and each litre read × 60 gives L/h. Both methods yield the maximum available flow at the source, which is exactly what irrigation capacity checks need.

The same equation as any liquid: Q = A·v. Halve the vessel diameter to get the radius, A = πr², then multiply by the mean blood velocity. A carotid artery of 7 mm diameter with a 25 cm/s mean velocity carries π·(3.5 mm)² × 25 cm/s ≈ 9.6 mL/s ≈ 0.58 L/min. Blood's density (1,060 kg/m³) and viscosity (≈ 3.5 mPa·s) put the Reynolds number near 500 — laminar flow, which is the healthy regime in most vessels.

It is the ceiling a system can deliver — beyond it, something else gives. For a tap it is set by supply pressure and pipe size; for a 3D-printer hotend it is the melt rate (10–17 mm³/s for a standard 40 W heater, 20–30 mm³/s for Volcano-class); for a pump it is the flat end of its curve. Sizing a system means staying under the ceiling with margin, not sitting on it.

Work the equation backwards: d = √(4Q/(πv)) after choosing a sensible velocity from the application band. A 40 GPM (2.52 L/s) domestic main at 2 m/s needs d = √(4 × 0.00252/(π × 2)) ≈ 40 mm — i.e. a 1½″ Schedule 40 pipe (40.9 mm ID). Enter that preset in the calculator and the velocity checker confirms the result sits in the green.

Time = volume ÷ flow. A 1,000 L buffer tank filling at 25 L/min takes 40 minutes. Draining through the same line takes longer in practice, because the head — and therefore the pressure and velocity — falls as the level drops. For a first estimate, though, t = V/Q is what batch times, dosing schedules, and tank turnover are built on.

SDR (Standard Dimension Ratio) is a plastic pipe's outside diameter divided by its wall thickness. Lower SDR means a thicker wall, a higher pressure rating, and a smaller bore. Inside and outside diameter are linked by dᵢ = d·(SDR − 2)/SDR: a 90 mm OD pipe at SDR 11 has a 73.6 mm bore, while at SDR 17 it opens up to 79.4 mm — and flow capacity grows with it. Match SDR to system pressure first, then check what the smaller bore costs you in flow.

Wall roughness. New plastic runs ≈ 0.0015 mm surface roughness against ≈ 0.045 mm for commercial steel — 30× smoother — so the Darcy friction factor f, the pressure drop per metre, and the pumping energy all come out lower. Steel also roughens with age as it corrodes and scales, while plastic stays put: a decades-old steel line can behave like cast iron (0.26 mm) or worse.

Cv is a valve's flow coefficient: the US gallons per minute of water that pass through the valve with a 1 psi pressure drop. Its metric cousin Kv is m³/h at 1 bar (Cv ≈ 1.16 × Kv). Once the manufacturer quotes Cv, required flow Q and specific gravity SG give the pressure drop: ΔP = SG·(Q/Cv)². The same square-root law governs orifices and spray nozzles — flow through a restriction always scales with √ΔP.

Linear flow (cm/h) is the speed of the mobile phase across the column's cross-section — volumetric flow divided by bed area: Q = u × π·(ID/2)². It is the quantity resin vendors quote because scale-up holds it constant: a 200 mm-ID column at 100 cm/h runs ≈ 524 mL/min where a 50 mm lab column needs ≈ 33 mL/min — sixteen times the volumetric flow at identical residence behaviour.

Gauge pressure (psig, barg) is measured relative to atmosphere; absolute pressure (psia, bara) counts from true zero: psia = psig + 14.696. Gas density calculations and standard-flow conversions (SCFM, Nm³/h) only work in absolute terms — an 80 psig compressed-air line is 94.7 psia, which is why actual-to-standard flow conversion multiplies by about 6.4, not 80.

Diameter gives area A = πd²/4; pressure gives the driving force but not the velocity directly. The missing link is either a known velocity (enter it in the calculator and read the friction loss it consumes over your pipe length — the velocity whose Darcy–Weisbach head matches your available pressure is the one to use, then Q = A × v) or the Hazen–Williams formula for water supply (Q = 0.435·C·d^2.63·(ΔP/L)^0.54, C = 150 for PVC, 120 for new steel). For a full step-by-step with worked examples see our <a href='/pressure-to-flow-rate-calculator'>pressure-to-flow-rate guide</a>.

Start with Darcy–Weisbach: ΔP = f·(L/D)·ρv²/2. First compute velocity v = Q/A, then Reynolds number Re = ρvD/μ to find the flow regime (laminar Re<2300 → f = 64/Re, turbulent Re>4000 → solve Colebrook–White iteratively for f). Multiply the head loss h_f = f·(L/D)·v²/2g by ρg to get pressure drop, or read it directly in the calculator's friction-loss panel. For the complete method including local losses (elbows, valves) and pipe roughness tables, see our <a href='/flow-rate-to-pressure-drop-calculator'>pressure-drop guide</a>.

Pick a recommended velocity band for your application (domestic water 1.0–2.5 m/s, compressed air 3–8 m/s, gravity drain ≥0.6 m/s self-cleansing), then compute the required area A = Q/v, and diameter d = √(4A/π). Round up to the nearest nominal pipe size, pick a schedule (Sch 40 vs Sch 80 — thicker wall = smaller bore = higher velocity = more friction), then verify the actual velocity and pressure loss with the calculator's preset. Full step-by-step with schedule trade-offs is in our <a href='/pipe-size-calculator'>pipe-sizing guide</a>.

Flow through any restriction scales with the square root of the pressure drop: Q = C_d·A·√(2ΔP/ρ). The discharge coefficient C_d captures the contraction and friction — sharp-edged orifice ≈ 0.60–0.62, nozzle ≈ 0.97–0.99, pipe entrance ≈ 0.82. Double ΔP and flow gains only 41%. For valves, the manufacturer's Cv (Kv metric) encodes this: Cv = GPM of water at 1 psi drop, ΔP = SG·(Q/Cv)². See our <a href='/orifice-flow-rate-calculator'>orifice guide</a> for ISO 5167 plate standards and our <a href='/orifice-flow-rate-calculator'>orifice calculator</a> for valve sizing.

ACFM is actual cubic feet per minute at your operating pressure and temperature; SCFM (standard cubic feet per minute) normalises to 14.696 psia and 68 °F — the reference conditions compressor nameplates use. The conversion is SCFM = ACFM × (P_op/14.696) × (528 °R/T_op). Pipe pressure drop for air needs the density correction (air density rises with absolute pressure), and recommended header velocities are 3–8 m/s. The calculator does all this automatically when you pick a gas preset and enter operating pressure. Full method with leak detection and compressor FAD is in our <a href='/compressed-air-flow-rate-calculator'>compressed-air guide</a>.

Hose inner diameter is not the nominal label — a 5/8" hose typically has a 12.7 mm ID, 3/4" has 15.9 mm, and 1" has 20.6 mm. Pressure drop through hose follows Hazen–Williams (C ≈ 150 for smooth rubber/PVC), and the sensitivity is stark: doubling the bore multiplies flow ≈ 7.8× at the same pressure and length; doubling the length only halves it (√ΔP/L law). Our <a href='/garden-hose-flow-rate-calculator'>hose guide</a> has GPM-by-size/length/pressure tables and drip-line sizing examples.

ṁ = ρ·Q is the bridge — mass flow (kg/s, lb/h) equals volumetric flow (m³/s, GPM) times density (kg/m³, lb/ft³). For liquids, density is nearly constant (water ≈ 998 kg/m³, gasoline ≈ 742, diesel ≈ 831) so the conversion is one multiplication. For gases, density depends on pressure and temperature via the ideal-gas law, which is why standard-vs-actual flow conversion exists (SCFM vs ACFM). The calculator reports both side by side for every preset, and our <a href='/calculators'>unit conversion guide</a> has the full table of every major unit.

Linear velocity u (cm/h) is the speed of the mobile phase across the column's cross-section — volumetric flow Q divided by bed area A: u = Q/A. To convert: Q (mL/min) = u (cm/h) × π × (ID/2)² (cm²) ÷ 60. Scale-up holds linear velocity constant — a 200 mm ID column at 100 cm/h needs 16× the volumetric flow of a 50 mm lab column (area ratio). Typical values: reversed-phase HPLC 2–10 cm/h, ion exchange 30–100 cm/h, GC helium 20–40 cm/s. Full conversion table is in our linear flow calculator</a> for chromatography velocity conversion.

Three ceilings exist. Engineering ceiling: recommended velocity limits to avoid erosion, noise, and water hammer — 2.5–3 m/s for PVC, 2.5 m/s for steel, 3 m/s for copper (see the velocity guide table). Physical ceiling for liquids: the velocity at which vapour pressure causes cavitation (rarely reached in design). Physical ceiling for gases: sonic or critical flow, reached when P₂/P₁ < 0.528 for air — beyond this, increasing supply pressure does not increase flow because sound is the information speed limit. The calculator's velocity guide checks against your application's ceiling. See our calculators page</a> for sonic vs erosion limits.

Start with two of the three core variables: cross-sectional area A, mean velocity v, or volumetric flow Q. If you know A and v, Q = A·v instantly — for a circular pipe A = πd²/4, the calculator's pipe-size presets fill the real ID from schedule tables. If you know pressure instead of velocity, run the friction-loss panel: enter a trial velocity, read the Darcy–Weisbach head it consumes, iterate until the loss matches your available pressure, then Q = A × v. If you know flow and need velocity, v = Q/A. The calculator reverses any pair of inputs into the third — there is no 'calculate' button, every keystroke updates.

Pick the gas preset in the calculator (natural gas ≈ methane, air, nitrogen, steam, propane…), enter operating pressure (absolute — psig + 14.696, barg + 1.013) and temperature, and the preset corrects density via the ideal-gas law (ρ = P·M/(Z·R·T)). Volumetric flow Q = A·v still works, but mass flow ṁ = ρ·Q will differ between ACFM and SCFM because the density at standard conditions is fixed. Steam additionally needs wetness fraction (quality) if it is saturated — the calculator's steam preset handles superheated and saturated steam at different pressures.

v = Q/A and Q = v·A are the same two-way equation — pick your known and compute the unknown. For a circular pipe v = Q/(πd²/4). For non-circular sections (rectangular duct, half-round channel) the calculator uses hydraulic diameter Dₕ = 4A/P (four times area over wetted perimeter) so velocity and Reynolds number are computed on the same basis as round pipes. Enter either flow or velocity and the panel reverses it for you — no manual area calculation needed.

Pick the trade you are in and stay consistent. Plumbing and irrigation speak GPM or L/min; HVAC speaks CFM or m³/h; process plants speak m³/h or m³/s; chromatography speaks mL/min; dosing speaks µL/min; mass flows speak kg/s, kg/h, or lb/h. The calculator reports every major unit simultaneously in the results panel so you never need to convert manually, and our <a href='/calculators'>calculators page</a> has the full cross-reference table for paper work.