Pipe Flow Rate Calculator
Calculate the volumetric flow rate through a full pipe from its inside diameter and the fluid velocity (Q = A × v), find the velocity for a known flow, or size a pipe for a target velocity. Choose a standard Schedule 40 pipe or type any inside diameter, and see the Reynolds number for water or air.
How to use the Pipe Flow Rate Calculator
- Choose what to find: flow rate, velocity, or pipe diameter.
- Pick a standard pipe size or choose Custom and enter the inside diameter.
- Enter the velocity or the flow rate with its unit.
- Select the fluid to get the Reynolds number (laminar or turbulent).
- Read the result in all the common units and compare pipe sizes in the table.
Pipe flow formulas
- Flow rate: Q = A × v, with the inside cross-section A = π D² ÷ 4
- Velocity: v = Q ÷ A = 4 Q ÷ (π D²)
- Diameter: D = √(4 Q ÷ (π v))
- US shortcut: v (ft/s) = 0.4085 × GPM ÷ D² (D in inches); GPM = 2.448 × v × D²
- Reynolds number: Re = v D ÷ ν (ν = kinematic viscosity; 1.004 × 10⁻⁶ m²/s for water at 20 °C)
These assume a full pipe and the average velocity across the section.
Schedule 40 pipe inside diameters
| Nominal size | Outside diameter (in) | Inside diameter (in) | Flow at 5 ft/s (US GPM) |
|---|---|---|---|
| 1/2" | 0.840 | 0.622 | 4.7 |
| 3/4" | 1.050 | 0.824 | 8.3 |
| 1" | 1.315 | 1.049 | 13.5 |
| 1-1/4" | 1.660 | 1.380 | 23.3 |
| 1-1/2" | 1.900 | 1.610 | 31.7 |
| 2" | 2.375 | 2.067 | 52.3 |
| 3" | 3.500 | 3.068 | 115.2 |
| 4" | 4.500 | 4.026 | 198.4 |
| 6" | 6.625 | 6.065 | 450.2 |
| 8" | 8.625 | 7.981 | 779.6 |
Flow units: 1 US GPM = 0.06309 L/s = 0.2271 m³/h; 1 L/s = 15.85 US GPM = 3.6 m³/h; 1 ft³/s = 448.83 US GPM.
Frequently asked questions
How do you calculate flow rate in a pipe?
Multiply the cross-sectional area by the average velocity: Q = A × v, with A = π × D² ÷ 4 for the inside diameter D. A pipe with a 50 mm bore and water moving at 1.5 m/s carries 0.0019635 m² × 1.5 m/s = 0.002945 m³/s ≈ 2.95 L/s ≈ 46.7 US GPM.
How do I find the velocity from GPM and pipe size?
Divide the flow by the area: v = Q ÷ A. In US units a handy shortcut is v (ft/s) = 0.4085 × GPM ÷ D², with D the inside diameter in inches. 20 GPM in a 1" Sch 40 pipe (ID 1.049") flows at about 7.4 ft/s.
Should I use the nominal or the inside diameter?
Always the inside diameter. Nominal pipe sizes are only names: a 1" Schedule 40 steel pipe has an inside diameter of 1.049", while 1" Schedule 80 is 0.957" and 1" copper or plastic tube differ again. The presets here use Schedule 40 inside diameters.
What is a good water velocity in a pipe?
A common design guideline for water in pressure pipes is roughly 1 to 2.5 m/s (3 to 8 ft/s). Slower flow needs bigger, more expensive pipes; faster flow causes more friction loss, noise, erosion and water hammer. Suction lines to pumps are usually kept slower.
What does the Reynolds number tell me?
Re = v × D ÷ ν compares inertial and viscous forces. In pipes, flow is usually laminar below about 2,300, turbulent above about 4,000, and transitional in between. Most water pipes in buildings run turbulent.
Does this calculator give pressure drop?
No. It calculates flow, velocity and size for a full pipe. Pressure loss depends also on pipe length, roughness, fittings and the friction factor (Darcy-Weisbach or Hazen-Williams equations).
How do I convert GPM to L/s?
1 US gallon per minute = 0.0630902 L/s = 3.78541 L/min = 0.227125 m³/h. 1 imperial gallon per minute = 0.0757682 L/s.