Pipe Diameter For Flow Calculator

Work out pipe diameter for flow instantly with clear inputs, formula shown and shareable results.

About Pipe Diameter For Flow Calculator

Pipe diameter follows from continuity: area is flow divided by the velocity limit, and diameter is the diameter of a circle with that area. Velocity limits are set by noise and erosion, typically 1.0-1.5 m/s for a distribution main and up to 2.5 m/s for short branches.

Formulas

Diameter

A = Q / V; D = sqrt(4A / pi)

Design flow

Q design = peak flow x (1 + growth margin)

Worked example

With the default values below, Pipe Diameter For Flow Calculator returns Internal diameter required of 96.6. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design flowminimum 0.01 L/s: 10 L/s
Maximum velocityminimum 0.1 m/s: 1.5 m/s
Flow margin for growthaccepts 0–50 %: 10 %

Results

Internal diameter required: 96.6 mm
Nominal size to use: DN 100
Velocity in the chosen size: 1.401 m/s
Design flow with margin: 11.00 L/s

Step by step

1Values used
2Diameter
3Design flow
4Internal diameter required= 96.6 mm
5Nominal size to use= DN 100
6Velocity in the chosen size= 1.401 m/s
7Design flow with margin= 11.00 L/s

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 3 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 the Pipe Diameter For Flow figure worked out?

A = Q / V; D = sqrt(4A / pi). Pipe diameter follows from continuity: area is flow divided by the velocity limit, and diameter is the diameter of a circle with that area.

Should I round up or down to the nominal size?

Up. Rounding down pushes velocity above the limit, and velocity drives both noise and friction loss, which is squared in the pressure penalty.

What values do I need to enter?

This calculator takes 3 inputs: Design flow, Maximum velocity, Flow margin for growth. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.