Flow Rate Converter

Overview

Convert volumetric flow rate between litres per minute (L/min, LPM), litres per second, cubic metres per hour (m³/h, CMH), cubic metres per minute, US and Imperial gallons per minute (GPM), and cubic feet per minute (CFM). Useful for comparing pump duty points, ventilation and range-hood airflow, air-compressor output, and water-meter usage when every manufacturer quotes a different unit. Enter a pipe inner diameter to also get the flow velocity from the continuity equation.

Formula

Every conversion goes through the base unit L/min.

1 m³/h = 16.666667 L/min
1 m³/min = 1,000 L/min
1 m³/s = 60,000 L/min
1 L/s = 60 L/min
1 US GPM = 3.785412 L/min (1 US gal = 3.785411784 L)
1 Imp GPM = 4.54609 L/min (exact UK gallon)
1 CFM (ft³/min) = 28.316847 L/min (1 ft³ = 28.316846592 L)
1 CFM = 1.699011 m³/h, 1 m³/h = 4.402868 US GPM

Pipe velocity: v (m/s) = Q (m³/s) ÷ A (m²), A = π × (inner diameter ÷ 2)²

How to Use

1. 1Enter the flow rate value (e.g. 100).
2. 2Choose the input unit (L/min, m³/h, US GPM, CFM, ...).
3. 3Or pick a preset (faucet, shower, range hood) to fill a common flow figure.
4. 4Read the value converted into every flow unit plus hourly and daily totals.
5. 5Enter a pipe inner diameter (mm) to see the resulting velocity and whether it falls in the recommended range.

Examples

Background

This calculator handles volumetric flow rate — the volume passing a point per unit time. The SI unit is m³/s, but practice favours L/min (LPM), m³/h (CMH), or CFM depending on scale. Gallon-based units differ by country: the US liquid gallon is 231 in³ = 3.785411784 L while the UK imperial gallon was fixed at 4.54609 L by the Weights and Measures Act 1985, about 20% larger, so a GPM figure is meaningless without knowing its origin. CFM follows from 1 ft = 0.3048 m, giving 1 ft³ = 28.316846592 L. Air and gas flows are often quoted as Nm³/h or SCFM, normalised to a reference temperature and pressure; those are not the same as actual volumetric flow and cannot be reached by unit conversion alone. Pipe velocity comes from the continuity equation Q = A·v for incompressible flow, and water supply pipes are typically designed for 1–2.5 m/s.

Tips

• ✔Multiply m³/h by 16.67 for L/min, or by 0.5886 for CFM. Going back, multiply CFM by 1.699 for m³/h.
• ✔US and Imperial GPM differ by about 20% — check which gallon an imported pump curve uses before sizing.
• ✔Read a pump's flow together with its head (m) on the performance curve; the duty point determines the real flow.
• ✔Velocity rises fast as bore shrinks: doubling the inner diameter cuts velocity to a quarter for the same flow.
• ✔Water bills are usually in m³, so multiplying L/min by 0.06 gives cubic metres per hour directly.
• ✔If a gas spec says Nm³/h or SCFM, it is a normalised figure — keep it separate from actual volumetric flow.

FAQ