Roof measurement
Roof runoff volume calculator
Estimate roof runoff volume. Rainfall event or rainwater planning.
Try the calculation
Use plan catchment area and event rainfall.
Adjustable example. Change these illustrative inputs to match your task, then recalculate. Reset restores the example and its result.
Understand the result
How this calculation works
Runoff volume estimates gallons from horizontal catchment area and rainfall depth.
Steps
- Enter plan area, event rainfall and runoff coefficient.
- Use the coefficient that matches the surface assumption you are checking.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
gallons = area × rainfall depth × runoff coefficient × 0.623.
Variables and units
- Plan catchment area (sq ft)
- Required. Use a positive value. Starting example: 1000. Replace it for your task.
- Rainfall depth (in)
- Required. Zero is accepted. Starting example: 1. Replace it for your task.
- Runoff coefficient (ratio)
- Required. Zero is accepted. Starting example: 0.8. Replace it for your task.
What the result means
This is event volume reaching the modeled collection path. It does not size storage or first-flush loss.
Worked example
Inputs: Plan catchment area = 1000; Rainfall depth = 1; Runoff coefficient = 0.8.
Calculation: 1,000 sq ft × 1 in × 0.8 × 0.623 = 498.4 gallons.
Result: 498.4 gallons.
Frequently asked questions
Why use plan area?
The constant is based on horizontal catchment area.
What does the coefficient represent?
The fraction of rainfall treated as runoff in this estimate.
Does the result guarantee barrel yield?
No, collection losses and overflow are outside this model.