Pipe Flow Calculator
Convert between flow rate, velocity and pipe size using continuity.
Please note: These are idealised textbook models using representative material data. They are for study, sizing and sanity-checking only — never for final design. Real engineering requires code-compliant analysis, verified material certificates and a qualified engineer’s sign-off.
What the Pipe Flow Calculator does
The continuity equation ties flow rate, pipe area and velocity together: Q = A v. Since area depends on the square of diameter, small changes in pipe size have an outsized effect on velocity — and therefore on noise, erosion and pressure loss.
Formula
Q = A × vA = π D² ÷ 4D = √(4Q ÷ (π v))Mass flow = Q × ρ
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Solve for | one of 3 options | Yes | — |
| Pipe inside diameter | number | In some modes | Shown Solve for is Flow rate from velocity, or Solve for is Velocity from flow rate. |
| Pipe inside diameter unit | one of 9 options | In some modes | Shown Solve for is Flow rate from velocity, or Solve for is Velocity from flow rate. |
| Flow velocity | m/s | In some modes | Accepts more than 0. Shown Solve for is Flow rate from velocity, or Solve for is Pipe diameter for a target velocity. |
| Flow rate | L/s | In some modes | Accepts more than 0. Shown Solve for is Velocity from flow rate, or Solve for is Pipe diameter for a target velocity. |
| Fluid | one of 7 options | Optional | — |
How to use it
- Choose Solve for and Fluid.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
Water at 2 m/s through a 50 mm pipe.
- Diameter
- 50 mm
- Velocity
- 2 m/s
A = π × 0.05² ÷ 4 = 1,963.5 mm². Q = 2 × 0.0019635 = 3.927 L/s, or 235.6 L/min.
Frequently asked questions
What velocity should I design for?
1 to 2.5 m/s suits most water services. Below 0.5 m/s sediment settles; above 3 m/s you get noise, erosion and a real risk of water hammer.
Why does a slightly smaller pipe make such a difference?
Area goes with diameter squared and pressure drop with velocity squared, so pressure loss scales roughly with the fifth power of diameter. A 10% smaller pipe loses about 60% more pressure.
Method and sources
Method. Continuity — flow rate as cross-sectional area times velocity — with the Reynolds number reported to identify the flow regime.
Assumptions
- Incompressible fluid, full pipe, and a uniform velocity profile assumed for the average.
Limitations
- Average velocity conceals the profile: laminar flow peaks at twice the average at the centreline, which matters for erosion and mixing.
- The calculation assumes a full pipe. Partially full gravity flow follows different relationships entirely.
- Compressible flow — gases at significant pressure drop — is outside this model.
Sources
- The continuity equation and Reynolds' flow-regime criterion — Classical fluid mechanics. The flow-rate relationship and the laminar-to-turbulent transition bands.