Inches of Water to Gallons Calculator

This calculator converts a volume of water measured in inches of depth over a rectangular area (like a pool, tank, or garden bed) into gallons. You enter the length and width of the area in feet, the water depth in inches, and the calculator returns the total volume in US gallons, along with cubic feet and cubic yards for reference.

Formula

The core calculation is simple: find the volume in cubic feet, then convert to gallons. The formula used is:

Gallons = (Length in feet × Width in feet × Depth in inches) / 12 × 7.48052

Where:

• Length (l) – the longer side of the rectangular area, in feet.
• Width (w) – the shorter side of the rectangular area, in feet.
• Depth (d) – the water depth, in inches.
• 7.48052 – the number of US gallons in one cubic foot.

If you prefer metric: 1 cubic meter = 1000 liters, and 1 inch = 0.0254 meters. So the same volume in liters would be l × w × d × 0.0254 × 1000 (where l and w are in meters).

Worked Example

Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 3 in.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft.
2. Calculate volume in cubic feet: 40 × 25 × 0.25 = 250 ft³.
3. Convert to gallons: 250 × 7.48052 = 1870.13 gallons.

The calculator rounds to 2 decimal places, so the displayed result is 1870.13 gallons.

How to Use the Calculator

This tool is handy for several real-world tasks:

• Filling a pool or pond – estimate how much water you need to add or how long a hose will take.
• Watering a garden – if you know the area and the inches of rainfall or irrigation, you can find the total gallons applied.
• Aquarium or tank volume – for a rectangular tank, enter the inside dimensions and water depth (not tank height).
• Flooding or runoff – estimate how much water falls on a roof or lawn during a storm.

To get the best result, measure the actual water depth, not the container height. For a pool, measure from the water surface to the bottom at the deepest point if the bottom is sloped – but remember this formula assumes a flat bottom, so for sloped pools it gives an overestimate. For a tank, use the internal dimensions, not external.

Common Mistakes and Misconceptions

The most frequent error is mixing units. For instance, entering the depth in feet but labeling it inches, or entering the length in yards. Always double-check that length and width are in feet and depth is in inches.

Another misconception is that “gallons� might mean Imperial gallons. This calculator uses US gallons (231 cubic inches). One Imperial gallon is about 1.2 US gallons. If you need Imperial, multiply the result by 0.83267.

Also, water density changes with temperature, but for volume calculations that doesn’t matter – a gallon is a fixed volume, regardless of temperature. Weight would change, but not volume.

Practical Examples

Let’s look at a few scenarios:

• Rainfall on a lawn: A 50 ft × 20 ft lawn receives 1 inch of rain. Volume = 50 × 20 × 1 / 12 × 7.48052 = 623.38 gallons. That’s a lot of water!
• Fish tank: A 48″ × 13″ × 20″ tank, but water is filled to 18″ deep. Convert inches to feet: 48″ = 4 ft, 13″ = 1.083 ft, 18″ = 1.5 ft. Volume = 4 × 1.083 × 1.5 × 7.48052 = 48.6 gallons. (Note: the calculator expects depth in inches, so you’d enter 18 for depth.)
• Pool top-up: A 20 ft round pool – but this calculator is rectangular. For a round pool, use the formula for a cylinder: Ï€ × radius² × depth. For a 20 ft diameter pool (radius 10 ft) with 6 inches of water added: Ï€ × 100 × 0.5 × 7.48052 ≈ 1175 gallons.

Limitations of the Formula

This formula assumes a perfectly rectangular container with a flat bottom and vertical sides. Real-world shapes like oval pools, sloped-bottom tanks, or irregular ponds will give inaccurate results. For those, you’d need a more complex integration or a different calculator.

Also, the calculator only works for water depth in inches. If you have a depth in feet, multiply by 12 first. If you have a depth in centimeters, divide by 2.54 to get inches.

For very large areas (like acres), you might prefer to convert to gallons per acre-inch: 1 acre-inch = 27,154 gallons. That’s a useful number for agricultural irrigation.

Frequently Asked Questions

Q: How many gallons is 1 inch of water over 1 square foot?
A: 1 square foot × 1 inch = 1/12 cubic foot. Multiply by 7.48052 to get 0.6234 gallons. So one inch of water over one square foot is about 0.62 gallons.

Q: Can I use this for a round pool?
A: No, this calculator is for rectangular shapes. For a round pool, use the cylinder formula: π × (radius in ft)² × (depth in ft) × 7.48052. For example, a 15 ft round pool with 2 ft depth: 3.1416 × 7.5² × 2 × 7.48052 ≈ 2646 gallons.

Q: What’s the difference between US and Imperial gallons?
A: US gallon = 231 cubic inches, Imperial gallon = 277.42 cubic inches. So an Imperial gallon is about 20% larger. To convert US gallons to Imperial, multiply by 0.83267.

Q: Does water temperature affect the gallon calculation?
A: No, a gallon is a unit of volume, not weight. The volume of water doesn’t change with temperature (except for negligible thermal expansion). If you need weight, remember that 1 US gallon of water weighs about 8.34 pounds at room temperature.

Q: How do I measure the depth if my tank has a gravel bed?
A: Measure the water depth above the gravel, not the total tank height. The water volume is what you need for fish stocking or medication dosing.

Q: Is there a way to convert inches of rain to gallons for a roof?
A: Yes, use the same formula: length × width of the roof footprint (in feet) × rainfall (in inches) / 12 × 7.48052. That gives you the runoff volume, assuming no losses.

Related

• How to Calculate A/R Days (with Formula)
• Liters to Gallons: How Many in a Gallon?
• Fl Oz to Gallons Calculator & Conversion Guide

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