About the Pipe Flow Rate Calculator
This pipe flow rate calculator links the three quantities of any full pipe: inside diameter, average fluid velocity and volumetric flow rate. Enter the diameter and velocity to get the flow in gallons per minute, liters per minute, cubic meters per hour and cubic feet per minute — or switch mode, enter a flow rate, and it tells you how fast the water or other liquid is moving.
Plumbers, irrigation designers, pump installers, HVAC hydronics techs and engineering students use it to check that a pipe is sized sensibly. Velocity matters: water lines are commonly designed for roughly 2–8 ft/s (about 0.6–2.4 m/s) because faster flow increases noise, friction loss and erosion, while very slow flow can let sediment settle.
The math is the continuity equation Q = A × v for a pipe running full. Use the true inside diameter, not the nominal size — a 1" Schedule 40 steel pipe is about 1.049" inside and 1" copper Type L about 1.025". Friction loss and pressure drop are not included.
How to use the pipe flow rate calculator
- 1Choose whether you want the flow rate or the velocity.
- 2Enter the pipe inside diameter and pick inches or millimeters.
- 3Enter the velocity (for flow mode) or the flow rate (for velocity mode) and its unit.
- 4Read the result in GPM, L/min, m³/h and ft/s or m/s.
- 5Check the note: water lines are usually kept between about 2 and 8 ft/s.
Formula and method
Volumetric flow rate equals the cross-sectional flow area multiplied by the average velocity (the continuity equation). For a round pipe running full the area is π × D² ÷ 4, using the inside diameter. To find velocity from a known flow, divide the flow by the area: v = Q ÷ A.
All units are converted to SI internally (meters and m³/s) and back. One US gallon is 3.785411784 liters, so the handy shortcut GPM = 2.448 × D² × v holds with D in inches and v in feet per second. The result is an average velocity; the real profile is faster at the center and slower near the wall.
- Q
- Volumetric flow rate
- A
- Inside cross-sectional area of the pipe
- v
- Average flow velocity
- D
- Pipe inside diameter
Worked examples
2" pipe at 5 ft/s
A 2-inch bore has an area of π × 2² ÷ 4 = 3.142 in². Moving at 5 ft/s that is 0.1091 ft³/s, or about 48.96 US gallons per minute (185.3 L/min).
Velocity of 10 GPM in a 3/4" line
A 0.75-inch inside diameter gives 0.4418 in² of flow area. Ten gallons per minute is 0.02228 ft³/s, and dividing by 0.003068 ft² gives about 7.26 ft/s — near the top of the usual range.
100 mm pipe at 1.5 m/s, in m³/h
A 100 mm bore has 0.007854 m² of area. At 1.5 m/s the flow is 0.01178 m³/s, which is 42.41 m³/h or about 707 liters per minute.
Frequently asked questions
How do you calculate flow rate from pipe size?+
Multiply the pipe inside area by the fluid velocity: Q = π × D² ÷ 4 × v. In US units, GPM ≈ 2.448 × D² × v with the diameter in inches and velocity in feet per second.
How many GPM can a 1-inch pipe carry?+
It depends on velocity. A 1" Schedule 40 pipe (1.049" inside) carries about 13.5 GPM at 5 ft/s and roughly 21.5 GPM at 8 ft/s, the upper end commonly used for water lines.
What is a good water velocity in a pipe?+
Many designers keep domestic water piping between about 2 and 8 ft/s (0.6–2.4 m/s), and often below 5 ft/s for hot water or copper, to limit noise, erosion and pressure loss.
Should I use nominal or inside diameter?+
Use the actual inside diameter. Nominal pipe sizes are labels; the real bore depends on material and wall thickness (schedule or type), and small differences change the area noticeably.
Does this calculator include pressure drop?+
No. It applies only the continuity equation for a pipe flowing full. Pressure loss depends on length, fittings and roughness, which need a friction-loss method such as Hazen-Williams or Darcy-Weisbach.