Flow Rate Calculator — L/min, gpm, m³/h, cfm, and pipe velocity

How it works

The converter works in two ways.

Unit conversion. Every flow is turned into cubic meters per second using NIST SP 811 volume factors, then into the unit you asked for. The US liquid gallon is exactly 231 cubic inches, which is 3.785411784 liters, so 1 gpm is exactly 3.785411784 L/min. A cubic foot is 0.3048³ cubic meters (the international foot is exact), so 1 cfm is 0.00169901079552 m³/s, or 1.69901079552 m³/h.

Pipe-velocity mode. For a circular pipe, volumetric flow and mean velocity are related by Q = v × A, with A = π(d/2)². Choose "flow from diameter and velocity" or "velocity from diameter and flow". This is the mean speed of an incompressible fluid in a full circular pipe. It is not a friction, pressure-drop, or pump-head calculation, and it does not apply unchanged to a compressible gas whose density changes along the pipe.

Worked example

Converting 10 US gallons per minute to liters per minute:

10 gpm × 3.785411784 = 37.8541 L/min

For a 2-inch interior diameter at a mean velocity of 2 m/s, the interior radius is 0.0254 m, the area is π × 0.0254² ≈ 0.00202683 m², and the flow is 2 × 0.00202683 = 0.00405366 m³/s, which is about 64.2518 gpm.

Frequently asked questions

How do you convert gallons per minute to liters per minute?

Multiply gpm by 3.785411784. The US liquid gallon is defined as exactly 231 cubic inches, which equals 3.785411784 liters, so the per-minute ratio is the same number. 10 gpm is 37.8541 L/min.

How many cubic meters per hour is 100 cubic feet per minute?

About 169.90 m³/h. One cubic foot is exactly 0.028316846592 m³, and there are 60 minutes in an hour, so 100 cfm × 0.028316846592 × 60 = 169.901079552 m³/h.

How is pipe velocity related to flow rate?

For a full circular pipe, Q = v × π(d/2)², where Q is volumetric flow, v is mean velocity, and d is the interior diameter, all in consistent units. Solving for velocity gives v = Q / A. This page uses that definition; it does not estimate friction losses.

Are these US gallons and standard cubic feet?

Yes. The gallon is the US liquid gallon (231 cubic inches), not the imperial gallon, which is larger (about 4.546 liters). Cubic feet use the international foot of exactly 0.3048 m. The "cfm" here is actual volumetric cubic feet per minute, not a mass-flow "standard" cfm that depends on a chosen temperature and pressure.

What does the pipe-velocity mode assume?

A circular, fully flowing pipe and an incompressible fluid, so volume is conserved and the velocity is the mean speed Q/A. Real pipes have roughness, fittings, and entrance effects that change pressure drop but not this mean velocity for a given flow and interior diameter. The mode does not size a pump or check whether the flow is laminar or turbulent.

Can I use this for mass flow or for a gas?

Not directly. This page converts volume per time. Mass flow also needs density (mass flow = density × Q). For a gas whose density changes with pressure or temperature, a single volumetric number is not enough; you would need the conditions at the point where the volume is measured, and often a standard-condition definition as well.