Gallons per Square Foot Calculator
Use this gallon per square foot calculator to determine how many gallons of liquid fit one square foot over a given depth.
Gallons per square foot0.62338at 1 in deep
Litres per square metre25.4
Per inch of depth0.623377 gal/sq ft144 cubic inches ÷ 231 per gallon
Total over 100 sq ft62.34 US gallons236 litres
Weight of that water520.2 lb0.26 US tons
Volume8.333 cu ft0.3086 cu yd
Load on the structure5.2 psfwater is about 5.2 lb per sq ft per inch of depth
One inch over an acre27,154 gallonsthe classic acre-inch
The method
gallons per square foot = depth in inches × 0.6234
0.6234 gallons per square foot per inch
A US gallon is 231 cubic inches and a square foot is 144, so an inch of water over a square foot is 144 ÷ 231 = 0.6234 gallons. Every irrigation, pond and flat-roof drainage calculation comes back to that number.
Water also weighs about 5.2 pounds per square foot per inch of depth, which is why ponded water on a flat roof is dangerous: four inches is over 20 psf, and a blocked drain during heavy rain can exceed what the roof was designed to carry.