Roof gutter directing rain into a collection barrel. Original AI-generated artwork produced for GreenCalc; the scene is not a measured construction drawing.
One inch of rain over one square foot represents about 0.6234 US liquid gallon before collection losses. Multiply that depth-volume relationship by the roof area draining to the collection point, then account separately for runoff losses and available tank space. Rainfall volume and stored water are not automatically the same quantity.
Use the horizontal catchment footprint
Rainfall depth is measured over a horizontal area. For an ordinary roof-volume estimate, use the horizontal projected area feeding the selected gutter or downpipe, rather than increasing the area simply because the roof surface is sloped. Separate roof sections when they drain to different collection points.
If a roof footprint is 30 feet by 20 feet and the whole footprint drains to your chosen collector, its catchment is 600 square feet. Overhangs, disconnected gutters and split drainage paths can change the contributing area. Record which surfaces actually feed the tank instead of treating the building outline as a guaranteed collection area.
Convert rainfall depth into a gross volume
One US liquid gallon equals exactly 231 cubic inches. One inch spread over a square foot occupies 144 cubic inches, so the ratio is 144 ÷ 231 = 48/77 US gallon per square foot. The decimal 0.6233766… is approximate; the fraction preserves the defined conversion.
In the Gallons per Square Foot Calculator, choose volume from area and depth. Enter 600 square feet and 0.75 inch. The gross volume is 600 × 0.75 × 48/77 = 280.5195 US gallons, or about 1,061.882 liters. The tool also reports imperial gallons separately so the larger imperial gallon cannot be mistaken for the US unit.
State collection efficiency as an assumption
Some rain may be lost to roof wetting, gutter overflow, deliberate first-flush diversion, leaks or other parts of the collection system. Those losses depend on the roof, rainfall event and installation. A single fixed efficiency is not correct for every project.
For a clearly labelled scenario, an assumed 80% collection efficiency applied to the gross 280.5195 gallons gives 224.4156 gallons reaching the tank. The 80% value is an illustration you replace with suitable project evidence. It is not a built-in recommendation from the volume calculator.
| Stage | Example quantity | Meaning |
|---|---|---|
| Gross rainfall on contributing area | 280.52 US gal | Geometric rain volume before losses |
| After assumed 80% collection efficiency | 224.42 US gal | Illustrative inflow reaching the tank |
| Available tank space | 150 US gal | Hypothetical remaining storage |
| Maximum added storage in this scenario | 150 US gal | Limited by space; remaining inflow overflows |
Check the storage limit separately
A tank's nominal capacity is not its empty space at the start of a storm. Subtract existing water from the actual usable capacity. The simple event estimate for additional stored volume is the smaller of effective inflow and available storage, before accounting for any simultaneous use or discharge.
A quarter-inch event and a prolonged intense storm can also behave differently even if a later arithmetic total looks similar. A static volume calculation does not size gutters, overflow outlets or drainage infrastructure. Those questions require flow and installation information beyond rainfall depth.
Keep the units on the record
- Write whether gallons are US liquid or imperial.
- Record the rainfall depth unit; inches and feet differ by a factor of twelve.
- Identify the horizontal area that contributes to the chosen downpipe.
- Keep gross rain volume, loss assumptions and available storage as separate lines.
- Use a measured event or an explicitly described design scenario rather than an unexplained annual total.
For a liquid container problem where volume is already known, use the separate gallons, footprint and waterline guide. The geometric conversion does not establish the quality or suitability of collected water for a particular use.
Use the Construction calculator collection to follow the measurement workflow, or visit the GreenCalc home page for the complete tool directory.
By GreenCalc Editorial Team · Updated 2026-10-22
Frequently asked questions
Should roof slope increase the horizontal rainfall catchment area?
For the ordinary rainfall-volume estimate described here, use the horizontal projected area draining to the collection point.
Is gross roof rainfall the amount added to the tank?
No. Collection losses and the tank’s available storage can reduce the amount actually retained.