This garden watering calculator converts watering depth over your garden’s area into gallons needed — and into sprinkler run-time from your system’s output rate.
The constant: 1 inch over 1 sq ft = 0.623 gallons. Measure your sprinkler’s true output with the tuna-can test (empty cans placed around the zone, run 15 minutes, measure depth × 4 = inches/hour) rather than trusting nozzle charts — pressure and spacing rewrite them.
Gallons = Area × Depth × 0.623 — Irrigation’s One Constant
An inch of water is a volume wearing a depth costume; the constant converts it. Capturing that water from the roof instead is our Rainwater Harvesting Calculator. Part of our Ecology Calculators collection.
Worked Example (Verified)
- 100 sq ft × 1 inch (defaults): 62.3 gallons per session; at 1 in/hr output, 60 minutes of run time; twice weekly = 124.6 gal/week
- Lawn scale shock: 5,000 sq ft at the standard 1 inch/week = 3,115 gallons/week — why lawns dominate suburban summer water bills
- Deep-and-infrequent logic: one or two deep sessions per week beat seven daily sprinkles of ~0.14 inch — depth drives roots down; daily sips train them shallow and thirsty
- Drip comparison: drip lines deliver ~90%+ to the root zone vs sprinklers’ 60–75% (wind/evaporation) — same gallons from this page, fewer needed in practice
Watering Wisdom in Brief
- Water early morning: less evaporation, foliage dries with the day (fungus dislikes the schedule).
- Cycle-and-soak on slopes/clay: split an hour into 3 × 20 min with rests — runoff is water billed but not banked.
- Mulch is irrigation you apply once: 2–4 inches cuts surface evaporation dramatically.
- Rain gauges and smart controllers convert this page’s open-loop math into closed-loop practice.
Related Tools
- Rainwater Harvesting Calculator — roof catchment supply
- Garden Soil Calculator — soil volumes for beds
- Ecology Calculators — the full collection
Frequently Asked Questions
How many gallons is 1 inch of water on my garden?
Area (sq ft) × 0.623 per inch: 100 sq ft needs 62.3 gallons for a full inch; 1,000 sq ft needs 623. This calculator multiplies by your target depth and converts to sprinkler minutes from your measured output rate.
How long should I run my sprinklers?
Depth wanted ÷ output rate: 1 inch at 1 in/hr = 60 minutes. Measure the rate with tuna cans — systems vary 0.25–2 in/hr by nozzle and pressure, and guessing is how lawns drown at the surface while roots stay thirsty.
How often should a lawn get its inch?
The standard guidance — ~1 inch per week total including rain, in one or two deep sessions — trains deep roots. Sandy soils take it in two half-inch sessions; clay may need cycle-and-soak splitting to absorb without runoff. Your grass species and climate move the number ±50%.
Is drip irrigation really more efficient?
Yes: 90%+ delivery to the root zone versus 60–75% for sprinklers (wind drift and evaporation tax the air route). The gallons computed here are what the plants should receive; sprinkler users effectively need to apply the total ÷ efficiency.
How do I measure sprinkler output?
The tuna-can test: place several identical straight-sided cans across the zone, run exactly 15 minutes, measure the depth in each, average it, × 4 = inches per hour. Uneven can depths expose coverage problems (head spacing, pressure) that no schedule can fix.
Is this garden watering calculator free?
Yes — free, instant, no sign-up.