Home & Construction
Rainwater Harvesting & Tank Calculator
Updated Aug 25, 2026 Reviewed Aug 25, 2026
Estimate roof runoff, first-flush loss, collectable rainwater, tank storage, overflow and simplified non-potable demand across multiple roofs.
Non-potable rainwater planning
Enter catchments, rainfall, storage and simplified demand
Result
Calculation summary
Enter values to see the result
Your result, breakdown, assumptions, and warnings will appear here.
Catchment-to-tank preview
Roof capture, storage and overflow
Collectable rain
478.75 US gal
Overflow
228.75 US gal
Estimated supply days
12.5
How to use this calculator
- 1Add each horizontal projected roof area.
- 2Enter rainfall for an event, month or year.
- 3Enter efficiency and first-flush depth.
- 4Set tank capacity, starting storage and optional demand.
Formula
Collectable = area × effective rain × efficiency
Effective rain is rainfall minus the entered first flush, not less than zero.
Calculation steps
- Calculate projected catchment area.
- Convert rain and first flush to depth.
- Apply roof efficiency.
- Apply tank capacity and starting storage.
- Deduct batch demand and report overflow or shortage.
Worked example
A 100 m² roof receiving 10 mm at 80% efficiency yields 0.8 m³ before tank limits when first flush is zero.
Assumptions
- Roof area is horizontal projection.
- Efficiency and first flush are user inputs.
- The period is one batch.
- Stored water is assumed non-potable.
- Conveyance, pumps and treatment are excluded.
Sources
Frequently asked questions
Should I use sloped roof area?
Use horizontal projected area because rainfall depth is vertical.
What is efficiency?
It represents collection losses.
What is first flush?
The initial depth diverted before storage.
Why is there overflow?
Starting storage plus collection exceeds capacity.
Is demand simulated daily?
No, this is a transparent single-batch balance.
Is the water safe to drink?
The tool assumes non-potable use and does not assess safety.
Related calculators
Roofing Calculator
Estimate pitched roof area, shingle bundles, underlayment rolls, and sheathing sheets from roof dimensions, overhang, pitch, and product coverage.
Drainage Slope & Fall Calculator
Solve drainage slope, required fall, available run or grade from elevations with mixed units, signed flow direction, ratio, angle, millimeters per meter and inches per foot.