GPM Calculator
Use this GPM calculator to turn pipe velocity and inside diameter into flow: 5 ft/s through a 2 in pipe moves about 48.96 gallons per minute.
How to calculate GPM from velocity and pipe size
A GPM calculator answers one question: how much water moves through a pipe when it flows at a given speed. Enter the water velocity in feet per second and the inside diameter in inches. The tool finds the pipe’s cross-section area and multiplies it by the velocity and by 448.831 to give gallons per minute.
Picture the water as a long cylinder sliding down the pipe. The cross-section is the end of that cylinder. The faster it slides, the more passes the end each minute.
The formula in plain words
- Divide the inside diameter in inches by 2 to get the radius, then divide by 12 to put it in feet.
- Area in square feet = π × radius × radius.
- Flow in cubic feet per second = velocity × area.
- GPM = cubic feet per second × 448.831, because one cubic foot per second is 448.831 gallons per minute.
The result page also shows the pipe cross-section in square feet, which is handy if you are comparing two pipe sizes.
Worked example: a 2 inch line at 5 ft/s
- Radius: 2 ÷ 2 ÷ 12 = 0.0833 ft
- Area: π × 0.0833² = 0.0218 ft²
- Flow: 5 × 0.0218 = 0.1091 cubic feet per second
- GPM: 0.1091 × 448.831 = 48.96 GPM
Worked example: a 1.5 inch line at 8 ft/s
- Radius: 1.5 ÷ 2 ÷ 12 = 0.0625 ft
- Area: π × 0.0625² = 0.0123 ft²
- Flow: 8 × 0.0123 = 0.0982 cubic feet per second
- GPM: 0.0982 × 448.831 = 44.06 GPM
The smaller pipe running faster still moves less water than the 2 inch pipe running slower. Diameter matters more than speed, because area grows with the square.
Flow at 5 ft/s by inside diameter
| Inside diameter | Area (ft²) | GPM |
|---|---|---|
| 0.75 in | 0.0031 | 6.88 |
| 1 in | 0.0055 | 12.24 |
| 1.5 in | 0.0123 | 27.54 |
| 2 in | 0.0218 | 48.96 |
| 3 in | 0.0491 | 110.16 |
| 4 in | 0.0873 | 195.84 |
Doubling the diameter from 2 in to 4 in quadruples the flow at the same speed. That is the square at work.
Where to get the numbers
For diameter, read the pipe’s marking or the manufacturer’s data sheet and use the inside diameter listed there. Copper, PVC, and PEX of the same nominal size have different inside diameters, so do not assume.
For velocity, you often do not know it. In that case, work backward: start from the GPM a fixture or pump delivers, and use the pipe velocity calculator to see how fast the water is moving.
What to do with the answer
A flow number is most useful when you compare it to something. A garden hose bib, a sprinkler zone, or a transfer pump each has a rated flow printed on the spec sheet. If the pipe can carry 48.96 GPM at your chosen speed but the pump only makes 20, the pump is the limit and the pipe is oversized.
Going the other way, if a fixture or zone needs 30 GPM, try a few diameters in the tool. Pick the smallest pipe that carries the flow at a speed you are comfortable with, then check friction loss over the full length.
Common mistakes
- Using the outside diameter. Wall thickness eats into the opening and this overstates flow.
- Entering the nominal size. A pipe called 2 in may measure a little more or less inside.
- Mixing units. Velocity must be in feet per second and diameter in inches. If your data is in meters per second, convert first.
- Reading the result as pump output. A pump curve, pipe length, fittings, and elevation all limit real flow. This tool gives the ideal figure for the velocity you typed.
- Ignoring friction. Long runs with many bends lose pressure, which lowers velocity. The pipe friction loss calculator shows how.
Related calculators
Once you know the flow, the pipe volume calculator tells you how much water the line holds, and the pipe velocity calculator runs the same math in reverse. For anything involving a licensed plumbing design, confirm sizes with a plumber or the engineer of record.
Frequently asked questions
How do you calculate GPM from velocity and pipe size?
Find the pipe's cross-section area in square feet from the inside diameter, multiply by the water velocity in feet per second, and multiply by 448.831. That last number converts cubic feet per second to gallons per minute. A 2 in pipe at 5 ft/s gives 48.96 GPM.
Should I use nominal pipe size or inside diameter?
Use the actual inside diameter. A pipe sold as 2 in has an inside diameter that depends on the material and wall thickness, and the difference changes the answer noticeably because area grows with the square of the diameter.
What velocity should I design for?
Many designers keep water in building supply piping at moderate speeds to limit noise and wear, but the right limit depends on the pipe material and the application. Check the pipe manufacturer's guidance or the plumbing code for your area.
Does this account for pressure or friction loss?
No. It only converts velocity and area into flow. To see how much pressure a long run loses, use the pipe friction loss calculator.
How do I go the other way, from GPM to velocity?
Divide flow in gallons per minute by 448.831 to get cubic feet per second, then divide by the pipe area in square feet. The pipe velocity calculator on this site does that for you.
Estimates only. Verify quantities with your supplier before purchasing.