Runoff calculator (rational method)
The Runoff calculator (rational method) computes runoff from the relation Q = C i A. It takes 3 inputs (runoff coefficient, rain intensity, area) and returns the peak runoff q. These calculators are written for earth and environmental science students, field and survey technicians, planners, land managers and anyone organising work or travel outdoors who has a figure in hand and needs to turn it into a usable quantity, answering questions such as how steep a stretch of ground is, how far apart two features lie, how much water a rainfall event delivers over an area, how much daylight a location receives on a given day, and how one recorded event compares in size with another. Enter your values below and the result updates instantly, and you can share a permalink that pre-fills the exact calculation. Match each entry to the unit the field asks for, keep any lengths, areas and depths you enter together in one measurement system, and be explicit about what each figure represents (horizontal run against distance along the ground, map distance against ground distance, depth of rainfall against volume over a catchment, and whether a latitude or declination needs its southern hemisphere sign); several of these scales are logarithmic as well, so equal steps in the answer do not stand for equal steps in the underlying physical quantity, and it pays to carry only as many significant figures as the figure you started with genuinely supports. For example, with runoff coefficient = 0.9, rain intensity = 100, area = 1, the peak runoff q works out to 90, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. This is a standard formula rather than a jurisdiction-specific rule, so it does not depend on an external rate or schedule, and the figure above each result carries the date it was last verified. The result reflects the simplified relation and the values you entered, not the setting they came from: terrain, catchments, soils, the atmosphere and the sea are uneven, shift with the season and with each individual event, and are seldom captured by a single figure, so treat the output as a working number for planning and interpretation rather than as a site survey, a hazard assessment or a statement of conditions on the day.
With Runoff coefficient = 0.9, Rain intensity = 100, Area = 1, the result is 90.
Applies to: any numeric inputs. Method: a standard formula, not a jurisdiction-specific rule. How we document and test our methods, checked 2026-07-09.
The formula
Q = C i A
Worked example
With Runoff coefficient = 0.9, Rain intensity = 100, Area = 1:
- Q = C i A
- Peak runoff Q = 90
This worked example is one of the automated golden-value tests this calculator must pass before it can publish.
What this assumes
- Inputs are real numbers in the units shown.
- The result is the exact value of Q = C i A; general information, not professional advice.
Frequently asked questions
What formula does this use?
Q = C i A. This is a standard formula rather than a jurisdiction-specific rule.
Does the result ever change over time?
No. This is a pure formula with no external rate, so the same inputs always give the same result.
Official sources and verification
- Method: a standard formula rather than a jurisdiction-specific rule; how we document and test our methods, checked 2026-07-09.
Reviewed by the CalculatorHub team, edited by James Graham, 2026-07-09. See our methodology. General information, not professional advice.