Water Flow

Free Water Flow calculator with clear step-by-step results.

About Water Flow

Flow rate is cross-sectional area times velocity, and the Darcy-Weisbach relation converts that velocity into friction head loss over the pipe run. The Reynolds number indicates the flow regime: below about 2,300 it is laminar, above 4,000 fully turbulent, which is where the friction factor assumption applies.

Formulas

Flow rate

Q = pi x (diameter / 2)^2 x velocity

Head loss

h = f x (length / diameter) x velocity^2 / (2 g)

Worked example

With the default values below, Water Flow returns Flow rate of 34.21. Change any input above and every figure here is recalculated in your browser.

Inputs used

Pipe internal diameterminimum 1 mm: 22 mm
Flow velocityaccepts 0.01–10 m/s: 1.5 m/s
Pipe run lengthminimum 0.1 m: 25 m
Darcy friction factoraccepts 0.005–0.1: 0.025

Results

Flow rate: 34.21 L/min
Flow rate: 2.0527 m3/h
Friction head loss over the run: 3.258 m
Reynolds number: 32,869

Step by step

1Values used
2Flow rate
3Head loss
4Flow rate= 34.21 L/min
5Flow rate= 2.0527 m3/h
6Friction head loss over the run= 3.258 m
7Reynolds number= 32,869

Assumptions and limits

The result is derived only from the 4 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 4 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

What velocity should I design for?

Typically 1-2 m/s for domestic supply. Below 0.5 m/s sediment settles; above about 2.5 m/s noise, erosion and water hammer become problems.

Where does the friction factor come from?

From the Moody chart or the Colebrook equation using Reynolds number and relative roughness. Around 0.02-0.03 is typical for copper and plastic at domestic velocities.