Building Drain Calculator

Inputs

Roof area (m²)

Projected roof catchment area contributing to the drain.

Rainfall intensity (mm/hr)

Use your design storm intensity for the chosen return period.

Runoff coefficient (C)

Typical roofs: 0.8–1.0. Lower for permeable areas.

Safety factor

Applies margin to peak runoff before sizing.

Pipe slope (%)

Slope along the drain run, as percent grade.

Pipe material

Roughness affects capacity for the same slope and diameter.

Drain length (m)

Straight run length used to estimate required vertical drop.

Fittings count

Elbows, tees, transitions, and major fittings.

Equivalent length per fitting (m)

Approximate added length per fitting for drop check.

Available vertical fall (m) (optional)

If set, compares available fall to required drop.

Check existing diameter (mm) (optional)

Optional verification of an already selected pipe size.

Minimum velocity (m/s)

Low velocity may increase sediment or debris accumulation.

Maximum velocity (m/s)

High velocity may require protection or energy dissipation.

Reset

Sample scenario showing typical inputs and computed sizing.

Roof area (m²)	Intensity (mm/hr)	C	Slope (%)	Material	Design flow (L/s)	Suggested diameter (mm)
220	75	0.90	1.0	PVC (smooth)	4.54	90

The calculator finds the smallest diameter whose Manning capacity meets the design flow at the selected slope and roughness.