Runoff Catchment Calculator
Turn roof, patio, driveway, and yard measurements into a runoff burden check so you can pick the right drainage branch before you buy trench or inlet materials.
Use when roof, hardscape, and yard surfaces all feed the same problem area.
Balanced residential default when local design data is unknown.
Very little infiltration. Usually drives basin/downspout capture decisions.
Absorbs more water, but can still overwhelm low spots.
Surface Coefficients Used For The Quick Check
| Surface type | Default coefficient | Why it matters |
|---|---|---|
| Roof | 0.95 | Very little infiltration. Usually drives basin/downspout capture decisions. |
| Asphalt / concrete | 0.90 | Fast surface runoff. Often needs channel or inlet capture. |
| Pavers / hardscape | 0.80 | Hardscape sheds quickly even when joints are permeable. |
| Lawn / loam | 0.35 | Absorbs more water, but can still overwhelm low spots. |
| Mulched bed / permeable soil | 0.20 | Lowest runoff of the common residential surface set. |
| Gravel | 0.45 | Often drains better than hardscape, but still sheds more than planted soil. |
Residential Rainfall Basis
Light storm
Quick residential screening for small storm events.
1.0 in depth
1.0 in/hr intensity
Moderate storm
Balanced residential default when local design data is unknown.
1.5 in depth
1.8 in/hr intensity
Heavy storm
Use when runoff is already causing frequent pooling or inlet overload.
2.5 in depth
2.8 in/hr intensity
How The Runoff Math Works
The calculator first builds a surface schedule: each roof, hardscape, yard, or permeable area is converted into square footage and then multiplied by a runoff coefficient that describes how much of the rainfall behaves like runoff instead of infiltration.
Weighted runoff coefficient = Σ(C × A) / Σ(A)
That weighted coefficient turns mixed surfaces into one residential runoff basis. From there, the calculator estimates a quick runoff volume for the selected storm depth and a peak runoff rate using a simplified residential rational-method screen.
Runoff volume = rainfall depth × weighted runoff area
Peak runoff ≈ C × i × A
The output is intentionally narrow. It does not replace municipal drainage design or a civil drainage report. It gives homeowners and contractors a trustworthy way to decide whether the next step should be a French drain, a basin/downspout branch, or a channel-drain branch.
Frequently Asked Questions
Why can a small puddle still need a large drainage system? +
When should I choose channel drain instead of a French drain? +
Why does roof area push the recommendation toward basins and downspouts? +
What changes in Pro mode? +
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