Pipe Flow Calculator

Size a pipe for a target flow rate and check the velocity against recommended limits.

About Pipe Flow Calculator

Excess velocity causes noise, erosion and steeply rising pressure loss, so pipes are sized to a velocity limit rather than simply to fit the flow. Typical limits are 1 to 3 m/s for water in building services. Pressure loss rises with roughly the square of velocity, so a pipe one size too small costs pumping energy for its entire life.

Formula

Pipe Flow

v = Q ÷ A; minimum diameter = √(4Q ÷ (π v_max))

v_max: Velocity ceiling set to limit noise, erosion and pressure loss

Worked example

With the default values below, Pipe Flow Calculator returns Resulting Velocity of 1.273. Change any input above and every figure here is recalculated in your browser.

Inputs used

Required flow rateminimum 0 L/s: 10 L/s
Pipe internal diameterminimum 0.1 mm: 100 mm
Recommended maximum velocityminimum 0.1 m/s: 3 m/s

Results

Resulting Velocity: 1.273 m/s
Minimum Diameter at the Velocity Limit: 65.15 mm
Cross-Sectional Area: 0.007854 m2
Headroom to the Velocity Limit: 57.6%
Verdict: Comfortably within the velocity limit

Step by step

1Values used
2Pipe Flow
3Resulting Velocity= 1.273 m/s
4Minimum Diameter at the Velocity Limit= 65.15 mm
5Cross-Sectional Area= 0.007854 m2
6Headroom to the Velocity Limit= 57.6
7Verdict= Comfortably within the velocity limit

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Pipe Flow calculated?

v = Q ÷ A; minimum diameter = √(4Q ÷ (π v_max)) Excess velocity causes noise, erosion and steeply rising pressure loss, so pipes are sized to a velocity limit rather than simply to fit the flow. Typical limits are 1 to 3 m/s for water in building services.

Why does Pipe Flow matter?

Pressure loss rises with roughly the square of velocity, so a pipe one size too small costs pumping energy for its entire life.

What values do I need to enter?

This calculator takes 3 inputs: Required flow rate, Pipe internal diameter, Recommended maximum velocity. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.