Flow Rate Converter

A flow rate converter translates any volumetric flow rate between ten common engineering units: cubic metres per second (m³/s), cubic metres per hour (m³/h), litres per second (L/s), litres per minute (L/min, also called LPM), litres per hour (L/h), US gallons per minute (US GPM), US gallons per hour (US GPH), Imperial gallons per minute (Imp GPM), cubic feet per second (ft³/s, often written CFS) and cubic feet per minute (ft³/min, often written CFM). Edit any one of the ten fields and the other nine update in real time through a single m³/s pivot, so you never have to do the arithmetic by hand. The tool also includes a secondary pipe-velocity panel: enter the pipe's internal diameter in millimetres and the converter computes the average cross-sectional fluid velocity v = Q / A in both m/s and ft/s, which is the highest-intent engineering question that follows from any volumetric flow reading.

The converter is built for plumbing, HVAC, pump sizing, irrigation, aquarium and pool work, fire-suppression design, fuel-system engineering, and any other context where volumetric flow rate has to be moved between metric and US/Imperial units without error. The single most damaging error in this field, conflating the US gallon with the Imperial gallon, is handled by keeping the two strictly distinct and labelling every field, so the user cannot make the classic cross-border mistake.

How to Use the Flow Rate Converter

1. Type a number into any one of the ten flow-rate fields. Each field is independently editable; type into any of them and the others all update.
2. The other nine fields recompute instantly, all pivoted through the same underlying m³/s value, so you cannot get inconsistent answers across fields.
3. Read the equivalent value in the unit your pump curve, datasheet, plumbing code, or HVAC specification calls for. Most US plumbing uses US GPM; most UK, Canadian and Commonwealth engineering uses Imp GPM or L/min; most international HVAC and process engineering uses L/s, m³/h or CFM.
4. For pipe velocity, set the pipe's internal diameter (the bore, not the nominal pipe size) in the secondary panel. The average cross-sectional velocity v = Q / A is shown in both m/s and ft/s.
5. Use the Quick presets row to load typical domestic and light-commercial flow rates as a starting point: domestic tap, low-flow and standard shower head, garden hose, and a small utility pump, each described as a typical range rather than a precise product specification.
6. Empty or non-numeric input is ignored so the form never displays NaN; clear a field and the rest hold their last valid conversion until you type into another field.

The Formulas: Conversion via the m³/s Pivot

Every conversion in this tool passes through the SI unit, the cubic metre per second. Once a value is expressed in m³/s, dividing by any unit's exact factor yields the equivalent in that unit. The factors below are exact (or exact to the displayed digits) under NIST SP 811, ISO 80000-4, the 1959 international yard-and-pound agreement, and NIST Handbook 44.

Defined conversion factors (each is exact):

• 1 L = 0.001 m³ (exact, SI definition)
• 1 ft³ = 0.028316846592 m³ (exact, since 1 ft = 0.3048 m exactly per the 1959 yd-lb agreement)
• 1 US gal = 0.003785411784 m³ (exact, NIST Handbook 44, US liquid gallon)
• 1 Imp gal = 0.00454609 m³ (exact, Imperial gallon)
• 1 m³ = 1 m³ (the SI base)

Direction A, Convert into m³/s: divide the source value by its unit factor (or, equivalently, multiply by the reciprocal). For example, 1 US GPM = 1 × 0.003785411784 / 60 = 0.00006309019640 m³/s.

Direction B, Convert out of m³/s: multiply the m³/s value by the unit factor (or divide by the reciprocal). For example, 1 m³/s × 60 / 0.003785411784 = 15850.323 US GPM.

Why a pivot at m³/s and not, say, L/min? Because m³/s is the coherent SI unit of volumetric flow, every conversion factors out cleanly. Going through any other unit would force an extra multiplication and a small rounding loss per step. The pivot also lets one definition feed every pair: m³/s ↔ L/s, m³/s ↔ m³/h, m³/s ↔ US GPM, m³/s ↔ Imp GPM, m³/s ↔ CFS, m³/s ↔ CFM, all are single-factor converts.

Sub-relationship between the two gallons: one Imperial gallon is exactly 1.2009499255 US gallons. This factor is not a nice round number, which is exactly why a converter that handles both is necessary: a flow labelled "10 GPM" without a qualifier could be 37.85 L/min (US) or 45.46 L/min (Imp), almost 8 L/min apart, and that is enough to mis-size a pump, a filter, a pipe, or a heat exchanger.

Sub-relationship between the two cubic-foot units: 1 ft³/s = 60 ft³/min exactly, since the only difference is the time unit. This identity is independent of the SI factors and is a useful sanity check: if a value in CFS reads sixty times the value in CFM, the input is consistent. Drop the identity and you lose an entire class of off-by-60 errors.

Sub-relationship between the two US gallon rates: 1 US GPM = 60 US GPH exactly. Same sanity-check role.

Sub-relationship between the two LPM-style rates: 1 L/min = 60 L/h exactly, and 1 L/s = 60 L/min exactly. These three units are pure time rescalings of each other.

Worked Examples

The five examples below cover the most common engineering conversions. Each one shows the arithmetic explicitly so you can reproduce the answer with a pocket calculator, and Example 5 demonstrates the secondary pipe-velocity panel.

Example 1, A flow of 1 m³/s expressed in m³/h.

A large municipal water treatment plant reports its throughput as 1 m³/s. The operations dashboard wants m³/h.

1 m³/s × 3600 s/h = 3600 m³/h exactly.

So 1 m³/s = 3600 m³/h. Use this identity as a sanity check whenever you see both units on the same screen.

Example 2, A flow of 1 L/s expressed in L/min.

A laboratory rotameter is calibrated in L/s; the bench test log records L/min.

1 L/s × 60 s/min = 60 L/min exactly.

Conversely, 1 L/min = 1/60 L/s ≈ 0.01667 L/s.

Example 3, A flow of 1 US GPM expressed in L/s.

A US-specification pump curve labels the operating point as 1 US GPM. The metric engineering team wants L/s.

1 US GPM × 0.003785411784 m³/gal ÷ 60 s/min = 0.003785411784 / 60 = 0.00006309019640 m³/s.

That value in L/s (multiply by 1000) is 0.06309019640 L/s. Drop the eight-digit precision and you have 0.0631 L/s, which is fine for a rough order-of-magnitude. Keep all eight digits when logging the value for pump selection or filter sizing.

Example 4, A flow of 1 Imp GPM expressed in L/s, plus a CFS/CFM pair.

A UK-specification pump curve says 1 Imp GPM. The metric engineering team wants L/s.

1 Imp GPM × 0.00454609 m³/gal ÷ 60 s/min = 0.00454609 / 60 = 0.00007576816667 m³/s.

That value in L/s is 0.07576816667 L/s, which is about 20% larger than the US GPM equivalent (0.0631 L/s), the exact size of the US/Imperial gallon ratio.

For the CFS/CFM pair: 1 CFM × 0.028316846592 m³/ft³ ÷ 60 s/min = 0.0004719474432 m³/s. And 1 ft³/s = 60 × 1 CFM = 60 ft³/min, so 1 ft³/s in US GPM = 60 × 0.028316846592 / 0.003785411784 = 448.831168831 US GPM to twelve significant figures.

Example 5, Pipe velocity for 1 L/s through a 25 mm internal-diameter pipe.

The fluid is water at modest temperature, the pipe is a small domestic copper tube with a 25 mm internal bore, and the flow is 1 L/s. What is the average cross-sectional velocity?

Step 1, convert Q to m³/s: 1 L/s = 0.001 m³/s. Step 2, compute the cross-sectional area: A = π × (d/2)² = π × (0.025/2)² = π × 0.0125² = π × 0.00015625 = 0.0004908739 m². Step 3, divide: v = Q / A = 0.001 / 0.0004908739 = 2.0372 m/s. Step 4, convert to ft/s: 2.0372 / 0.3048 = 6.6837 ft/s.

For typical domestic plumbing, 2 m/s is at the upper end of the recommended velocity range, many plumbing codes target roughly 1.0 to 2.5 m/s in copper or PEX to keep noise, erosion and water-hammer risk low. If the velocity comes out much higher than that, the next step is usually to upsize the pipe, not to throttle the pump.

Where Volumetric Flow Rate Conversion Shows Up

Domestic plumbing and water supply. A typical cold-water tap in a domestic kitchen runs somewhere in the range of 4 to 10 L/min (about 1 to 2.6 US GPM), a low-flow shower head delivers roughly 8 to 15 L/min (about 2 to 4 US GPM), and a standard shower head delivers roughly 10 to 20 L/min. A garden hose at normal mains pressure lands near 20 to 30 L/min. None of these are exact specifications, they are typical ranges that vary with local mains pressure, pipe diameter, and outlet restriction, and the converter lists them as ranges rather than as precise numbers for that reason. Hot-water recirculation systems in larger houses run higher; a small utility pump for irrigation or pool turnover commonly delivers 100 to 200 L/min.

HVAC, CFM in ducts and fans. Building HVAC uses cubic feet per minute (CFM) almost exclusively in North America and L/s (or m³/h) in metric-system markets. A residential supply register might be sized for 100 to 150 CFM per room, a bathroom exhaust fan around 50 to 110 CFM, and a kitchen range hood 100 to 600 CFM depending on type. Commercial rooftop units commonly deliver thousands of CFM. Duct sizing depends directly on the velocity target: low-velocity residential ductwork is typically designed for 600 to 900 FPM (feet per minute) in the trunk, while high-velocity industrial systems may run 2000 to 4000 FPM, with a hard upper limit set by noise and duct erosion.

Pump curves and pump selection. Every centrifugal pump has a published curve that plots flow (in GPM, m³/h, or L/s) against head (in metres or feet). Converting between these flow units is a daily task in pump selection. A residential booster pump commonly delivers 5 to 15 US GPM at 30 to 60 feet of head; a small irrigation pump delivers 20 to 100 US GPM at modest head; a fire pump can deliver hundreds to thousands of US GPM at 100+ psi.

Municipal water and wastewater. Large plants report flow in MGD (million gallons per day, US) or m³/day. A small wastewater treatment plant for a town of 10 000 people might handle 1 to 5 MGD; a large city plant handles hundreds of MGD. Converting between MGD and m³/day is straightforward once the US gallon factor is locked in: 1 MGD = 3785.411784 m³/day.

Process engineering, chemical, food, oil and gas. Process flowmeters (magnetic, vortex, Coriolis, ultrasonic) report in the unit the plant standardises on. Cross-border equipment often reads in a mix of units, so the converter's job is to keep the arithmetic consistent.

Fire suppression. Sprinkler system flow tests report flow in US GPM at a given residual pressure. A light-hazard office occupancy might need 150 to 250 US GPM at the design area; a high-hazard warehouse can need 1000+ US GPM. Hydrant flow tests compare the measured GPM against the required GPM, with the residual pressure ensuring the system can deliver that flow at usable pressure.

Natural gas and fuel. Although natural gas is sometimes measured in standard cubic feet per minute (SCFM) or standard cubic metres per hour (Nm³/h), the conversion is structurally identical to water-flow conversion once the standard temperature and pressure are pinned down. This tool handles the unit conversion only, it does not adjust for gas compressibility, which is a separate calculation governed by ISO 6976 or AGA reports.

Common Mistakes

The single most damaging error: confusing the US gallon with the Imperial gallon. They look almost identical in print, and the ratio is roughly 1.2, which is exactly the kind of factor that survives a sloppy glance. A flow labelled "10 GPM" without a country qualifier could be 37.85 L/min (US) or 45.46 L/min (Imperial), almost 8 L/min apart. A pump, filter, pipe or heat exchanger selected off the wrong gallon will be noticeably under- or over-sized. The converter keeps both labelled at all times so the user cannot pick the wrong one by accident.

Confusing volumetric flow rate with mass flow rate. Volumetric flow rate is volume per unit time (m³/s, L/min, GPM, CFM). Mass flow rate is mass per unit time (kg/s, lb/hr). Converting between them requires the fluid density, for water at room temperature, 1 L/min is very close to 1 kg/min, but for fuels, oils, slurries, and gases the ratio can vary widely. A volumetric flow reading alone does not let you convert to mass; you also need density (and, for gases, temperature and pressure). ISO 80000-4 distinguishes the two quantities explicitly for exactly this reason.

Confusing GPM with CFM. Both are "gallons or cubic feet per minute", but the volumes differ by a factor of roughly 7.48 (one ft³ ≈ 7.4805 US gal). A pump spec in GPM cannot be compared directly with a fan spec in CFM unless you convert the volumes first. The converter does this directly: 1 CFM ≈ 7.4805 US GPM, so a 100 CFM fan moves roughly 748 US GPM of air.

Using nominal pipe size as the internal diameter. A 1″ nominal pipe is not 1 inch bore. NPS (nominal pipe size) is a trade designation, and the actual internal diameter depends on the schedule (wall thickness). Schedule 40 (standard) 1″ steel pipe has an internal diameter of about 1.049″ (26.6 mm), while Schedule 80 1″ steel pipe has an internal diameter of about 0.957″ (24.3 mm). A 25 mm nominal copper tube (Type M, used in domestic water) has a 25 mm outside diameter and an internal diameter of about 22.9 mm. Using NPS or outside diameter instead of the true bore will under-estimate the velocity by a few percent, enough to push the calculation outside the safe range for erosion or water-hammer.

Confusing CFS and CFM, or GPM and GPH, etc. These are off-by-60 errors. 1 CFS = 60 CFM exactly; 1 GPM = 60 GPH exactly; 1 L/s = 60 L/min exactly. The typo is always catastrophic in one direction or the other: a 50 GPM spec misread as 50 GPH is off by 60×, which can completely undersize a fire system, an irrigation main, or a process pump.

Reading CFM as SCFM, or Nm³/h as m³/h. "CFM" by itself is ambiguous: standard (SCFM, at standard temperature and pressure) and actual (ACFM, at the actual temperature and pressure in the duct) can differ by 10 to 30%. Always check the qualifier. The converter handles the unit conversion only, for SCFM/ACFM differences, consult the appropriate gas-flow reference.

Rounding too aggressively too early. A 100 US GPM spec rounded to 100 L/min is off by about 26% (100 US GPM ≈ 378.54 L/min). Round only at the end of the calculation, and keep at least three significant digits in the intermediate steps.

Volumetric vs Mass Flow Rate

Volumetric flow rate Q measures how much volume of fluid crosses a section per unit time. The SI coherent unit is m³/s, with L/s and L/min common in practice and US GPM / CFM common in the United States and the UK construction industry. Mass flow rate ṁ measures how much mass crosses a section per unit time. The SI coherent unit is kg/s. The two are related by the fluid density ρ:

ṁ = ρ × Q

For liquid water at standard conditions, ρ ≈ 1000 kg/m³, so 1 L/min ≈ 1 kg/min to within a fraction of a percent. For fuels (gasoline ρ ≈ 740 kg/m³, diesel ρ ≈ 830 kg/m³), the ratio is noticeably different from 1. For air at standard conditions, ρ ≈ 1.225 kg/m³, so a 100 CFM air flow is roughly 100 × 0.0004719 × 1.225 ≈ 0.0578 kg/s, or about 208 kg/hr. For hot or pressurised gases the density can vary by a factor of 5 or more. Density must be known to convert between volumetric and mass flow, this converter handles the volumetric side, and a density lookup is needed for mass.

The Continuity Equation

For an incompressible fluid in a closed pipe, the volumetric flow rate is constant along the pipe:

Q = A × v

where A is the cross-sectional area of the pipe and v is the average cross-sectional velocity. This is the continuity equation in its simplest form. The converter's secondary panel implements exactly this relationship: given Q (from any of the ten unit fields) and an internal diameter d, it computes A = π × (d/2)² and v = Q / A.

The practical implication is that shrinking the pipe diameter increases the velocity. Halving the diameter quarters the area and roughly quadruples the velocity for the same flow. This is why undersizing a pipe in a high-flow system causes noise, erosion, and excess pump head, the velocity climbs through the constriction even though the volumetric flow rate stays the same.

For a compressible fluid (gas), the continuity equation has a density term: ṁ = ρ × A × v, and the mass flow is constant along the pipe even as the volumetric flow changes with pressure and temperature.

Laminar vs Turbulent Flow

The Reynolds number Re = ρ × v × d / μ (or Re = v × d / ν in kinematic form) determines the flow regime:

• Re < ~2300: laminar flow, smooth and predictable, velocity profile is a clean parabola across the diameter.
• 2300 < Re < ~4000: transitional, unstable, sensitive to disturbances upstream.
• Re > ~4000: turbulent flow, chaotic, with eddies at many scales; velocity profile is much flatter across the core and drops sharply near the wall.

Most domestic water pipes and most HVAC ducts operate in turbulent flow. A 25 mm copper tube at 2 m/s with water at 20 °C has Re ≈ 50 000, well into turbulence. A small syringe at very low flow can be laminar. The Reynolds number does not affect the converter's arithmetic, Q, A and v are the same regardless of regime, but it affects the engineering interpretation: a turbulent pipe needs more head for the same flow, and the head loss scales differently with velocity (roughly v¹·⁷ to v² in turbulent flow versus just v in laminar flow, per the Darcy-Weisbach and Hagen-Poiseuille relations respectively).

Common Measurement Methods

Volumetric flow is rarely read directly; it is usually inferred from a velocity or volume measurement. The three methods below are the ones most commonly encountered in plumbing, HVAC, and light industrial work.

Bucket and stopwatch (or graduated container and stopwatch). The oldest and most accessible method. A container of known volume is filled from the source; the time to fill is measured; Q = volume / time. For domestic taps, a 1 L or 2 L jug and a phone stopwatch give a flow reading to within a few percent, which is good enough for sizing a garden pump or checking a water softener. For larger flows (pumps, mains), a calibrated tank or weir is used.

Rotameter (variable-area flowmeter). A tapered vertical tube with a float inside. The float rises to a height at which the drag force on it balances its weight. The higher the float sits, the larger the annular area around it, and the higher the flow. Rotameters are cheap, simple, and need no external power; they are widely used in laboratory gas flow, medical oxygen, and process water. They are calibrated for a specific fluid at a specific temperature and pressure, so using them outside their calibrated condition introduces error.

Ultrasonic flowmeter. Two transducers on the outside of the pipe send ultrasonic pulses diagonally through the flowing fluid; the difference in transit time between the upstream and downstream pulses is proportional to the average fluid velocity. Clamp-on ultrasonic meters are non-intrusive (no pipe penetration, no pressure drop), which makes them attractive for retrofits and for fluids that must not be contaminated by a wetted sensor. They are widely used in HVAC, district heating/cooling, and large water mains.

Other common methods include magnetic flowmeters (for conductive liquids like water and slurries), vortex-shedding flowmeters (for steam, gas, and clean liquids), Coriolis flowmeters (the high-accuracy reference for both volumetric and mass flow in process plants), and turbine flowmeters (a rotor whose speed is proportional to flow).

Frequently Asked Questions

What is the difference between volumetric flow rate and mass flow rate?

Volumetric flow rate measures volume per unit time (m³/s, L/min, GPM, CFM). Mass flow rate measures mass per unit time (kg/s, lb/hr). They are linked by the fluid density: ṁ = ρ × Q. For liquid water at room temperature the ratio is close to 1 L/min ≈ 1 kg/min, but for fuels, oils, gases, and slurries the conversion is non-trivial, you must know the density (and for gases, the temperature and pressure) to convert accurately.

How do I convert GPM to L/min without mixing up US and Imperial gallons?

Multiply US GPM by 3.785411784 to get L/min. Multiply Imperial GPM by 4.54609 to get L/min. The two factors differ by about 20%, so picking the wrong one shifts the answer by roughly that amount. The converter keeps both units separate and labelled so the user cannot pick the wrong gallon by accident.

Why does 1 ft³ equal exactly 0.028316846592 m³?

Because 1 foot is exactly 0.3048 metres (a definition fixed by the international yard-and-pound agreement of 1959 between the US, the UK, Canada, Australia, New Zealand, and South Africa). Cubing that exact value gives 1 ft³ = (0.3048)³ = 0.028316846592 m³ exactly. There is no rounding anywhere in the conversion.

What is pipe velocity, and why does it matter?

Pipe velocity v is the average cross-sectional speed of the fluid in the pipe, computed from the volumetric flow Q and the pipe's internal cross-sectional area A as v = Q / A. It matters because too-low velocity encourages sediment deposition and poor heat transfer; too-high velocity causes noise, erosion, water hammer, and excess pumping head. Most plumbing codes target 1.0 to 2.5 m/s in copper or PEX domestic water lines, 1.5 to 3.0 m/s in steel process lines, and similar ranges elsewhere.

Is the nominal pipe size the same as the internal diameter?

No. Nominal pipe size (NPS) is a trade designation, not a measured bore. The actual internal diameter depends on the schedule (wall thickness) and the material. A 1″ NPS Schedule 40 steel pipe has an internal diameter of about 1.049″ (26.6 mm); a 1″ NPS Schedule 80 steel pipe has an internal diameter of about 0.957″ (24.3 mm). Copper and PEX tubes are usually specified by outside diameter with a known wall thickness, so a 22 mm copper tube has a smaller bore than a 25 mm copper tube. Always use the true internal bore, not the nominal designation.

What does "CFM" mean in HVAC?

CFM stands for cubic feet per minute, a unit of volumetric flow rate widely used in North American HVAC. It is structurally the same as CFS scaled by 60: 1 CFS = 60 CFM. CFM can refer to either standard CFM (SCFM, at standard temperature and pressure) or actual CFM (ACFM, at the actual conditions in the duct); the two can differ by 10 to 30%. Always check the qualifier in the relevant datasheet.

can the Flow Rate Converter be used for professional or commercial purposes?

yes, the Flow Rate Converter provides mathematically correct results that are suitable for professional, commercial, and educational use. For the Flow Rate Converter, For the Flow Rate Converter, For high-stakes applications (medical, legal, financial), verify results with a domain expert. For the Flow Rate Converter, the Flow Rate Converter formulas used are well-established and validated against reference standards.

For the Flow Rate Converter, How often are the underlying formulas updated?

For the Flow Rate Converter, the Flow Rate Converter formulas are based on established scientific, mathematical, or industry-standard references and rarely require updates. when standards change, the Flow Rate Converter is updated to reflect the current authoritative source. For the Flow Rate Converter, For the Flow Rate Converter, Each calculator's references section lists the specific sources used.

References

• NIST Special Publication 811, Guide for the Use of the International System of Units (SI). The authoritative US reference for SI usage and unit conversion, including the litre-cubic-metre relationship (1 L = 0.001 m³ exactly) and the relationship between the SI and the customary units used in the US.
• ISO 80000-4, Quantities and units, Part 4: Mechanics. The international standard for the quantities, symbols, and units of mechanics, including volumetric flow rate (symbol q, unit m³/s) and the distinction between volumetric and mass flow rate.
• International yard-and-pound agreement of 1959, agreement between the US, the UK, Canada, Australia, New Zealand, and South Africa defining the yard as exactly 0.9144 metres, and therefore the foot as exactly 0.3048 metres. The cubic-foot factor 1 ft³ = 0.028316846592 m³ follows from cubing 0.3048 exactly.
• NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. The US specification that defines the US liquid gallon as exactly 3.785411784 litres, which underpins the US GPM, US GPH and US gallon conversions used throughout commerce in the United States.