Horizontal roof catchment
Catchment event · m²
Allowed range: 0.01 to 1000000
Rainwater Harvest Calculator
Estimate collected rain volume, first flush loss and remaining tank space.
Open this toolRain falling on a catchment is not all stored in the tank. Collection losses and first flush reduce the available volume.
Enter rainfall, catchment area and collection assumptions, then compare the remaining tank space.
Inputs and controls
Event rain depth
Catchment event · mm
Allowed range: 0 to 10000
Runoff collection share
Catchment event · %
Allowed range: 0 to 100
First flush volume
Catchment event · L
Allowed range: 0 to 100000000
Tank capacity
Tank state · L
Allowed range: 0.1 to 100000000
Water already in tank
Tank state · L
Allowed range: 0 to 100000000
How to use
- Enter horizontal roof catchment, event rain depth, runoff collection share and first flush volume.
- Enter tank capacity and water already in tank.
- Select Calculate result. Read the returned values alongside the assumptions below before copying, exporting or acting on the result.
Why use this tool
Collection and overflow are separated, useful when checking whether a larger tank could capture more of the event.
Before relying on the result
Collection arithmetic does not assess water quality. Overflow is water that does not fit after the first flush.
Tool limitations
Collection arithmetic does not assess water quality. Overflow is water that does not fit after the first flush.
Questions
Does collected volume mean potable water?
No. The calculation does not assess water quality or treatment needs.
What should I check before using Rainwater Harvest Calculator?
Check Horizontal roof catchment, Event rain depth, Runoff collection share and First flush volume. Use the exact units shown beside the controls and replace any example values with your own inputs.
Source page last modified: 2026-10-04T13:08:12+00:00. This profile describes the tool; use its page to run it.
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