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One inch of rain falling on one square foot of surface equals about 0.623 gallons of water.
That means you can estimate rainwater with this simple rule:
1 inch of rain × 1 square foot = 0.623 gallons
For rainwater harvesting, this number lets you estimate how much water a roof can collect from a storm.
How Many Gallons Is 1 Inch of Rain per Square Foot?
One square foot covered by one inch of water holds about:
0.623 US gallons
You may also see this rounded to 0.62 gallons per square foot.
Here are a few examples:
| Catchment Area | 1 Inch of Rain |
|---|---|
| 1 sq. ft. | 0.623 gallons |
| 10 sq. ft. | 6.23 gallons |
| 100 sq. ft. | 62.3 gallons |
| 500 sq. ft. | 311.5 gallons |
| 1,000 sq. ft. | 623 gallons |
| 2,000 sq. ft. | 1,246 gallons |
These figures describe the amount of rain that falls on the area. A real rainwater collection system normally captures somewhat less.
The Rainwater Collection Formula
To estimate how many gallons fall on a roof or other catchment area, use:
Gallons = catchment area in square feet × rainfall in inches × 0.623
For example, suppose you have a 1,000-square-foot roof and receive 1 inch of rain:
1,000 × 1 × 0.623 = 623 gallons
About 623 gallons of rain fall on that roof.
If the same roof receives half an inch:
1,000 × 0.5 × 0.623 = 311.5 gallons
If it receives 2 inches:
1,000 × 2 × 0.623 = 1,246 gallons
Why One Inch of Rain Equals 0.623 Gallons per Square Foot
A square foot measures 12 inches by 12 inches.
If water is one inch deep, its volume is:
12 × 12 × 1 = 144 cubic inches
One US gallon contains 231 cubic inches.
Divide the two:
144 ÷ 231 = 0.623
So one inch of water spread over one square foot equals about 0.623 gallons.
How to Calculate Rainwater From a Roof
For a roof collection system, you need two main numbers:
- The horizontal catchment area of the roof in square feet
- The amount of rainfall in inches
Then multiply both by 0.623.
For example, a 1,500-square-foot catchment receiving 0.75 inches of rain would get:
1,500 × 0.75 × 0.623 = about 701 gallons
That is the theoretical rainfall volume.
It does not mean exactly 701 gallons will reach your storage tank.
Actual Collection Will Be Lower
The 0.623 figure tells you how much rain falls on a square foot. Real rainwater harvesting systems lose some water along the way.
Water may be lost because of:
- Roof wetting
- Splashing
- Gutter overflow
- Leaks
- Debris screens
- First-flush diversion
- Water left in pipes
- Tank overflow
A first flush is a system that diverts the first portion of roof runoff. It can help keep some roof debris and contamination out of storage.
Because losses vary from one system to another, it is better to treat the basic calculation as the maximum rainfall available rather than a promise of how much your tank will receive.
Does Roof Slope Change the Calculation?
Usually, you should calculate rainwater using the roof's horizontal footprint, not the total sloped surface area.
Rain falls onto the horizontal area covered by the roof.
For example, imagine a simple building that is 20 feet wide and 40 feet long:
20 × 40 = 800 square feet
One inch of rain over that footprint represents:
800 × 0.623 = about 498 gallons
A steep roof has more actual roofing material than a flat roof covering the same building footprint, but it does not automatically collect more rainfall simply because the surface is longer.
Factor in gallons produced by one inch of roof rainfall to understand a safe route for moving roof water into storage.
Roof shapes with large overhangs or several sections can make the calculation more complicated. In those cases, calculate the horizontal catchment area of each section that drains into your collection system.
How Much Rain Does It Take to Fill a Rain Barrel?
You can rearrange the formula to estimate how much rainfall could supply a certain amount of water.
For example, consider a 55-gallon rain barrel connected to 100 square feet of roof.
One inch of rain provides:
100 × 0.623 = 62.3 gallons
So, in theory, less than one inch of rain can provide enough runoff to fill the barrel.
In practice, some water will be lost before it reaches the barrel. The barrel may also already contain water, and excess water must have a safe overflow path once it becomes full.
This is why even a relatively small roof area can supply a rain barrel quickly during a strong storm.
Quick Rule for Estimating Rainwater
For quick estimates, remember:
Every 100 square feet of roof receives about 62 gallons of water from 1 inch of rain.
Or:
Every 1,000 square feet receives about 623 gallons per inch of rain.
You can then adjust for rainfall.
For a 1,000-square-foot catchment:
- 0.25 inch = about 156 gallons
- 0.5 inch = about 312 gallons
- 1 inch = about 623 gallons
- 2 inches = about 1,246 gallons
- 3 inches = about 1,869 gallons
These are theoretical rainfall amounts before system losses.
Storage Capacity Still Matters
A roof may produce much more runoff than your rain barrel, IBC tote, or cistern can hold.
For example, one inch of rain on a 2,000-square-foot catchment represents about:
2,000 × 0.623 = 1,246 gallons
A small storage system would fill well before all of that runoff could be stored.
When planning a system, consider both:
How much water the roof can supply and how much water the storage tank can safely hold.
The tank also needs a properly sized overflow that sends excess water away from foundations and other areas where it could cause problems.
Rainfall Volume Does Not Tell You Water Quality
The 0.623-gallon calculation only tells you the volume of rainfall. It says nothing about whether collected roof runoff is suitable for a particular use.
Roof runoff can pick up dirt, animal waste, roofing material residues, microorganisms, and other contaminants.
Water used for garden irrigation may need a different level of treatment than water connected to household plumbing. Drinking-water use requires much more careful system planning, suitable collection and treatment, current laboratory testing, maintenance, and compliance with applicable local requirements.
Do not assume that rainwater is safe to drink simply because it looks clear.
Frequently Asked Questions
How many gallons is 1 inch of rain on 1 square foot?
One inch of rain on one square foot equals about 0.623 US gallons.
How many gallons is 1 inch of rain on 100 square feet?
Multiply 100 by 0.623:
100 × 0.623 = 62.3 gallons
So 100 square feet receives about 62.3 gallons from one inch of rain.
How many gallons is 1 inch of rain on a 1,000-square-foot roof?
One inch of rain over 1,000 square feet equals about 623 gallons before collection losses.
What is the formula for converting roof area and rainfall to gallons?
Use:
Square feet × rainfall in inches × 0.623 = gallons
This gives the theoretical amount of rain falling on the catchment area.
Does all of that rain reach my storage tank?
No. Some water is normally lost through roof wetting, first-flush diversion, screens, gutters, pipes, leaks, splash, and overflow. Actual collection depends on the design and condition of the system.
Should I use the sloped roof area when calculating rainwater?
Usually, use the roof's horizontal catchment area rather than the sloped surface measurement. If several roof sections drain to different gutters or tanks, calculate the contributing area for each section separately.
How many gallons does half an inch of rain produce per square foot?
Half an inch produces about:
0.5 × 0.623 = 0.312 gallons per square foot
So a 1,000-square-foot catchment receives about 312 gallons from half an inch of rain.
