Flow Rate Conversion Calculator — GPM, L/min, CFM, m³/h, kg/h

How the Converter Works: Exact Definitions First

Every volumetric and mass factor on this page is derived, not memorized. The 1959 international yard-and-pound agreement fixed the inch at exactly 25.4 mm, and that single decision chains three exact definitions together:

• The US liquid gallon is exactly 231 in³, so 1 gal = 231 × (25.4 mm)³ = 3,785.411784 mL = 3.785411784 L — which is why 1 GPM = 3.785411784 L/min with no measurement uncertainty at all.
• The foot is exactly 0.3048 m, so 1 ft³ = 0.3048³ m³ = 28.316846592 L — which is why 1 CFM = 28.316846592 L/min and 1 CFS = 448.831 GPM.
• The avoirdupois pound is exactly 0.45359237 kg — which is why 1 kg/h = 2.204622622 lb/h and 1 kg/s = 7,936.6414 lb/h to every digit shown.

Time factors are trivially exact (60 min/h, 3,600 s/h), so conversions among GPM, GPH, L/min, L/s, mL/min, m³/h, m³/s, CFM, CFS, ft³/h, kg/h, kg/s, lb/h, lb/min, and g/s are pure arithmetic. No fluid property, no measurement, no approximation — which means a conversion quoted from this page can be defended line-by-line in a datasheet review.

The one place convention enters is the gas-standard group. A “standard cubic foot” and a “normal litre” are volumes at conditions someone chose — the number is meaningless until the temperature and pressure of those conditions are named. This page quotes SCFM at 60 °F and 14.696 psia, and NL/min and Nm³/h at 0 °C and 101.325 kPa, and says so everywhere.

🧪Exact definitions make conversions auditable

Exact definitions make conversions auditablePhoto: Unsplash

Volumetric Flow Conversions in Practice

The pairs that cross desks most often — plumbing in GPM, HVAC in CFM, process engineering in m³/h — arranged as a one-glance check table. Every cell is the exact factor applied to the GPM column and rounded once, at display time, to three decimals.

Two worked examples show how the table is built. First, 100 GPM of water is 100 × 3.785411784 = 378.541 L/min, which is 378.541 L/min × 60 min/h ÷ 1,000 L/m³ = 22.712 m³/h, and 100 × 0.13368056 = 13.368 CFM. Second, the HVAC bridge: 1 CFM = 28.316846592 L/min = 1.699011 m³/h = 7.4805 GPM, so a 2,000 CFM air handler moves 3,398 m³/h — the number a European duct schedule expects.

The same table logic answers the mixed-unit questions that come up on job sites: a fire pump rated 250 GPM delivers 946.353 L/min; a well yield of 10 GPM is 2.271 m³/h per hour of running time; an irrigation main carrying 500 GPM is moving 66.840 CFM of water volume — a unit pairing that only makes sense as volume, never as mass, because the fluid at each end of the comparison has a different density.

When the conversion is done and the next question is “can the line carry it?”, take the converted figure to the application presets, where each scenario carries a recommended velocity band built in.

Standard and Actual Gas Flow: SCFM, Nm³/h, ACFM

Why the conversion is multiplicative

Gases have no fixed volume — a fixed amount of gas expands and contracts with pressure and temperature. The ideal-gas law, PV = nRT, says the amount of gas in a volume scales with P/T: density at line conditions is density at standard conditions multiplied by (P_act/P_std) × (T_std/T_act). Mass conservation then flips that ratio into the flow conversion:

ACFM = SCFM × (P_std/P_act) × (T_act/T_std)

Two properties follow, and both matter in practice. First, there are no additive terms — only ratios of absolute pressure and absolute temperature, so gauge pressures must be converted to absolute (psig + 14.696, barg + 1.013) and temperatures to absolute scales before the ratio is formed. Second, the same multiplicative structure handles every caliber pair: NL/min and Nm³/h share the 0 °C basis so they differ only by the time factor (×60), while SCFM sits at 60 °F and differs from them by 288.71/273.15.

Worked example, the one compressor salesmen argue about: a compressor rated 500 SCFM (60 °F, 14.696 psia caliber) delivering into a line at 100 psig and 70 °F. Absolute line pressure is 114.696 psia, so the pressure ratio is 14.696/114.696 = 0.12814, the temperature ratio is 530/520 °R = 1.01923, and the actual flow is 500 × 0.12814 × 1.01923 = 65.30 ACFM. The gas occupies about one-eighth of the volume its rating implies — a 7.8:1 compression ratio doing exactly what it should.

Reference calibers compared

Every “standard” gas volume needs its conditions named, and industry chose differently across borders and eras. The multiplier column converts one unit of flow quoted at that caliber into Nm³/h at true normal conditions (0 °C, 101.325 kPa) — each row states the basis unit it applies to.

The datasheet caliber trap — the detail official tutorials skip

The number-one cause of wrong gas-flow conversions is not bad arithmetic — it is an unnamed caliber. The US compressor tradition (60 °F) and SI-aligned practice (20 °C) differ by exactly 293.15/288.71 = 1.0154, about 1.5%. That sounds small until it decides something: a fan selected at the wrong caliber is 1.5% short of its duty point, two instruments compared across calibers disagree by 1.5% with neither one broken, and a compressor acceptance test contested at the wrong caliber becomes a dispute with no arithmetic error to point at. The trap is silent because every conversion looks right on its own.

The discriminator is cheap: read the standards cited on the datasheet. A document citing ASME PTC 9 or ANSI practice, or stating “60 °F, 14.696 psia”, is on the US caliber. A document citing ISO 1217 (or CAGI performance data, which follows ISO 1217 practice) is on 1 bar/20 °C. A European sheet quoting Nm³/h is on DIN 1343 conditions — 0 °C/1 atm, which is the normal basis. If the sheet says only “SCFM” or “Sm³/h” with no citation and no conditions, the caliber is unknown and the numbers are unquotable until the vendor answers one question.

Where the ideal-gas shortcut breaks

The multiplicative relation is exact for an ideal gas and therefore excellent for compressed-air work at moderate conditions. Its failure boundary is set by the compressibility factor Z: once Z departs meaningfully from 1, volume no longer scales linearly with P/T and the ratio method inherits that error. The typical thresholds engineers use: above ≈10 bar absolute, or below ≈−50 °C, ideal-gas conversion can drift more than 1% — enough to matter in custody transfer, high-pressure bottle banks, and cryogenic or LNG-adjacent service. Past the red line, use a real-gas method (a Z-factor chart or an equation of state) instead of the ratio shortcut; the structure of the conversion stays the same, but the density ratio must be computed from real-gas densities rather than P/T alone. This page flags any Tab 3 result computed past those thresholds.

Before a Cv or orifice calculation, every gas figure must live on one basis: take the datasheet’s standard flow, correct it to actual line conditions here, then feed the actual flow to the orifice flow calculator — Cv math mixes units silently, and a standard/actual mismatch inside it is invisible in the result.

🌬️Standard flow is a convention — name it or lose it

Standard flow is a convention — name it or lose itPhoto: Unsplash

Which Units to Quote Where

A datasheet is a promise to a reader you will never meet, in an office that speaks a different unit dialect. The convention that works in practice: quote the pair the reader’s industry speaks and its SI twin, with the basis named. Plumbing and irrigation speak GPM or L/min. HVAC speaks CFM or m³/h. Process plants speak m³/h for volume and kg/h for mass balance. Compressed air speaks SCFM and Nm³/h — always with the caliber. Dosing and chromatography speak mL/min and, at the small end, µL/min.

Three basis rules prevent most disputes. For liquids, state the fluid and the temperature that fixes its density — “100 L/min of water at 20 °C” is a complete specification, “100 L/min” is not. For gases, name the reference conditions next to every standard-volume figure, and mark actual figures as actual. For mass flows, mass needs no basis statement — that is the quiet advantage of kg/h and lb/h in mixed-vendor procurement, and the reason process balances are written in mass.

The deeper reasoning — why density inherits temperature, how viscosity and regime interact with the converted figure — lives in the guide library, which pairs every tool on this site with the reading that explains it.