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TOOL-016 / STORMWATERReleased calculator

Composite Runoff Coefficient Calculator

Calculate an area-weighted composite runoff coefficient from the land-cover components you supply.

Method & assumptionsWorked exampleFAQ
Composite C only. Peak runoff, rainfall, time of concentration, and routing stay outside this calculator.
01

Land-cover inputs

Supply an ID, name, area, and coefficient for each component.

Input units
Component 1
Component 2

Switching units converts entered areas and keeps the full converted value for the next calculation. The field shows 6 significant figures. A blank area stays blank. The previous result is cleared when an input changes.

02

Composite result

Enter components and calculate.

Ready for calculation

Review the land-cover areas and coefficients, then calculate the area-weighted mean.

Ccomp = Σ(Ci × Ai) / ΣAi
03 / METHOD & LIMITS

Area-weighted composite coefficient

The weighted mean uses only the areas and coefficients you supply.

Ccomp = Σ(Ci × Ai) / ΣAiComposite coefficient = sum of coefficient times area, divided by total area

Each land-cover component contributes its runoff coefficient in proportion to its area. The calculator converts US areas to square metres with 1 ft² = 0.09290304 m², then evaluates the weighted mean in canonical SI area. Area fractions are Ai / ΣAi.

Input guidance

Component area · A
Use a finite area greater than zero. Square metres in SI, square feet in US.
Runoff coefficient · C
Supply a dimensionless coefficient from 0 to 1, including both bounds. The calculator does not select it.
Component identity
Each component needs a unique ID and a name so the area fractions stay traceable.

Applicability and limitations

This calculation is the area-weighted composite runoff coefficient only. It does not calculate peak runoff, rainfall intensity, time of concentration, or routing. It does not size pipes, select a coefficient from a land-cover table, or establish code or permit compliance.

Method reference

Formula FORMULA-TOOL016-AREA-WEIGHTED-COMPOSITE-C-001. The area-weighted coefficient is FHWA HEC-22 Equation 4.2 in the 4th edition (FHWA-HIF-24-006, February 2024, Section 4.2.2.1) and Equation 3-2 in the 3rd edition (FHWA-NHI-10-009, September 2009, revised August 2013, Section 3.2.2.1): weighted C = Σ(Cx Ax) / Atotal.

Mixed-cover Rational Method coefficients are also discussed in ASCE Manual of Practice No. 77 / WEF Manual of Practice FD-20, Design and Construction of Urban Stormwater Management Systems (1992), Chapter 5, Section II.G.2 Runoff Coefficient. That manual updates ASCE/WPCF Manual of Practice No. 37, Design and Construction of Sanitary and Storm Sewers (1969). Source records SRC-AREA-WEIGHTED-COMPOSITE-RUNOFF-TOOL016-001 and SRC-PROJECT-METHOD-TOOL016-001, both version 1.0.0. This page does not copy a coefficient table from those manuals.

FAQ

Frequently asked questions

What formula does this calculator use?

Composite runoff coefficient equals the sum of each supplied coefficient times its area, divided by the total area: Ccomp = Σ(Ci × Ai) / ΣAi. The primary reference is FHWA HEC-22 Equation 4.2 in the 4th edition (FHWA-HIF-24-006, February 2024, Section 4.2.2.1). The same weighting is Equation 3-2 in the 3rd edition (FHWA-NHI-10-009, September 2009, revised August 2013, Section 3.2.2.1).

What does the composite runoff coefficient represent?

It is the area-weighted mean of the runoff coefficients you supply for each land-cover component. The result is dimensionless.

Which area units can I enter?

SI areas are square metres. US areas are square feet and are converted with 1 ft² = 0.09290304 m² before the weighted average is calculated. The reported total area is canonical square metres.

Are runoff coefficients of 0 and 1 accepted?

Yes. A coefficient of 0 or 1 is valid. A blank or whitespace-only coefficient is required and is not treated as 0. A value that is not a number is rejected with a request to enter a number. A coefficient below 0 or above 1 is rejected before a result is shown.

What happens if an area is zero, negative, or the component list is empty?

Each component area must be a finite number greater than zero, and at least one component is required. An empty list, a zero area, or a non-positive area is a validation error, and any previous result is cleared.

Does this calculator produce peak runoff?

No. Peak runoff, rainfall intensity, time of concentration, and routing are outside this calculator. Composite C can be reviewed before a separate Rational Method calculation.

Will the calculator choose a runoff coefficient for me?

No. You supply every component coefficient. The tool does not look up land-cover tables or apply jurisdictional criteria.

Does a composite coefficient prove code compliance?

No. The result is a preliminary area-weighted calculation. Soil, slope, connectivity, and local drainage criteria still need engineering review.

CONTINUE YOUR WORKFLOW

After the composite coefficient

Separate peak-flow estimateRational Method peak runoff

Q = C × i × A uses a coefficient you still have to justify.

Read the methodRational Method guide

Units, assumptions, and a worked peak-flow example.

Related

Continue from this result

ReleasedRational Method peak runoff

Peak flow stays a separate calculation

ReleasedDrainage planner

Preliminary runoff and pipe check

ReleasedRational Method guide

How a supplied coefficient enters Q = C i A

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