The Formula
It comes down to one equation: Imperial gallons collected = Rainfall (inches) × Roof Area (sq ft) × 0.5187. The 0.5187 factor converts the volume of water falling on one square foot to Imperial gallons — the unit used throughout Bermuda for tanks, truck deliveries, and water pricing. Evaporation and roof condition reduce collection slightly in practice, but the formula gives a reliable ballpark.
All gallon figures on this page — and across the Tankrain BDA site — are Imperial gallons. 1 Imperial gallon = 1.20 US gallons. If a Bermuda tank is described as 12,000 gallons, that's 14,400 US gallons; the difference matters when comparing equipment specifications or US-sourced rainwater-harvesting guides.
Worked Examples
Typical Bermuda household: 1,500 sq ft roof, 15,000 Imperial gallon tank (above the 12,000 Ig code minimum set by the Bermuda Building Code 2014), currently 40% full (6,000 gallons). Forecast: 1 inch of rain.
Collection: 1 × 1,500 × 0.5187 ≈ 778 Imperial gallons. New level: 6,778 gallons = 45%. A 5-point gain — meaningful, but not a full recovery. With 2 inches: 1,556 Ig, pushing you to 51%. A proper tank rain event makes a real dent. A light shower does not.
Tank size changes everything: 778 Ig into a 5,000 gallon tank = a 15.6% boost. Into a 20,000 gallon tank = only 3.9%. 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 drop your effective collection efficiency by 15–20%, costing you a fifth of every rainfall event.
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.
Converting Millimetres to Inches
The tank refill formula uses inches, because roof area in Bermuda is measured in square feet. But weather forecasts in Bermuda and internationally often express rainfall in millimetres. To convert: divide millimetres by 25.4 to get inches. So if the forecast says 20mm, that's 0.79 inches. On a 1,500 sq ft roof: 0.79 × 1,500 × 0.5187 = 614 Imperial gallons.
If you prefer to work entirely in millimetres, the equivalent formula is: litres = rainfall (mm) × roof area (m²). For a 139 m² roof (1,500 sq ft) and 20mm of rain: 20 × 139 = 2,780 litres = about 734 gallons. The slight difference is rounding. Both approaches give the same practical answer. The calculator on the Rainfall page handles the conversion automatically when you switch between units.
Planning Through the Dry Season
The most useful application of the tank calculator isn't single-storm planning — it's seasonal planning. If you're entering December with a tank at 60%, and January through April average only 65mm per month, here's how to think about it:
Starting inventory: 60% of a 15,000 gallon tank = 9,000 gallons. Daily consumption for a family of four: ~300 gallons. Days of supply: 30 days. Monthly rainfall (average): 65mm = 2.56 inches × 1,500 sq ft × 0.5187 = 1,991 Imperial gallons collected. Monthly consumption: 9,000 gallons. Net monthly drawdown: approximately 7,000 gallons. At that rate, you would burn through the tank in about six weeks of average conditions.
The calculation isn't meant to be precise — actual rainfall varies enormously from the average — but it gives you a planning framework. If the numbers show you're likely to hit 10–15% by March without a delivery, ordering in January is far better than scrambling in March when everyone else is also running low. The Rainfall page shows the 7-day forecast updated daily, which you can use to track whether the season is tracking above or below average and adjust accordingly.
Why Knowing Your Roof Size Matters
Most Bermudians don't know their exact roof size off the top of their head, but it's worth finding out. It's on your property plans, often in the property tax assessment, and a reasonable estimate can be derived from your footprint: a 1,200 sq ft footprint house with a standard roof overhang has roughly 1,350–1,450 sq ft of catchment area. Once you have the number, every rain forecast for the rest of the time you own the house gives you a directly actionable collection estimate.
Two houses on the same street getting the same rainfall have completely different collection outcomes if one has a 900 sq ft roof and the other has a 2,200 sq ft roof. The family in the smaller house needs to order water deliveries far more frequently, particularly during dry stretches. Knowing your roof area is the starting point for understanding your true water security position — and the calculator is only as useful as the inputs you give it.
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