Drainage Calculator

What size drain pipe do I need?

sq ft
Surface
in/hr
%
Pipe type

Plan for

3inch pipe

Rational method with a typical runoff coefficient of 0.85, and Manning full-pipe capacity at n = 0.011. A planning estimate only; large or critical drainage needs a civil engineer and local approval.

2,000 sq ft35 gpm in the storm1% slope3 in pipe carries 47 gpm
Peak flow35 gpm
Flow in cfs0.078
Pipe capacity full46.9 gpm
Runoff coefficient0.85

Enter the area draining to a pipe and a storm intensity. This compares peak runoff against what each pipe size can carry.

How it works

Peak flow uses the rational method, Q = C × i × A, converted to gallons per minute. Pipe capacity uses the Manning equation for full flow. The result is the smallest common size that carries it.

Before you order

  • Use your local design storm intensity, available from your county or NOAA Atlas 14.
  • Corrugated pipe carries about half of what smooth pipe does at the same size.
  • Add a cleanout where pipes change direction.

Common questions

Is bigger drain pipe always better?

Oversizing rarely hurts, but it costs more and needs a steeper grade to stay clean.

Do I need an engineer for drainage?

For small residential runs usually not, but large areas, property line discharge and anything touching a public system often need one.