What Is the Gallons per Square Foot Calculator?
This tool tells you how many US gallons of water it takes to cover a flat area to a specific depth. It's handy for filling a pool, flooding a garden bed, estimating irrigation, sizing a pond liner, or figuring out water on a flat roof. You provide the surface area in square feet and the water depth in inches, and the calculator returns the total volume in US gallons.
How to Use It
Enter the Area of the surface in square feet and the Depth of water you want in inches. The result updates to show the total number of US gallons required to reach that depth across the whole area.
The Formula Explained
The calculation is:
$$\text{Gallons} = \text{Area (ft}^2\text{)} \times \text{Depth (in)} \times 0.623$$
The constant 0.623 comes from unit conversion. One square foot one inch deep equals 144 cubic inches, and one US gallon equals 231 cubic inches, so \(144 \div 231 \approx 0.6234\) gallons per square foot per inch of depth.
Worked Example
Suppose you have a 200 ft² area and want 3 inches of water. Multiply: $$200 \times 3 \times 0.623 = 373.8 \text{ US gallons}.$$ So you'd need roughly 374 gallons to cover that surface to a 3-inch depth.
FAQ
Are these US or Imperial gallons? The 0.623 factor gives US gallons. For Imperial gallons, use a factor of about 0.519 instead.
Does it work for any shape? Yes — as long as you know the total surface area in square feet and the depth is uniform.
Can I use this for a pool? Yes for flat-bottomed areas. For sloped or irregular pools, use the average depth for a closer estimate.
