Flow Rate Converter
Convert GPM, litres per minute, CFM and cubic metres per hour — volumetric flow only, with US and Imperial gallons kept twenty per cent apart where they belong.
Flow Rate Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Volumetric flow only; mass flow requires density and is reported separately.
- CFM here is actual volumetric flow, not SCFM corrected to standard conditions.
- Converting flow to velocity or pressure drop needs pipe geometry and is outside a unit conversion.
- Gas volumetric flow varies with pressure and temperature; mass flow is the stable quantity.
In short: Flow conversion routes through the cubic metre per second. 1 L/s is 15.850323 US GPM and 1 CFM is exactly 28.316847 L/min. Two cautions: US and Imperial GPM differ by 20.095%, so a pump rated “50 GPM” is ambiguous; and this is volumetric flow — mass flow in kg/s is a different quantity that needs the fluid's density.
Formula
Each factor is the number of cubic metres per second in one of that unit. Every factor is exact, since the litre, the US gallon at 3.785411784 L, the Imperial gallon at 4.54609 L and the foot at 0.3048 m are all exactly defined.
Worked Example
- Identify the source factor. One US GPM is 6.3090196×10-5 m³/s.
- Convert to the SI unit. 100 × that = 0.0063090196 m³/s.
- Identify the target factor. One m³/h is 1/3600 m³/s.
- Divide. 0.0063090196 × 3600 = 22.712471 m³/h.
- Check the gallon. 100 Imperial GPM would be 27.2765 m³/h instead — 20% more.
The ambiguous pump. A pump advertised at 50 GPM moves either 189.27 or 227.30 litres per minute depending on whose gallon the catalogue used. That is a twenty per cent difference in duty, easily enough to undersize a system or fail a commissioning test. British and American equipment specifications regularly sit side by side in the same project documentation, so the gallon should be named explicitly wherever the figure matters.
Strengths & Limits Of This Model
Where this engine is strong
- US and Imperial gallons per minute reported side by side, never merged.
- Every factor derived from exact gallon, litre and foot definitions.
- Mass flow offered as an explicit derived readout rather than a fake unit row.
- Includes CFM and the oil barrel per day for cross-industry work.
Where it stops
- Not a hydraulic calculator — no head loss, velocity or pipe sizing.
- Cannot distinguish actual from standard conditions for compressible flow.
Practical Use Cases
Pump and pipework sizing
Duty points are quoted in GPM, litres per second or cubic metres per hour depending on the supplier's origin, and comparing them requires a common unit.
HVAC and ventilation
Air handling is specified in CFM in America and litres per second or cubic metres per hour elsewhere; one CFM is exactly 28.316847 L/min.
Process and chemical engineering
Volumetric duty must often be converted to a mass basis using density — see the Density Converter when the fluid is not water.
Oil and gas reporting
Production is reported in barrels per day, where the barrel is 42 US gallons, a unit that appears nowhere else in engineering practice.
Methodology & Editorial Standards
All conversions route through the cubic metre per second. Every factor derives from exact definitions: the litre as a cubic decimetre, the US liquid gallon as exactly 3.785411784 litres, the Imperial gallon as exactly 4.54609 litres and the international foot as exactly 0.3048 metres. The oil barrel is 42 US gallons. US and Imperial gallons per minute are offered as separate rows and reported together in a single card, because the 20.095 per cent difference between them is a genuine source of specification error rather than a rounding. This page handles volumetric flow only. Mass flow is a different physical quantity, obtained by multiplying volumetric flow by density, and it is therefore provided as a derived readout requiring an explicit density rather than being offered as a convertible unit alongside the volumetric ones. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Flow Rate Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I convert GPM to litres per minute?
Multiply US gallons per minute by 3.785411784, the exact NIST definition of the US liquid gallon in litres. So 50 US GPM is 189.27 L/min. For Imperial gallons multiply by 4.54609 instead, giving 227.30 L/min for the same number — which is why the gallon must be identified.
What is the difference between US and Imperial GPM?
The gallon itself. A US gallon is 3.785412 litres and an Imperial gallon is 4.54609, about 20.095 per cent larger. A flow of 100 Imperial GPM therefore moves twenty per cent more liquid than 100 US GPM. Equipment catalogues frequently omit which is meant.
How many litres per minute is one CFM?
Exactly 28.316846592 litres per minute, since a cubic foot is exactly 0.3048 cubed cubic metres. CFM is the standard unit for air movement in American HVAC work, while most of the world uses litres per second or cubic metres per hour.
What is the difference between volumetric and mass flow?
Volumetric flow measures the volume passing a point per unit time; mass flow measures the mass. They are related by density: mass flow equals volumetric flow times density. The same cubic metre per second of air and of water differ in mass by a factor of about 830, so the two cannot be converted without knowing the fluid.
Why does this converter ask for density separately?
Because mass flow is a different physical quantity from volumetric flow, and putting kg/s in the same dropdown as L/min would imply a conversion that does not exist. Supplying a density makes the assumption explicit rather than hidden.
What is a barrel per day?
The standard unit of oil production, where a barrel is 42 US gallons, about 159 litres. One barrel per day is roughly 1.84 microlitres per second — a tiny flow, which is why production is quoted in thousands or millions of barrels.
Is CFM the same as SCFM?
No. CFM is actual volumetric flow at the prevailing conditions; SCFM is standard cubic feet per minute, corrected to a reference temperature and pressure. Compressed air specifications almost always mean SCFM, and the two can differ substantially. This converter handles actual volumetric flow.
How do I convert flow rate to velocity?
Divide volumetric flow by the cross-sectional area of the pipe. A flow of 0.01 m³/s through a pipe of 0.01 m² gives 1 m/s. The relationship needs the pipe geometry, so it belongs to hydraulic calculation rather than unit conversion.
What is a cusec?
One cubic foot per second, a unit still used in water resource management and irrigation, particularly in South Asia and parts of the United States. One cusec is 28.316847 litres per second, or about 449 US gallons per minute.
Does flow rate change along a pipe?
For an incompressible fluid in a pipe with no branches, the mass flow is constant, so the volumetric flow is too, and velocity rises wherever the pipe narrows. For gases, which compress, volumetric flow changes with pressure while mass flow stays constant — another reason to be clear about which quantity is being specified.