How to use this calculator
- Enter Roof footprint in sq ft.
- Enter Rainfall in in.
- Enter Efficiency in %.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
Yield = roof area × rainfall inches × 0.623 gal, × 0.85 collection efficiency. A 1,000 sq ft roof yields about 530 usable gallons per inch of rain. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Where this fits in the project
Efficiency upgrades interact: better insulation and correct R-values shrink the heating and cooling load, which shrinks the solar array needed to offset it. Size loads first, generation second.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the rainwater harvesting calculator:
- Roof Square Footage Calculator — Rainfall is collected from the roof surface area.
- Gutter Calculator — Gutters direct rainwater to collection system.
- French Drain Calculator — Rainwater collection vs French drain dispersal.
Frequently asked questions
How much rain can one roof collect yearly?
Footprint × annual inches × 0.52 usable — a 1,500 sq ft roof in a 40 in climate nets ~31,000 gallons.
Is harvested rainwater potable?
Not without treatment — use it for irrigation as-is; drinking requires filtration and disinfection, and some states regulate use.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.