Rainfall Calculator

A rainfall calculator turns roof size and rain depth into gallons of water. The rule is roof area in square feet × rainfall in inches × 0.623. A 1,500 sq ft roof catching 1 inch of rain sheds 1,500 × 1 × 0.623 = 934.5 gallons, which the calculator rounds to 935.

Where 0.623 comes from

Picture one square foot of roof with 1 inch of water standing on it. That is a slab 12 inches by 12 inches by 1 inch, or 144 cubic inches. A gallon holds 231 cubic inches.

So each square foot gives 144 ÷ 231 = 0.623 gallons for every inch of rain. The calculator multiplies that by your area and your rainfall.

Worked examples

Example 1, the default: 1,500 sq ft of roof, 1 inch of rain.

• 1,500 × 1 × 0.623 = 934.5 gallons
• In 55 gallon barrels: 934.5 ÷ 55 = 17.0 barrels

Example 2, a small outbuilding with a bigger storm: 800 sq ft, 1.5 inches.

• 800 × 1.5 × 0.623 = 747.6 gallons
• If only 80% reaches storage: 747.6 × 0.80 = 598 gallons

Example 3, the same house across a wet month of 4 inches: 1,500 × 4 × 0.623 = 3,738 gallons.

Gallons by roof size at 1 inch of rain

Roof footprint	Gallons
1,000 sq ft	623
1,200 sq ft	748
1,500 sq ft	935
2,000 sq ft	1,246
2,400 sq ft	1,495

Gallons scale with rainfall, so multiply by the inches. The 1,500 sq ft roof gives 467 gallons at half an inch, 935 at one inch, and 1,869 at two.

How to measure

Measure the footprint of the roof from above. For a simple house, that is the outside length and width of the roof edges, overhangs included. For a roof with several sections draining to different gutters, measure each section and the gutter that serves it.

Find rainfall from local weather records for a storm, a month, or a year. Use the number that matches your question. A storm total sizes the tank. An annual total estimates yearly supply.

Common mistakes

• Using the sloped surface area. A 6/12 roof has about 12% more surface than footprint, and rain does not care.
• Treating the result as what you will collect. Subtract for gutter spill, first flush, and leaks.
• Sizing a tank for the biggest storm of the year when you will use the water quickly. Match the tank to your use.
• Ignoring local rules. Some places limit or regulate rainwater collection, so check the rules where you live.
• Drinking harvested water without treatment. Roof runoff carries dirt and debris, and it is not drinking water on its own.

Matching storage to use

Start with what you need. A 1,000 sq ft vegetable garden that wants 1 inch of water a week uses 1,000 × 1 × 0.623 = 623 gallons a week.

The 1,500 sq ft roof yields 935 gallons from 1 inch of rain, which is 1.5 weeks of that garden’s water. At 80% collection it is 748 gallons, or 1.2 weeks. So a storage volume near 750 gallons, about 14 barrels of 55 gallons, holds one decent storm.

For a yearly view, a place with 40 inches of rain a year gives the 1,500 sq ft roof 1,500 × 40 × 0.623 = 37,380 gallons in theory, and about 29,904 at 80%. Storage is what turns the wet months into water for the dry ones.

What this calculator does not do

It takes one rainfall number and gives one volume. It does not track the tank as it fills and empties, apply evaporation, or add up months. For that, repeat the calculation for each period using local rainfall records, and compare with your water use over the same period. A simple table of monthly rain and monthly demand shows how big a tank you need to bridge the dry months.

Related calculators

Once you know the gallons, the tank volume calculator checks whether your barrel or cistern is big enough. The gutter size calculator helps you check that gutters can carry the flow, and the roof area calculator finds the sloped surface if you need it for other jobs.