Watering advice is usually given in inches of water per week, and hoses deliver gallons. This calculator converts between them and subtracts the rain you have already had, so what comes out is the number of gallons or liters still to give.
How to use it
- Enter the garden area. Just the planted beds, not the paths between them.
- Set the weekly target. One inch, or 25 millimetres, suits most established vegetables. Sandy soil, hot weather and windy sites push it up; heavy soil and cool weather pull it down.
- Enter the rain so far this week. A straight-sided jar left in the open is a perfectly serviceable gauge.
If the rain has already met the target the calculator says so rather than producing a number — but check the soil a few inches down before you believe it, because a heavy shower on dry ground can run off rather than soak in.
How it is calculated
In imperial:
gallons = area (sq ft) x net inches x 0.6234
An inch of water over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches by definition. 144 / 231 = 0.6234, which is where the familiar 0.623 comes from.
In metric it is simpler, because the units were designed to make this easy:
liters = area (m2) x net millimetres
One millimetre over one square metre is exactly one liter.
The "net" figure is the target minus the rainfall, and never goes below zero — rain in excess of the target does not create a negative requirement.
A worked example
A 100 square foot bed, one inch a week target, 0.4 inches of rain so far:
- Net requirement: 1 − 0.4 = 0.6 inches
- Water needed: 100 x 0.6 x 0.6234 = 37.4 gallons
- Split over two soakings: 18.7 gallons each
The same bed with no rain at all needs 62.3 gallons for the week.
Gallons per week, at one inch
| Garden area | Full inch | Half inch | Per soaking, twice weekly |
|---|---|---|---|
| 32 sq ft (4 x 8 bed) | 20 gal | 10 gal | 10 gal |
| 50 sq ft | 31 gal | 16 gal | 16 gal |
| 100 sq ft | 62 gal | 31 gal | 31 gal |
| 200 sq ft | 125 gal | 62 gal | 62 gal |
| 400 sq ft | 249 gal | 125 gal | 125 gal |
| 1,000 sq ft | 623 gal | 312 gal | 312 gal |
Tips and common mistakes
Find out what your hose or can actually delivers. Time how long a watering can takes to fill, then how many cans go on a bed. Most people substantially overestimate how much water a few minutes with a hose puts down.
Two long drinks beat seven short ones. Deep watering pulls roots downward, where the soil stays damp longer. Light daily sprinkling keeps them near the surface, where the first hot day finds them.
Check the soil, not the surface. Push a finger in two or three inches. Dry dust on top over damp soil beneath is normal and needs no water; damp crust over dry soil is a sign of light watering.
Mulch before you water more. Two or three inches of mulch cuts evaporation from the soil surface noticeably, and does it every day without your involvement.
Water the soil, not the leaves. A watering can with the rose removed, a soaker hose or drip line all put water where roots are and keep foliage dry, which matters for tomatoes and squash in particular.
Containers are a different problem. They hold so little that a large pot in high summer can need water daily whatever a weekly figure says. Use the container calculator to see how little soil a pot actually holds.