Gallons Per Inch of Rain: How to Calculate

This calculator works out the volume of rainwater (in gallons) that falls on a rectangular area—like a lawn, garden, or roof—for a given rainfall depth. You enter the length and width of the area in feet, plus the rainfall depth in inches, and the calculator returns the total volume in gallons and cubic feet. The first result, gallons, is the headline answer you’re looking for.

Formula

The volume of rainwater is simply the area of the surface times the depth of rainfall. In symbols:

Volume (gallons) = Length (ft) × Width (ft) × Depth (in) × 0.623

Where:

• Length (l) – the length of the area in feet (ft)
• Width (w) – the width of the area in feet (ft)
• Depth (d) – the rainfall depth in inches (in)
• 0.623 – the conversion factor: 1 inch of rain on 1 square foot equals 0.623 gallons (this comes from 1 cubic foot = 7.48052 gallons, divided by 12 inches per foot)

The calculator also gives the volume in cubic feet (ft³) and cubic yards (yd³) for convenience, using the relationships 1 ft³ = 7.48052 gallons and 1 yd³ = 27 ft³.

Worked Example

Let’s use the default values from the calculator: a lawn that is 40 feet long, 25 feet wide, and receives 3 inches of rain.

1. Calculate the area: 40 ft × 25 ft = 1,000 ft²
2. Multiply by the depth in inches: 1,000 ft² × 3 in = 3,000 ft²·in
3. Convert to gallons: 3,000 × 0.623 = 1,869 gallons
4. For cubic feet: 1,000 ft² × (3 in ÷ 12 in/ft) = 250 ft³
5. For cubic yards: 250 ft³ ÷ 27 = 9.26 yd³

The calculator’s first output shows 1,869 gallons (rounded to the nearest whole number), matching our step-by-step calculation.

Why 0.623 Gallons per Square Foot?

This constant is derived from unit conversions, not from rainfall intensity. One inch of rain falling on one square foot of surface creates a volume of 1/12 of a cubic foot (since 1 inch = 1/12 foot). A cubic foot of water holds 7.48052 US gallons. Therefore:

1/12 ft³ × 7.48052 gal/ft³ = 0.62338 gallons

So for any area, the formula is: Area (ft²) × Rainfall (in) × 0.623 = Gallons. This works for any rectangular or even irregular shape, as long as you know the total area in square feet.

How to Use This Calculator

This tool is useful for several practical purposes:

• Rainwater harvesting – Estimate how much water you could collect from a roof during a storm. For example, a 1,000 ft² roof with 1 inch of rain yields about 623 gallons.
• Garden irrigation – Know how much water your lawn or garden receives naturally, so you don’t over-water.
• Flood planning – Understand the volume of water that might accumulate on a driveway or patio during heavy rain.
• Educational projects – Demonstrate the relationship between rainfall depth and volume.

To get the most accurate estimate, measure the length and width of the area in feet (use a tape measure or a wheel). For irregular shapes, break the area into rectangles and add the volumes together.

Common Mistakes and Limitations

Here are a few pitfalls to avoid:

• Mixing units – Always convert all measurements to feet and inches before using the formula. If you measure in meters, convert to feet first (1 m = 3.28084 ft).
• Using the wrong gallon – This calculator uses US liquid gallons. The imperial gallon (used in the UK) is larger (1 imperial gallon = 1.20095 US gallons). If you need imperial gallons, multiply the result by 0.83267.
• Ignoring runoff and absorption – The formula gives the total volume of rain that falls on the surface. In reality, some water soaks into the ground, evaporates, or runs off. For a roof, most water goes to gutters, but for a lawn, much is absorbed.
• Assuming uniform depth – Rainfall is rarely perfectly uniform, especially over large areas. The formula assumes the same depth everywhere.

Rainfall Intensity and Duration

This calculator treats rainfall as a total depth over an unspecified time. However, you might also want to know the rate of rainfall. For example, a storm that drops 2 inches in 1 hour is much more intense than one that drops 2 inches over 2 days. The intensity affects how much water runs off versus soaks in. The US National Weather Service classifies rainfall rates as:

• Light rain: less than 0.1 inches per hour
• Moderate rain: 0.1 to 0.3 inches per hour
• Heavy rain: more than 0.3 inches per hour

For drainage design, engineers use “design storms� with specific durations (e.g., 1-hour, 24-hour) and return periods (e.g., 10-year, 100-year). This calculator does not account for time, but you can use it to find the total volume for any single storm event.

Frequently Asked Questions

How many gallons of water are in 1 inch of rain per square foot?

Exactly 0.623 gallons (about 2.36 liters). This is the standard conversion used in rainwater harvesting and irrigation calculations.

How many gallons is 1 inch of rain on a 1,000 square foot roof?

Multiply 1,000 by 0.623 to get 623 gallons. That’s enough to fill about 12 standard 55-gallon drums.

Does the type of surface affect the volume?

No, the total volume of rain falling is the same regardless of surface. However, the usable volume differs: a roof directs water to gutters, while a lawn absorbs much of it. The calculator gives the gross rainfall volume, not the collectable amount.

How do I convert the result to liters?

Multiply the gallons by 3.78541. For example, 1,869 gallons × 3.78541 = 7,075 liters.

What if my area is not rectangular?

Break the area into rectangles, calculate the volume for each, and add them together. Or, if you know the total area in square feet (from a survey or mapping tool), you can enter that area as length × width (e.g., for a 500 ft² area, use length=25, width=20).

Is this formula accurate for very heavy rain?

Yes, the volume calculation is independent of rainfall intensity. The only inaccuracy comes from measurement errors in area or depth, not from the rain rate.

Related

• Growing Degree Days Calculator (GDD)
• Snow to Rain Calculator: Convert Snowfall to Rain
• Average Annual Rainfall: Calculate It Step by Step

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1