The Rational Method: flow in litres per second from the runoff coefficient, rainfall intensity in mm/hr and catchment area, with unit conversion handled internally.

Does it choose a pipe diameter?

No. You enter a candidate capacity and the tool reports the percentage margin against the estimated flow; selecting the pipe remains a design decision.

Where do the coefficient and intensity come from?

From the local requirement or the approved climatic records for your site. No coefficient table or rainfall data is bundled with the tool.

Is it suitable for foul drainage?

No. The Rational Method addresses rainfall runoff. Foul drainage loading uses fixture-based methods that this engine does not implement.

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Drainage Estimator

Estimate drainage system requirements.

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Drainage reviews start with a flow figure before any pipe is chosen. This Professional calculator applies the Rational Method to the catchment area, runoff coefficient and rainfall intensity you provide, then reports how much margin a candidate capacity leaves against that flow.

Method and assumptions

Rational Method

Q(L/s) = 2.7778 × C × i(mm/hr) × A(ha)
Margin% = (candidate − Q) ÷ Q × 100

The runoff coefficient and design rainfall intensity are user-entered from the local code or the approved climatic records for the project.
All percentages and inputs are user-defined; the calculation is purely arithmetic and assumes no particular code or specification.

Frequently asked questions

What method is used?: The Rational Method: flow in litres per second from the runoff coefficient, rainfall intensity in mm/hr and catchment area, with unit conversion handled internally.
Does it choose a pipe diameter?: No. You enter a candidate capacity and the tool reports the percentage margin against the estimated flow; selecting the pipe remains a design decision.
Where do the coefficient and intensity come from?: From the local requirement or the approved climatic records for your site. No coefficient table or rainfall data is bundled with the tool.
Is it suitable for foul drainage?: No. The Rational Method addresses rainfall runoff. Foul drainage loading uses fixture-based methods that this engine does not implement.

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