GPM Calculator (Gallons per Minute)

Work out gallons per minute from a bucket test, from pressure through a nozzle, from pipe velocity, or convert between gpm, L/min, cfs and m³/h.

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The GPM Calculator gives flow in US gallons per minute (gpm) from a few measurements, so you can work out a garden hose flow rate, the flow through a nozzle at a set pressure, or the flow in a pipe. It is free and runs entirely in your browser, with no account or sign-up. It has five modes: a bucket test, pressure to flow at a nozzle (a psi to gpm calculator), flow to pressure (a gallons per minute to psi calculator), unit conversion, and pipe velocity with inside diameter. Each result shows the formula and the numbers used, so you can check the working. Results are estimates, so check critical designs against the applicable standard.

How to use the GPM Calculator

Under Calculate gpm from, choose the mode that matches what you measured: bucket test, pressure at a nozzle or orifice, flow through a nozzle, unit conversion, or pipe velocity with inside diameter.
With a bucket test, enter the volume collected and the time taken. Volume can be in gal, L, qt, fl oz, mL or m3, and time in seconds or minutes.
Nozzle modes need the nozzle diameter in in, mm, cm or ft, plus the pressure in psi, bar or kPa. Pressure is the gauge pressure measured just behind the opening while water flows.
Conversion works from the flow value and its unit: gpm, L/min, m3/h, cfs, Imperial gpm, L/s or cfm.
In pipe mode, type the velocity in ft/s and the inside diameter of the pipe.
Set each quantity’s unit in its own menu. You can mix metric and US customary units because the tool converts them before calculating.
Read the result. It updates as you type. Open “How it was calculated” under the result to see the formula, the values substituted and the answer with units.

Preset buttons above the form load typical cases, and Reset restores the starting example. Less common inputs sit under More options. Copy result places the result lines and a link on the clipboard, while Copy link grabs the page address; it stores every input so the calculation reopens later. Print / save as PDF creates a one-page record with the inputs, results, steps and date.

How do you calculate gpm?

To work out gpm from a bucket test, divide the gallons collected by the minutes taken: gpm = gallons ÷ minutes. For instance, 5 gal in 30 s is 5 ÷ 0.5 = 10 gpm, or 37.85 L/min, so convert seconds to minutes first.

When sizing a nozzle, the calculator relies on the smooth-bore formula GPM = 29.84 × Cd × d² × √P, with d in inches and P in psi (Freeman fire-service hydraulics). Cd is the discharge coefficient. The default is 0.97, a round value slightly below the fire-service smooth-bore constant (published charts use 29.7 instead of 29.84, which is about Cd 0.995), and a sharp-edged hole is about 0.61. A 1 in nozzle at 50 psi gives 29.84 × 0.97 × 1² × √50 = 204.7 gpm.

Pressure by itself won’t fix a flow rate. The opening’s size matters too, since flow depends on a known diameter and coefficient.

Pipe flow uses GPM = 2.448 × v × d², where v is in ft/s and d in inches. It is Q = A × v with the unit factors folded in. Take a 2 in inside-diameter pipe at 5 ft/s: 2.448 × 5 × 2² = 48.96 gpm.

How do you calculate gallons per minute from pressure?

To find gallons per minute from a required nozzle pressure, apply the nozzle formula and solve for flow. Going the other way, from a target flow to the pressure it needs, rearrange to P = (GPM ÷ (29.84 × Cd × d²))². For example, 250 gpm through a 1.25 in nozzle with Cd 0.97 needs 30.56 psi, or 2.11 bar.

Conversions rely on fixed factors. A US gallon equals 3.785411784 L. A cubic foot per second (cfs) equals 448.83 gpm. The Imperial gallon measures 4.54609 L (NIST SP 811). So 100 L/min is 100 ÷ 3.785 = 26.42 gpm, and 1 cfs is about 448.8 gpm, or 1,699 L/min.

A pipe result above 20 ft/s raises a warning, since velocities that high usually signal a design problem.

Sources

Unit factors: NIST SP 811
Nozzle flow: Freeman form Q = 29.84 Cd d² √P (fire-service charts use 29.7, Colorado DFPC pump chart)
Pipe form GPM = 2.448 v d²: Crane TP-410

These results are estimates, so check critical designs against the applicable standard.

Worked examples

Input	Result
5 gal collected in 30 s	10 gpm (37.85 L/min)
1 in nozzle at 50 psi, Cd 0.97	204.7 gpm (774.8 L/min)
250 gpm through a 1.25 in nozzle, Cd 0.97	30.56 psi (2.11 bar)
1 cfs	448.8 gpm (1,699 L/min)
100 L/min	26.42 gpm (100 L/min)
v = 5 ft/s, pipe ID 2 in	48.96 gpm (185.3 L/min)

Calculated by the tool above with these values.

Frequently asked questions

Can I use the results in commercial projects or client reports?

Yes. You may use results in commercial work, such as quotes, design notes, reports and client projects, without attribution. Treat the result as an estimate: the tool does not certify compliance with any code, so check critical designs against the applicable standard.

Is the GPM Calculator free, and is anything I type uploaded?

There is no charge and no account or sign-up. Every calculation runs in your browser, and nothing you type is uploaded.

Yes, in two ways. Print / save as PDF produces a one-page record, while Copy link stores your inputs in the page address so a colleague can open the same calculation.

How to calculate gpm of a pump?

To measure it, run the pump into a container of known size and time the fill. Then use the bucket test mode: gpm = gallons collected ÷ minutes. A 5 gal bucket filled in 30 s, for instance, means 10 gpm. Take the reading at the outlet you care about, since flow shifts with the head the pump works against.

Why does the calculator warn when the bucket fill time is under 5 seconds?

A very short fill leaves little time to measure, so a small error in starting or stopping the timer has a large effect on the gpm result. A longer fill gives a more reliable reading.

What discharge coefficient should I use for a nozzle?

The default is 0.97, a round value slightly below the fire-service smooth-bore constant (about 0.995). If your nozzle maker publishes a coefficient, enter that instead. A sharp-edged hole comes in around 0.61. The calculator flags any Cd outside 0.5 to 1.0 with a warning.

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