Flow Rate Calculator
Work out water flow in GPM or L/min from a bucket test or from pipe size and velocity, and convert feet of head to pressure. Use it as a GPM calculator for a bucket test, size up a pipe from its inside diameter, or use the head pressure calculator to turn feet of water into psi and back.
Your numbers
A 5-gallon bucket is the usual one; mark the fill line if it is not full to the brim.
Time it with the tap fully open, from the moment the water hits the bucket.
Gallons per minute
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- Gallons per hour
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- Litres per minute
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- Cubic metres per hour
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- Cubic feet per second
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A bucket test gives the flow at the pressure the tap had during the test; open other taps and it drops.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How to calculate flow rate
Flow rate is a volume divided by a time. The bucket test measures it directly: gallons ÷ seconds × 60 gives gallons per minute (GPM), and GPM × 60 gives gallons per hour. The calculator also shows litres per minute, cubic metres per hour and cubic feet per second, using the US gallon of 231 cubic inches (3.785 L).
Worked example: a 5-gallon bucket fills in 12 seconds. 5 ÷ 12 × 60 = 25 GPM, which is 1,500 GPH, 94.6 L/min or 5.68 m³/h. Run it at the tap the job depends on: the well's pressure tank outlet, the hose bib for an irrigation zone or the fixture that is short of water.
Flow from pipe size and velocity
Flow is the inside area of the pipe times the speed of the water: area = π × d² ÷ 4. In US units that works out to GPM = 2.448 × d² × v, with d the inside diameter in inches and v in feet per second. Turned round, v = GPM ÷ (2.448 × d²), which is how fast a known flow moves through a given pipe.
The pipe presets are the inside diameters of Schedule 40 steel pipe from ASME B36.10, as tabled by Engineering ToolBox: 1/2 in is 0.622 in inside, 1 in is 1.049 in and 2 in is 2.067 in. Other pipe has other bores, so for copper, PEX or anything else, type the inside diameter from the maker's data.
On speed: Engineering ToolBox gives 8 ft/s as the most for cold water in copper tube, 5 ft/s for hot water below 140 °F and 2 ft/s above it. The calculator flags a velocity above 8 ft/s.
Head pressure: feet of water to psi
A column of water presses down by its height alone, whatever the pipe size. NIST lists an inch of water at 60 °F as 248.84 pascals, which makes a foot of water 0.433 psi and 1 psi equal to 2.31 feet, the same figures the California Water Boards print on their operator exam formula sheet. In metric, a metre of water is 9,806.65 pascals, 0.0981 bar.
Worked example: a storage tank's water surface sits 60 ft above a pressure gauge. 60 × 0.433 = 26 psi on the gauge with no water flowing. Turned round, a pump that must deliver 50 psi at a tap 20 ft above it has to make 50 × 2.31 + 20 = 135.5 ft of head before friction loss.
What this calculator does not do
It does not work out friction loss, the pressure drop through fittings and meters, or the flow a fixture count calls for. Sizing a water service or a pump runs through those, under the plumbing code that applies where you work. Use these numbers to check a measurement, rough out a quote or explain one.
For plumbers, irrigation and well contractors
The numbers in a water quote come from the service call: what the bucket test gave, the fixture count, the run lengths, the height to the top floor or the tank, and what the customer says goes wrong and when. Miss one and the quote, or the fix, comes back.
Sources, each read on 2026-10-25. All results are estimates for checking and quoting:
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement (1 gallon = 231 cubic inches)
- NIST SP 811, Appendix B.8: Factors for units (inch of water at 60 °F, psi, mm of water, bar)
- California Water Boards: distribution operator exam formula sheet (0.433 psi/ft, 2.31 ft/psi)
- Engineering ToolBox: ASME/ANSI B36.10 steel pipe dimensions (Schedule 40 inside diameters)
- Engineering ToolBox: Copper Tubes, Maximum Water Velocities (8, 5 and 2 ft/s)
- EPA WaterSense: Showerheads (2.5 GPM standard, 2.0 GPM labeled)
- EPA WaterSense: Bathroom Faucets (2.2 GPM standard, 1.5 GPM labeled)
More for trades and home services sales: Home improvement sales, Quote follow-up email.
The fixture count gets said on the service call
So do the run lengths, the bucket test result and what the customer says goes wrong. Record the call in ParrotNotes and get a structured note, a summary and the action items, so the quote matches the job. Free for 100 minutes of recording a month.
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