The Formula
It comes down to one equation: Gallons collected = Rainfall (inches) × Roof Area (sq ft) × 0.623. The 0.623 factor is a unit conversion (1 inch × 1 sq ft = 0.623 gallons) and is the standard figure used in Bermuda and international rainwater harvesting. Evaporation and roof condition reduce it slightly in practice, but it gives a reliable ballpark.
Worked Examples
Typical Bermuda household: 1,500 sq ft roof, 15,000 gallon tank, currently 40% full (6,000 gallons). Forecast: 1 inch of rain.
Collection: 1 × 1,500 × 0.623 = 935 gallons. New level: 6,935 gallons = 46%. A 6-point gain — meaningful, but not a full recovery. With 2 inches: 1,869 gallons, pushing you to 52%. A proper tank rain event makes a real dent. A light shower does not.
Tank size changes everything: 935 gallons into a 5,000 gallon tank = an 18.7% boost. Into a 20,000 gallon tank = only 4.7%. Two neighbours experience the same storm very differently.
The Tank Rain Scale
The Tank Rain Scale translates raw totals into practical labels calibrated to typical Bermuda tank sizes: No Rain — nothing collected. Light Shower — negligible impact. Minor Tank Boost — under 0.25 inches. Good Tank Rain — 0.25–0.5 inches, noticeable refill. Proper Tank Rain — 0.5–1 inch, what households need regularly through the dry season. Big Tank Refill — over 1 inch, moves the needle even on large tanks.
What Affects Actual Collection
Blocked gutters cut your effective collection area — clearing them before the rainy season is the highest-return maintenance task for your water supply. Roof condition matters too: cracks, moss, or debris can push effective collection from 0.623 down to 0.50–0.55.
Using the Calculator
The tank calculator on the rainfall page takes three inputs: roof size, tank capacity, and current tank level. It returns estimated collection today, tomorrow, and over the 7-day forecast, plus your projected tank level after each event. The goal: turn an abstract rain forecast into a concrete answer about your water supply.
More Guides
See the 7-day rainfall forecast and run your tank calculation now.
Open the Calculator