Plant Watering Calculator
Most garden plants need 1 to 1.5 inches of water per week, but it is not always obvious how many gallons that equates to for your specific bed or lawn. This calculator converts inches-per-week watering needs into gallons per week and per day, making it easy to program a timer or plan manual watering sessions. It also lets you subtract expected rainfall so you only apply what is actually needed. The conversion factor (0.6233 gallons per sq ft per inch) is the standard US value used in USDA and university extension irrigation guides.
Plant watering formula
Irrigation (gal/week) = Area (sq ft) x (Water needed - Rainfall) x 0.6233
Irrigation (gal/day) = Irrigation (gal/week) / 7
The factor 0.6233 converts square feet times inches to US gallons (1 sq ft x 1 in = 144 cu in = 0.6233 gal). Negative results are clamped to zero, meaning rainfall alone meets the target.
Weekly water requirements by plant type
- Established lawn: 1.0 to 1.5 inches per week.
- Vegetable garden: 1.0 to 2.0 inches per week (more for fruiting crops in heat).
- Annual flowers: 1.0 inch per week.
- Established perennials: 0.5 to 1.0 inch per week once roots are established.
- Trees (first 2 years): 1 gallon per inch of trunk diameter, twice per week.
Plant watering calculator: frequently asked questions
How many gallons of water does a garden need per week?
Most vegetable gardens need 1 to 1.5 inches of water per week. To convert to gallons: multiply area in sq ft by inches per week, then multiply by 0.6233. For a 100 sq ft garden needing 1 inch per week: 100 x 1 x 0.6233 = 62.33 gallons per week.
Where does the 0.6233 conversion factor come from?
One inch of water over one square foot equals 0.6233 US gallons (144 sq in x 1 in = 144 cu in; 144 / 231 = 0.6234 gal, using 231 cu in per gallon). This is the standard US irrigation conversion used by USDA and university extension programs.
How do I account for rainfall?
Subtract weekly rainfall from the target weekly water requirement before irrigation. If your garden needs 1.5 inches and you received 0.5 inches of rain, apply 1.0 inch of supplemental irrigation.
What is evapotranspiration (ET)?
Evapotranspiration is the combined water loss from soil evaporation and plant transpiration. USDA NRCS publishes ET rates by region to help growers determine precise irrigation needs. For home gardens, 1 to 1.5 inches per week covers most conditions.
How long should I run a sprinkler to apply 1 inch of water?
Place rain gauges or tuna cans around your lawn while the sprinkler runs. Time how long it takes to collect 1 inch. Most residential sprinklers apply 0.5 to 1 inch per hour. Run the system until the gauge reads the target depth.
Official sources
- USDA NRCS, irrigation water management: nrcs.usda.gov.
- USDA Agricultural Research Service, water management: ars.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.