Tool

Water calculator

Gallons a week for your beds, minutes of hose time from a timed five-gallon pail, and a drip run time from the emitters you have.

Gallons and hose time

Fill a 5-gallon pail from your hose at the pressure you water with, and time it. That one number turns gallons into minutes (the pail method from University of Minnesota Extension).

Area (sq ft) Target (inches per week) Rain this week (in) Seconds to fill a 5-gal pail

Assumptions behind the numbers

Gallons = area (sq ft) x inches x 0.623. One inch of rain on an acre is 27,154 gallons (USGS); 0.623 per square foot is that figure divided by 43,560 (University of Arizona uses the same factor).
One inch a week is a starting convention for established plants in typical soil (University of Minnesota Extension). Heat, wind, sand and containers need more; the finger test two inches down overrules the number.
Hose minutes assume the flow you timed holds for the whole run. Recheck if the pressure changes.

Fictional example

The Okafor garden: 64 sq ft. One inch is 64 x 0.623 = 39.9 gal. With 0.3 in of rain this week, 0.7 in is still needed: 27.9 gal. Their hose fills the pail in 66 s, so 27.9 / 5 x 66 = 368 s, about 6 min 8 s, given slowly at the base of the plants. Before watering, they check the soil two inches down; if it is still moist, they wait.

Drip run time

For drip line or emitters in a bed: the application rate in inches per hour from emitter flow, emitter spacing and the width of soil the line waters (University of Arkansas FSA6174), then the minutes for your target.

Emitter flow (gal per hour) Emitter spacing (in) Width watered (in) Target (in per week)

Rate (in/hr) = gph x 231.1 / (emitter spacing in x width in). 231 cubic inches make one gallon (NIST Handbook 44).
Emitters are commonly 0.5, 1 or 2 gph and need a filter; most drip systems run at 20 to 30 psi with a pressure regulator (Clemson HGIC 1811). Use your product's figures.
The tablespoon test (UC Master Gardeners): a 1 gph emitter fills a tablespoon in about 14 seconds. Time a few emitters to see what yours deliver.
Split a long weekly total into several runs, especially on sandy soil, so water soaks in rather than draining past the roots (Module 8).

Limits

These are starting numbers. The finger test two inches down decides whether to water, and containers in hot weather can need water every day whatever the calculator says. Rain barrel water: check your state, city or HOA rules first (Module 8).