What Size Rainwater Tank Do I Need?

Two questions decide a rainwater system: how much you can catch, and how big a tank to store it in. The first is a tidy formula from your roof and rainfall; the second is about the dry spell you want to bridge — and it is where most tanks are sized wrong.

How much you can harvest

Gallons per year ≈ roof plan area (sq ft) × annual rainfall (inches) × 0.62 × collection efficiency. A 1,000 sq ft roof in a 24-inch rainfall area at 85% efficiency yields about 12,600 gallons (48,000 L) a year. Use the roof’s footprint (plan) area, not the sloped surface — rain falls vertically.

Collection efficiency

Around 80–90% for a clean metal or tiled roof with decent gutters. Losses come from evaporation, gutter overshoot in downpours, and the first-flush diverter discarding the dirty first run-off. Use the lower end for flat or rough roofs and undersized gutters.

What size tank?

Size for the dry period you want to bridge: daily demand × days of storage. Using 50 gallons (200 L) a day through a 30-day dry spell needs 1,500 gallons (6,000 L) — round up to the next standard tank (common sizes run 250, 500, 1,000, 2,500 and 5,000 gallons). A tank bigger than your roof can refill just stores air, so watch for demand outrunning harvest.

Is it safe to drink?

Not without treatment. Straight off a roof it suits gardens, toilets, laundry and washing down. For drinking it needs filtration and disinfection (UV or chlorination), and many jurisdictions regulate potable use — check local rules. Always screen the inlet and use a first-flush diverter to keep the tank clean.

Why the tank runs dry anyway

Rain is seasonal — an annual average hides a dry season that can run for months. If your use continues year-round, size the tank for the dry months (or accept topping up from mains), and remember the tank starts the dry season only as full as the last rains left it. Two smaller linked tanks also beat one big one for cleaning and redundancy.