Flow rate links pipe size and speed: the same flow through a smaller pipe must move faster.
How to use the Pipe Flow Rate Calculator
- Inner diameter
- Fluid velocity
- Want to work backwards? Type a value into Flow rate and Fluid velocity is calculated for you.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Multiply the pipe’s cross-sectional area by the average velocity.
Inputs explained
| Field | What to enter |
|---|---|
| Inner diameter | Units: mm, in, cm, m |
| Fluid velocity | Units: m/s, km/h, mph, ft/s, knots |
Example values
How the main result changes when Fluid velocity changes and everything else stays at the example values above:
| Fluid velocity (m/s) | Flow rate (L/min) |
|---|---|
| 0.75 | 22.09 |
| 1.13 | 33.13 |
| 1.5 | 44.18 |
| 1.88 | 55.22 |
| 2.25 | 66.27 |
| 3 | 88.36 |
Formula
Q = π (d ÷ 2)² × vWorked example
Inputs — Inner diameter: 25 mm; Fluid velocity: 1.5 m/s.
Results — Flow rate = 44.18 L/min.
Frequently asked questions
What velocity is typical for water pipes?
About 0.5–2.5 m/s in domestic plumbing.
Why use inner diameter?
Only the open bore carries fluid.
What formula does the Pipe Flow Rate Calculator use?
Q = π (d ÷ 2)² × v.
Can I work backwards from the result?
Yes. Type the result you want into Flow rate and the calculator finds the matching fluid velocity.