Gallons Per Square Foot Calculator — Coverage Rate

Formula

How this is calculated

Gallons per square foot is a coverage rate — it answers "how deep would this liquid be if it formed a uniform layer?" One inch of rainfall on one square foot equals (1/12) cubic foot of water, and since one US gallon equals approximately 0.1337 cubic feet, that works out to about 0.6234 US gallons per square foot per inch of rain. Dividing the coverage rate by 0.6234 gives the equivalent rainfall depth in inches, which is useful for irrigation scheduling.

For lawn and garden irrigation, typical application rates are 0.5–1 inch of water per week (roughly 0.31–0.62 gal/ft²). Agricultural drip systems may apply far less — sometimes under 0.05 gal/ft² per day — while flood irrigation can temporarily apply several gallons per square foot. Knowing the coverage rate lets you compare your irrigation against reference evapotranspiration (ET) data from local agricultural extension services.

To convert the result to litres per square metre (the metric equivalent), the calculator multiplies by 3.7854 (litres per US gallon) and by 10.7639 (square feet per square metre). One litre per square metre equals one millimetre of depth, so the L/m² figure doubles as rainfall depth in millimetres.