Gallons per Square Foot Calculator

How to use the gallons per square foot calculator

What this calculator does. It converts the footprint and liquid depth of a rectangular tank, pool, planter reservoir, containment tray, or similar space into cubic feet and US liquid gallons. It also reports gallons per square foot, which is the volume held above each square foot of base at the chosen depth. In reverse mode, it divides a known gallon volume by the gallon capacity per square foot to estimate the required base area. It does not account for sloped floors, rounded corners, internal equipment, displacement, freeboard, or irregular walls.

When to use it. Use it to estimate pool or tank capacity from floor area and depth, size a temporary liquid containment zone, convert a known delivery volume into the floor area it will cover at a uniform depth, or check material and transport planning before detailed field measurements. The conversion rests on the US gallon definition maintained in the NIST guidance on volume units.

How to calculate. The calculator opens with a complete demonstration: 120 ft² of base area and 1.5 ft of depth. Its Excel workbook is ready immediately. Follow these steps:

1. Select Calculation mode. Choose “Area and depth → gallons” when you know the footprint, or “Gallons and depth → area” when you know the total liquid volume.
2. Replace the demonstration value in Base area or Known volume, depending on the selected mode.
3. Enter the vertical Liquid depth in feet. Results update as you type.
4. Read Total volume, Gallons per square foot, Base area, and Cubic volume, then review the calculation table.
5. Select Download Excel to save the current inputs and results as a real XLSX workbook. Reset clears the demonstration and disables export until a complete valid state is entered again.

Input guide. Calculation mode is required and determines which value the calculator solves for. Base area is required in area-to-gallons mode; enter a positive decimal in square feet, such as 120. Increasing it increases total gallons in direct proportion. Do not enter a length here unless the footprint is exactly one foot wide. Known volume is required in reverse mode; enter a positive US-gallon value, such as 500. Increasing it increases the required area in direct proportion. Liquid depth is always required; enter a positive depth in feet, such as 1.5. Greater depth raises gallons per square foot and total gallons, but in reverse mode it reduces the required area for a fixed volume. Commas, unit text, scientific notation, zero, negative values, and ambiguous decimal-comma entries are rejected.

Output guide. Total volume is the estimated US liquid gallons in the modeled rectangular prism. Gallons per square foot is an exact conversion identity for a uniform depth: depth in feet multiplied by 7.4805194805. Base area is either your input or the area solved from known gallons. Cubic volume is area multiplied by depth, measured in cubic feet. A zero result is not displayed because all physical dimensions must be positive. High values are not inherently wrong, but they should prompt a check that feet, square feet, and US gallons were not mixed with inches, meters, or imperial gallons.

Worked example. With the startup values, 120 ft² × 1.5 ft = 180 ft³. Multiplying 180 ft³ by 7.4805194805 US gal/ft³ gives 1,346.4935 US gallons, displayed as 1,346.49 US gal. The per-area value is 1.5 × 7.4805194805 = 11.2208 gal/ft². These same typed values appear in the initial XLSX workbook.

Learn more. The NIST overview of SI units is useful when measurements must first be converted from metric dimensions, while the USGS water measurement conversion guide provides practical context for common water-volume units.