Enter the inside pipe diameter in inches and the flow velocity in feet per second. The formula, gallons per minute equals 2.448 times the diameter squared times the velocity, comes from the pipe's cross-sectional area. Velocity limits guide design: about 5 feet per second is common for domestic supply lines and up to 8 for short runs, since higher velocities cause noise, erosion and water hammer.
Use the actual bore of the pipe.
The formula assumes full flow at the given velocity, but real flow depends on the pressure available and the friction of the pipe and fittings. Pressure drops along the pipe reduce flow at the far end. Different fluids and temperatures change the numbers.
Plumbing codes set minimum pipe sizes by fixture units.
Flow rate (GPM) from pipe diameter
| Pipe diameter | 0.75 |
|---|---|
| Flow velocity | 5 |
| Flow rate (GPM) | 6.9 |
| Flow rate (gallons per hour) | 413.1 |
Common error: Sizing a pipe from the nominal size alone. The inside diameter sets the flow, and it is smaller than the nominal size for most materials.
Flow Rate (GPM): formula
Flow Rate (GPM): changing Pipe diameter
| Pipe diameter | Flow rate (GPM) | Flow rate (gallons per hour) |
|---|---|---|
| 0.6 | 4.4 | 264.4 |
| 0.675 | 5.6 | 334.6 |
| 0.75 (example) | 6.9 | 413.1 |
| 0.825 | 8.3 | 499.9 |
| 0.9 | 9.9 | 594.9 |
Flow Rate (GPM): compare input ranges
| Input | Tested from | To | Flow rate (GPM): low input | Flow rate (GPM): high input | Swing |
|---|---|---|---|---|---|
| Pipe diameter | 0.675 | 0.825 | 5.6 | 8.3 | 2.8 (40%) |
| Flow velocity | 4.5 | 5.5 | 6.2 | 7.6 | 1.4 (20%) |
Flow Rate (GPM): individual input comparisons
| Input | Value | Flow rate (GPM) | Flow rate (gallons per hour) |
|---|---|---|---|
| Pipe diameter | 0.6 | 4.4 | 264.4 |
| 0.675 | 5.6 | 334.6 | |
| 0.75 | 6.9 | 413.1 | |
| 0.825 | 8.3 | 499.9 | |
| 0.9 | 9.9 | 594.9 | |
| Flow velocity | 4 | 5.5 | 330.5 |
| 4.5 | 6.2 | 371.8 | |
| 5 | 6.9 | 413.1 | |
| 5.5 | 7.6 | 454.4 | |
| 6 | 8.3 | 495.7 |
Flow rate (GPM): Pipe diameter against Flow velocity
| Pipe diameter / Flow velocity | 4 | 5 | 6 |
|---|---|---|---|
| 0.6 | 3.5 | 4.4 | 5.3 |
| 0.675 | 4.5 | 5.6 | 6.7 |
| 0.75 | 5.5 | 6.9 | 8.3 |
| 0.825 | 6.7 | 8.3 | 10 |
| 0.9 | 7.9 | 9.9 | 11.9 |
Flow Rate (GPM): values and definitions
| Quantity and definition | Value used |
|---|---|
| Pipe diameter — Inside diameter of the pipe, in inches. | 0.75 |
| Flow velocity — Water velocity in the pipe, in feet per second. | 5 |
| Flow rate (GPM) | 6.9 |
| Flow rate (gallons per hour) | 413.1 |
Pipe diameter: half and double
| Pipe diameter | Flow rate (GPM) |
|---|---|
| 0.375 | 1.7 |
| 0.75 | 6.9 |
| 1.5 | 27.5 |
The flow through a pipe is its cross-section times the velocity of the water. A three-quarter inch line at 5 feet per second carries about 6.9 gallons a minute, and a one inch line at the same speed carries about 12.2. Doubling the diameter quadruples the capacity at the same velocity.
Velocity matters too: faster water makes noise and wears fittings, so designers keep it within a range and pick a larger pipe rather than push more through a small one.
Check the velocity against the limit for the pipe material and the service. Hot water lines and long runs usually call for lower speeds than a short cold line.
Calculate the pressure drop on the pipe friction loss calculator, the water held on the pipe volume page and hot water needs on the water heater sizing calculator. The drainage fixture unit and drain slope pages cover waste piping, and the volume converter changes units.
How do I convert gallons per minute to gallons per hour?
Multiply by 60. So 6.9 gpm is about 413 gallons per hour.
What flow does a typical shower use?
About 2 gallons per minute for a modern low-flow showerhead, with older ones using 2.5 or more.