Rainwater Harvesting Calculator

A roof collects far more water than most people expect: an inch of rain on 1,000 square feet yields about 623 gallons. Enter your roof size, roofing material and local rainfall, and this calculator shows how much you can collect each month and year, then tests tank sizes against how much water you plan to use, month by month, so you can choose a tank that is big enough without overspending.

The rainwater formula

The 0.623 comes from the volume of one inch of water over one square foot: 144 cubic inches, which is 0.623 US gallons. In metric it is even simpler — 1 mm of rain on 1 m² is exactly 1 litre. Use the roof’s footprint (the area it covers on the ground), not the sloped surface area.

Runoff coefficients

Not all rain that lands on a roof reaches the tank: some splashes off, soaks in, evaporates, or goes to the first-flush diverter. The coefficient accounts for these losses.

Worked example

A 1,500 sq ft asphalt-shingle roof (coefficient 0.85) in a place with 42.4 inches of rain a year collects 1,500 × 42.4 × 0.623 × 0.85 ≈ 33,700 gallons a year — about 794 gallons for every inch of rain. With a monthly use of 1,200 gallons for the garden and toilets (14,400 a year), even a 1,500-gallon tank meets nearly all of that demand in this steady-rainfall example; in climates with dry summers, a much larger tank is needed to carry water from the wet months.

Sizing a tank

1. List monthly rainfall for your area (national weather services publish averages).
2. Estimate monthly use: garden irrigation, toilet flushing, laundry, livestock.
3. Test a tank size: the table shows each month’s collection, use, overflow and what’s left.
4. Increase the tank until the dry months are covered or the extra cost isn’t worth it.
5. Consider a dry-year scenario — enter lower rainfall figures to test resilience.

Uses for harvested rainwater

• Garden and lawn watering — the most common use, needing little treatment.
• Toilet flushing and laundry, with filtration and a separate pipe system.
• Washing cars and equipment.
• Livestock and animals, with appropriate filtering.
• Drinking water only with proper treatment and in line with local health regulations.

System components

Keeping water clean

• Keep gutters and screens clear so leaves don’t rot in the tank.
• Use a first-flush diverter; a common rule of thumb discards roughly the first 10 gallons per 1,000 sq ft of roof (or around 0.5–1 mm of rain), though designs vary.
• Choose an opaque tank and keep the lid sealed to block light and mosquitoes.
• Draw water from just above the bottom, above the sediment layer.
• Clean the tank every few years or when sediment builds up.

Garden watering needs

A common rule of thumb for lawns and vegetable beds in the growing season is about an inch of water a week, which is roughly 0.62 gallons per square foot (25 litres per m²). A 200 sq ft vegetable garden therefore uses around 125 gallons a week in dry spells, or about 500 gallons a month — useful for setting the “water use per month” figure.

Tank types

Rules and permits

Rainwater harvesting is encouraged in many places and restricted in a few, and indoor use usually has plumbing rules to prevent cross-connection with drinking water. Check local regulations, building codes and any rebates before installing a large system.

Planning pack

The optional pack includes an Excel rainwater balance workbook with your local rainfall, a tank sizing comparison sheet, a system maintenance checklist and a water use log. The calculator above is free.

AI-assisted content

This page was drafted with AI assistance and reviewed by Kedop. Results are estimates; rainfall varies and local rules apply.