Water & Irrigation
Rainwater Tank Calculator: How Much Can You Collect?
Work out how much rainwater you can collect from your roof, and size a rainwater tank. Enter your roof area and local rainfall to estimate litres collected.
Estimated collection
Rainwater results
For 1,000 sq ft of roof area and 1 in of rain, this setup can collect about 449 gallons before storage limits.
Formula used
For U.S. inputs, this calculator uses:
gallons = roof area sq ft x rainfall inches x 0.623 x runoff coefficient x efficiency
For metric inputs, it uses:
litres = roof area m2 x rainfall mm x runoff coefficient x efficiency
Efficiency is entered as a percentage and converted to a decimal. For example, 90% efficiency becomes 0.9.
Roof material runoff table
| Roof surface | Typical coefficient | Planning note |
|---|---|---|
| Metal roof | 0.9 | Smooth surfaces often shed water efficiently. |
| Asphalt shingle | 0.8 | A practical default for many residential roofs. |
| Tile roof | 0.75 | Surface shape and texture can reduce collection. |
| Flat roof | 0.6 | Standing water, slope, and drains can affect runoff. |
Rain barrel sizing tips
A small 50 to 60 gallon barrel can fill quickly from a moderate roof area. If the calculator shows frequent overflow, consider a larger tank, multiple barrels connected safely, or a planned overflow route away from foundations. A rain barrel size calculator is most useful when you compare typical storm amounts with realistic storage space.
How to use collected rainwater safely
Collected rainwater is commonly used for garden watering, but it can pick up roof debris, dust, animal waste, and other contaminants. Do not treat this calculator as drinking water guidance. Check local rules and use proper filtration and treatment for non-garden uses.
Common mistakes
- Using total house floor area instead of the roof area that drains to the collection point.
- Forgetting that gutters, filters, first-flush diverters, and leaks reduce collection efficiency.
- Choosing a tank size without planning overflow away from the foundation.
- Assuming all roof materials have the same runoff coefficient.
- Using average monthly rainfall when you need storm-by-storm storage planning.
FAQ
Rainwater collection calculator FAQ
How much rainwater can I collect?
It depends on roof area, rainfall, roof material, and collection efficiency. The calculator estimates gallons and litres from those inputs.
How does a roof runoff calculator work?
It multiplies roof area by rainfall and then adjusts for runoff coefficient and collection efficiency.
Why does the U.S. formula use 0.623?
One inch of rain on one square foot equals about 0.623 gallons of water before runoff and efficiency losses.
What runoff coefficient should I choose?
Use the roof material closest to your situation, or enter a custom value if you have a better estimate.
Can this help size a rain barrel?
Yes. Enter a tank or barrel size to see how many full barrels the storm could fill and whether overflow is likely.
Can I use metric units?
Yes. Enter roof area in square metres and rainfall in millimeters. Results still show both gallons and litres.
Can collected rainwater be used for drinking?
This page does not provide drinking water guidance. Check local rules and use proper filtration and treatment for non-garden uses.
Why is collection efficiency less than 100%?
Water can be lost through splashing, gutter overflow, leaks, debris screens, first-flush systems, and imperfect routing.