How to estimate runoff volume from rainfall
Rain depth spread over an area represents a volume of water. A specified runoff fraction C estimates the part that becomes runoff for this simple event-volume exercise.
What the formula is saying
First calculate rainfall volume PA, then multiply by the specified fraction C: V = CPA. Here C is a supplied event runoff fraction, not a universal property of every surface.
Read the symbols in plain language
- C
- Runoff coefficientratio / no unit
- P
- Rainfall depthm
- A
- Catchment aream²
Sort out the units first
Rainfall depth must be in metres: 30 mm = 0.03 m. Area is in m². Enter C = 0.8 for an assumed 80% runoff fraction.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
- C · Runoff coefficient
- 0.8
- P · Rainfall depth
- 0.03 m
- A · Catchment area
- 200 m²
Find the rain volume over the area
Depth times area makes a volume, not a flow rate.
(0.03) × (200) = 6 m³Keep the specified runoff fraction
The remaining fraction is not counted as runoff by this simple model.
(0.8) × (6) = 4.8 m³
Does this worked answer make sense?
The example rain volume is 6 m³, so 80% gives 4.8 m³. With C between 0 and 1, runoff cannot exceed the rain volume in this model.
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Use these new values. Work it out first, then check your answer.
- C · Runoff coefficient
- 0.6
- P · Rainfall depth
- 0.05 m
- A · Catchment area
- 500 m²
Find: How to estimate runoff volume from rainfall
A hint, not the answer
First calculate rainfall volume PA, then multiply by the specified fraction C: V = CPA. Here C is a supplied event runoff fraction, not a universal property of every surface.
Rainfall depth must be in metres: 30 mm = 0.03 m. Area is in m². Enter C = 0.8 for an assumed 80% runoff fraction.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Find the rain volume over the area
Depth times area makes a volume, not a flow rate.
(0.05) × (500) = 25 m³Keep the specified runoff fraction
The remaining fraction is not counted as runoff by this simple model.
(0.6) × (25) = 15 m³
Avoid the common trap
Using millimetres as metres makes the answer 1,000 times too large. A volume in m³ is not peak discharge in m³/s.
When this method applies — and when it does not
Event-volume estimate with uniform rainfall and a stated fraction between 0 and 1. It does not size drains or replace a calibrated rainfall–runoff model.
For study and understanding, not approval of a real structure, site operation or design. Apply the correct standard, National Annex and professional review to actual engineering work.
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Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
- OpenStax · Prealgebra 2e · Volume and surface area
- FAO · Rainfall–runoff analysis · Event runoff coefficient
Lesson updated: · Worked examples checked against the implemented formula; not an independent engineering certification.