How do I measure a pump’s flow in GPM?

The simplest way is a timed catch: run into a container of known volume and note the seconds to fill it. Gallons divided by minutes is your GPM. Filling a 5-gallon bucket in 20 seconds is 5 ÷ (20/60) = 15 GPM. This tool does that, and it also works out flow from a pipe size and a velocity if you have those instead.

How is flow related to pipe size and velocity?

Flow is the pipe’s cross-section area times how fast the fluid moves through it. A bigger bore or a faster velocity means more flow. Enter the inside diameter and the velocity and the tool multiplies the bore area by the speed, then converts the result into gallons or litres per minute.

Does this size a pump for me?

No. It measures or expresses a flow rate — it does not choose a pump, account for head, friction loss, or suction lift, and it does not tell you a pump will deliver this against a real system. Those are design questions. Use the number to check what you are getting, not to specify what to buy.

Pump GPM calculator

This measures or expresses a flow rate and nothing else. It does not size a pump, allow for head, friction or lift, or promise a pump will hit this figure against a real system — those are design calls for a qualified person.

Choose a method. A timed catch turns a container volume and the seconds to fill it into a flow rate; the pipe method turns an inside diameter and a velocity into the same thing. Either way you get gallons and litres per minute.

Worked example

Filling a 5-gallon bucket in 20 seconds is 5 gallons in a third of a minute, so 5 ÷ (20 ÷ 60) = 15 GPM, about 57 litres a minute. By the pipe method, a 2-inch bore at 5 ft/s carries π × (1/12)² × 5 ≈ 0.109 ft³/s, and one cubic foot per second is 448.8 GPM, giving roughly 49 GPM.

The formula

catch: GPM = volume ÷ (seconds ÷ 60) · pipe: flow = π × (bore ÷ 2)² × velocity

A flow rate is a volume over a time, so the catch method just divides the caught volume by the fill time in minutes. The pipe method builds the volume rate from geometry — bore area times velocity gives cubic feet or metres per second, which then converts to per-minute fluid units.

Questions

How do I measure a pump’s flow in GPM?: The simplest way is a timed catch: run into a container of known volume and note the seconds to fill it. Gallons divided by minutes is your GPM. Filling a 5-gallon bucket in 20 seconds is 5 ÷ (20/60) = 15 GPM. This tool does that, and it also works out flow from a pipe size and a velocity if you have those instead.
How is flow related to pipe size and velocity?: Flow is the pipe’s cross-section area times how fast the fluid moves through it. A bigger bore or a faster velocity means more flow. Enter the inside diameter and the velocity and the tool multiplies the bore area by the speed, then converts the result into gallons or litres per minute.
Does this size a pump for me?: No. It measures or expresses a flow rate — it does not choose a pump, account for head, friction loss, or suction lift, and it does not tell you a pump will deliver this against a real system. Those are design questions. Use the number to check what you are getting, not to specify what to buy.

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