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.

Exemple détaillé

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.

In metric: a 20-litre pail filled in 15 seconds is 20 ÷ (15 ÷ 60) = 80 L/min (21.1 US GPM, 4.8 m³/h). By the pipe method, a 50 mm bore at 1.5 m/s carries π × 0.025² × 1.5 ≈ 0.00295 m³/s, which is 177 L/min — about 10.6 m³/h.

Sur le chantier

Testing a well or a hose bib. Five-gallon bucket, stopwatch, three runs: 12 s, 13 s, 12 s. Average 12.3 s → 5 ÷ (12.3 ÷ 60) = 24 GPM. Do it three times because the first run catches the pressure tank, not the pump. Time it to the second — at 12 s a one-second slip is an 8% error.

Dewatering a hole. 2,000 gallons in the excavation and the crew is due in 20 minutes: you need 100 GPM at the head you are lifting. The pump curve gives GPM at that head; if the curve says 60, the hole is empty in 33 minutes, not 20. Flow in and flow out are the same arithmetic run backwards — minutes = gallons ÷ GPM.

Flow from a pipe you cannot catch. A 3 in line at a measured 6 ft/s: π × (1.5 ÷ 12)² × 6 = 0.295 ft³/s × 448.8 = 132 GPM. The velocity comes from a meter or a dye timing; the bore is the inside diameter of the actual pipe.

Exercez-vous

Quick referencefor flow

GPM from a catch: gallons ÷ (seconds ÷ 60) · a 5-gal bucket in 10 s = 30 GPM · in 20 s = 15 · in 30 s = 10
GPM from bore and velocity: 2.45 × bore² × ft/s (bore in inches)
L/min from bore and velocity: 0.0471 × bore² × m/s (bore in mm) · 50 mm at 1.5 m/s = 177 L/min
At 5 ft/s: 1 in 12 GPM · 2 in 49 · 3 in 110 · 4 in 196 · 6 in 441
Minutes to fill or empty: gallons ÷ GPM
Unités: 1 ft³/s = 448.8 GPM · 1 GPM = 3.79 L/min · 1 m³/h = 4.4 GPM

Mistakes to catchbefore you trust the number

Timing the bucket from the moment you open the valve, before the flow is steady — start the watch once the stream is full.
Using a 5-gallon pail that holds 5 gallons only to the rim; measure the fill line you stop at.
Typing a pipe's outside diameter for the velocity method — flow uses the bore.

La formule

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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