Free planning tool

Rainwater collection and tank size calculator

Compare what your roof could collect in a year with the storage volume needed for your intended use. The two figures answer different planning questions.

Rainwater planning estimate

Collection potential and storage target

Estimate annual roof collection separately from the storage needed to cover your chosen daily demand and reserve period.

Roof catchment area (sq ft) State or territory Annual rainfall (inches) Statewide historical averages are starting estimates. Enter a verified local value when available. Runoff efficiency Intended use Planned daily demand (gallons) Desired reserve (days)

Collection formula: roof area × annual rainfall × 0.6233 × runoff efficiency. Storage target: daily demand × reserve days. This screening estimate does not model monthly droughts, tank overflow, local roof geometry, water quality, permits, or structural requirements. Rainfall source: NOAA National Centers for Environmental Information, U.S. Monthly Climate Normals, 1971-2000; verified 2026-09-14.