Garden watering calculator

From inches to gallons

Watering advice is given as a depth — "an inch a week" — because it scales with area. Converting it to volume uses the same 0.623 gallons per square foot per inch as rainwater math: the USGS Water Science School puts 1 inch of rain on an acre at about 27,154 gallons, and an acre is 43,560 sq ft.

gallons per week = area (sq ft) × inches × 0.623
litres per week = area (m²) × mm
hose: minutes = gallons per watering ÷ flow (gal per min)
sprinkler system: minutes = inches per watering ÷ precipitation rate (in/hr) × 60

A typical family plot is worked through in how much water a 10×20 vegetable garden needs.

Sprinkler systems: run time from the precipitation rate

An in-ground system is easier to think about as a depth per hour than as gallons per minute. That depth per hour is the precipitation rate, and it depends mostly on the head type. Colorado State University Extension gives pop-up spray heads 1 to 2½ in per hour (typically about 1½) and rotors about ¼ to ¾ in per hour; Hunter rates its standard MP Rotator nozzles at about 0.4 in per hour. So half an inch takes about 20 minutes under spray heads but more than an hour under rotors, which is why CSU says not to mix them on one zone.

Catch-cup test. Set six or more straight-sided cans (tuna or cat-food tins work) around one zone, run it for 15 minutes, and measure the water in each. Average them: 0.25 in in 15 minutes is 1 in per hour. Pick "I did a catch-cup test" above and the calculator does the rest. If some cans hold twice what others do, fix the heads before blaming the grass.

How much water do plants need?

Around 1 in per week is a common starting point for lawns and vegetable gardens in active growth; NC State Extension’s Gardener Handbook says 1 inch a week is usually adequate for established lawns, applied so it wets the soil 6 to 8 inches down. Sandy soils drain fast and may need water more often; clay holds it longer. Hot, windy weather can push needs well above an inch. Established shrubs and trees usually need deep, infrequent watering rather than a weekly inch.

New seed and new sod are different

The weekly inch is for established plants. New grass seed needs the top half-inch of soil kept damp, which means short, light waterings once or twice a day until it is up; for a small lawn that is sketched out in how much grass seed for 100 m². New sod needs a deep soak straight after laying and frequent watering until its roots knit, as covered in how much sod for 1,000 sq ft. Use this calculator for the routine once they are established.

Watering tips

• Water deeply and less often. Two soakings a week encourage deeper roots than daily sprinkles.
• Water early in the morning to reduce evaporation and leaf-disease risk.
• Measure your hose flow: time how long it takes to fill a 5-gallon bucket. 5 gallons in 75 seconds is 4 gal/min.
• Soaker hoses and drip lines put water at the roots; they typically deliver far less per minute than a sprinkler, so run times are longer.
• A rain gauge tells you how much of the weekly inch nature already covered.