Gallons Per Minute Calculator (GPM)

Gallons Per Minute Calculator turns a timed bucket fill, a pipe size and velocity, or a nozzle pressure into GPM. It also converts other flow rate units and gives a tank fill time.

How the Gallons Per Minute Calculator Finds GPM

The Gallons Per Minute Calculator has four methods under How to Find GPM. Each one suits a different job, and each needs different measurements. Pick the one that matches what you can measure on site.

Every method in the Gallons Per Minute Calculator ends in the same result panel. It shows the flow in metric and US units and the time to fill a tank. It also gives the volume used in a set run time and how many fixtures that flow can serve.

Bucket Test Formula for GPM

A bucket test needs a container of known volume and a timer. Divide the gallons by the seconds it takes to fill, then multiply by 60 to scale the result to one minute.

$$GPM=\frac{V}{t}\times 60$$

V is the container volume in gallons and t is the fill time in seconds. A 5 gallon bucket that fills in 30 seconds gives 10 GPM. The same bucket filled in 14 seconds gives 21.4 GPM.

Timing is the weak point. On a 30 second fill, being off by 1 second moves the answer by 3.3%. On a 10 second fill the same slip is 10%, so the Gallons Per Minute Calculator warns you under 10 seconds and suggests a bigger container.

Rain Bird's flow check for sprinkler planning uses this same method. Shut off all other water in the house, open the faucet all the way, and time a 5 gallon bucket. Their example then sizes a zone from the result, with three rotors of 3.11 GPM each on a 10 GPM supply.

A bucket test reads the flow from a wide-open outlet, which is the most that outlet will give. Sprinklers and appliances need pressure left over to work. The flow you can use while holding that pressure is lower than the bucket figure.

Gallons Per Minute Calculator for Pipe Size

When you cannot catch the water, flow comes from the pipe's inside diameter and the water velocity. The Gallons Per Minute Calculator multiplies the pipe's cross-section area by the velocity. For inches and feet per second that reduces to one line.

$$GPM=2.448\times d^2\times v$$

Here d is the inside diameter in inches and v is the velocity in feet per second. A 1 inch bore at 5 ft/s carries 12.24 GPM. Doubling the diameter gives four times the flow, because area grows with the square of the diameter.

Velocity is the number to choose with care. The Copper Tube Handbook limits copper tube to 8 ft/s for cold water and 5 ft/s for hot water up to about 140°F, to avoid noise and erosion. Those two limits set the flow each size can carry.

Use the true inside diameter, not the nominal size printed on the pipe. This method gives the flow at the velocity you enter and says nothing about pressure loss. The Pipe Flow Calculator works the same diameter and velocity problem on its own page, and the Hazen Williams Equation Calculator finds the head loss along the run.

GPM From PSI and Nozzle Size

The third method in the Gallons Per Minute Calculator works from pressure. It uses the discharge formula in NFPA 291, the fire service practice for flow testing hydrants. The same relation holds for any round nozzle or orifice that discharges to open air.

$$GPM=29.84\times C_d\times d^2\times \sqrt{P}$$

P is the pressure of the flowing stream in psi, d is the opening diameter in inches, and Cd is the discharge coefficient. A 0.5 inch nozzle at 40 psi with a coefficient of 0.90 flows 42.46 GPM.

The coefficient depends on the shape of the outlet. NFPA 291 uses 0.90 for a smooth, rounded outlet, 0.80 for a square, sharp one, and 0.70 for an outlet that projects into the barrel. The WSRB hydrant flow testing guide lists all three.

A 2.5 inch hydrant outlet reading 20 psi on a pitot gauge flows 751 GPM at 0.90. With a square outlet at 0.80 it drops to 667 GPM. Picking the wrong coefficient changes the answer by 11%.

The pressure must be read in the stream while water is flowing. Static pressure, taken with the water shut off, is always higher and gives a flow the opening cannot deliver. For the flow a building needs, use the Fire Flow Calculator.

Converting GPM to Other Flow Units

Convert Another Unit turns the Gallons Per Minute Calculator into a converter for ratings in liters, cubic meters, cubic feet or UK gallons. The base is the US gallon of 231 cubic inches, which works out to exactly 3.785411784 liters. The NIST conversion tables round it to 3.785412.

Going the other way, 40 L/min is 10.57 GPM. A UK gallon is 4.54609 liters, about 20% larger than the US gallon, so 10 UK gpm equals 12.01 US GPM. Choose UK gpm for pumps rated in imperial gallons. For work that starts in metric, the Liters per Minute Calculator runs the volume and time test with liters as the result.

Fill Time, Run Volume and Fixtures

Open Tank Fill and Run Time to set a tank size and a run time. At 10 GPM a 5,000 gallon tank takes 8 hours 20 minutes. A 15 minute run uses 150 gallons, and a full day of nonstop flow is 14,400 gallons.

Fill time is the tank volume divided by GPM, and it assumes the flow stays steady. If you do not know the volume, get it from the Tank Volume Calculator or the Pool Calculator first. The table under the cards lists fill times from a 5 gallon bucket up to a 20,000 gallon pool.

The last card in the Gallons Per Minute Calculator compares the flow with fixture ratings. EPA WaterSense labels showerheads that use no more than 2.0 GPM, against a standard of 2.5 GPM. WaterSense bathroom faucets are capped at 1.5 GPM.

At 10 GPM that is five WaterSense showerheads, four standard ones, or six bathroom faucets running at once. Treat the count as a ceiling. Real fixtures share pressure, so fewer will run well when the supply pressure sags.

GPM FAQ

How do I calculate the GPM of a pump?

Run the pump into a container of known volume and time the fill, then use Bucket Test in the Gallons Per Minute Calculator. A pump that fills a 55 gallon drum in 2 minutes 45 seconds delivers 20 GPM.

Test it at the height and pipe length it will work against. Pump output falls as lift and friction rise, so a reading taken at ground level overstates the flow at the tank. The Pump Pressure Calculator converts that lift into pressure.

How do I convert gallons per minute to feet per second?

You need the pipe's inside diameter, because feet per second is a speed and GPM is a volume rate. Divide the flow by the pipe area with this form of the pipe equation.

$$v=\frac{0.4085\times GPM}{d^2}$$

At 10 GPM, a 1 inch bore runs at 4.09 ft/s and a 0.75 inch bore runs at 7.26 ft/s. To check a result, enter the diameter and velocity under Pipe Size and Velocity and confirm the GPM matches.

How do I convert gallons per minute to gallons per day?

Multiply by 1,440, the number of minutes in a day. A 10 GPM flow is 14,400 gallons per day. For a set number of minutes, multiply GPM by the minutes instead, so 10 GPM for 15 minutes is 150 gallons.

The same math shows what a small flow adds up to. A leak of 0.1 GPM is 144 gallons per day. Pick gal/day under Convert Another Unit in the Gallons Per Minute Calculator to turn a daily meter reading back into GPM.