Last Updated on September 17, 2026

How Many Gallons Is 1 Inch of Water? (Garden Beds and Lawns)

1 inch of water over 1 square foot is 0.623 gallons. That comes to about 10 gallons (9.97) for a 4×4 raised bed, 19.9 gallons for a 4×8 bed, 62.3 gallons for 100 square feet and 27,154 gallons for an acre. For any other size, multiply your square feet by 0.623, or use the calculator below, which also turns those gallons into minutes on your hose, drip line or sprinkler.

Inch of water calculator

Pick a bed size or type your own, set the inches you want to put down, and tell it how fast your water comes out. It also works backward: gallons or minutes in, inches out.

Gallons for 1 inch of water, by garden size

Every number here is square feet times 0.623. The half-inch column is for gardens watered twice a week.

For several beds, multiply: three 4×8 beds take 59.8 gallons for an inch. The acre rows match the USGS Water Science School, which gives 27,154 gallons for 1 inch of rain on one acre and 13,577 gallons on a half-acre lot, and University of Minnesota Extension rounds 100 square feet to 62 gallons.

Where 0.623 gallons comes from

It is plain geometry. A square foot is 12 inches by 12 inches, or 144 square inches. Cover it 1 inch deep and you have 144 cubic inches of water. The NIST Handbook 44 tables of units define a US gallon as exactly 231 cubic inches and an acre as exactly 43,560 square feet. So:

• Per square foot: 144 ÷ 231 = 0.6234 gallons for each inch of depth.
• Per acre: 43,560 × 0.6234 = 27,154 gallons.
• Backward: 1 gallon covers 1.6 square feet an inch deep (231 ÷ 144), so inches = gallons × 1.6 ÷ square feet.

Iowa State University Extension uses the same 0.623 figure: multiply the area of your garden by 0.623 to get the gallons for an inch. The backward version is handy for a rain barrel. Mine holds 50 gallons. Emptied onto one 4×8 bed, that is 50 × 1.6 ÷ 32 = 2.5 inches of water. Spread over three 4×8 beds, it is about 0.84 inch.

How long to run the hose: the bucket test

Gallons only help if you know how fast your water comes out. Iowa State’s method is to time how long it takes to fill a five-gallon bucket, then divide the bucket’s gallons by that time in minutes. Their example bucket filled in 2 minutes 26 seconds, which works out to about 2.1 gallons per minute. University of Minnesota Extension adds two checks: measure water into the pail first and mark the real 5-gallon line, and run the test a second time to confirm.

A worked example: your hose fills the bucket in 40 seconds. That is 5 ÷ 40 × 60 = 7.5 gallons per minute. A 4×8 bed needs 19.9 gallons for an inch, and 19.9 ÷ 7.5 = 2.66 minutes, about 2 minutes 40 seconds. Enter 40 seconds in the calculator’s bucket test and it gives the same run time.

Drip lines and soaker hoses

For a soaker hose, Iowa State says the flow rate is printed on the package in gallons per hour. If it is given per foot, multiply by the length of hose, then divide by 60 for gallons per minute. In the calculator, pick “Drip or soaker”, enter the gallons per hour, and multiply by the feet of line or the number of emitters it lists.

Treat the result as a starting point. Colorado State University Extension says drip timing varies with the brand of hose, the water pressure and the spacing of emitters and plants, and to set the run time by checking soil moisture. My raised bed drip irrigation runs on a Raindrip R675CT water timer, which has a runtime dial in minutes and a frequency dial, so the minutes from this calculator are the number to start that dial on. How I plumb the beds is in How to Build a Raised Garden Bed with Drip Irrigation.

The tuna can test for sprinklers

A sprinkler throws water over a pattern, so measure depth instead of flow. University of Illinois Extension suggests timing how long your sprinkler takes to fill a tuna can; that time is about one inch of water. No tuna can? A 1-inch-deep dish works, or mark 1 inch inside an empty butter tub. Iowa State sets several shallow, flat-sided containers in the spray, times how long they take to reach half an inch, and doubles it.

Use more than one can. Colorado State measures with several straight-sided cans placed around the garden and says delivery depends on the type of sprinkler head, the pressure and the head spacing. Their examples: pop-up spray heads put down about 1.5 inches an hour, around 40 minutes for an inch, and rotor heads about half an inch an hour, around 120 minutes. In the calculator, pick “Sprinkler can test” and enter the minutes and the depth your cans caught.

Is 1 inch a week the right amount?

It is the common rule of thumb, not a fixed need. Iowa State says most garden areas, the lawn and vegetable garden included, do best with an inch of water each week, and University of Minnesota Extension says the vegetable garden needs one inch of rain per week. What moves the number:

• Rain counts. Check a rain gauge and supply only the difference. Minnesota’s example: if the week brought three-eighths of an inch, add the other five-eighths. For a 4×8 bed that is 12.5 gallons instead of 19.9. The calculator’s rain box does this subtraction.
• Soil. On sandy, well-drained soil, Minnesota suggests watering twice a week with half an inch each time, 31 gallons per 100 square feet. Heavier clay soils and loam rich in organic matter hold moisture, and once a week is fine.
• Mulch. Minnesota notes that mulched soil holds water better, so you may water less often.
• Heat, wind and plant size. Colorado State says vegetables use around a quarter inch of water a day in typical summer weather (1.75 inches a week), depending on temperature, wind and stage of growth; that hot, windy weather raises demand significantly; and that use is low in spring while crops are small and temperatures are cool.

The soil settles it. Minnesota says that if the soil is dry two inches below the surface, it is time to water. That is why the calculator treats inches as your number to set, with 1 inch only as the starting value.

Quick questions

How many gallons is 1 inch of water per week?

The same 0.623 gallons per square foot, counted over the whole week with rain included: about 19.9 gallons for a 4×8 bed and 62 gallons for 100 square feet. If it rained, subtract the rain first. University of Minnesota Extension gives the 1 inch per week figure for vegetable gardens.

How many gallons is 1 inch of water on an acre?

27,154 gallons. An acre is 43,560 square feet, and 43,560 × 0.6234 = 27,154, the same figure the USGS gives for 1 inch of rain on one acre.

How long do I run a sprinkler to put down 1 inch of water?

Measure it with cans. Set several straight-sided cans in the spray and time how long they take to collect an inch, or half an inch and double it. Colorado State University Extension gives pop-up spray heads about 40 minutes and rotor heads about 120 minutes for an inch, but your heads and pressure will differ.

How long do I run drip irrigation for 1 inch of water?

Divide the gallons by the flow. Add up the gallons per hour of your drip line or emitters and divide by 60 for gallons per minute. A 4×8 bed needs 19.9 gallons for an inch, so a line delivering 30 gallons per hour takes about 40 minutes. Then check the soil and adjust.

How many gallons is half an inch of water?

0.312 gallons per square foot, which is about 10 gallons for a 4×8 bed or 31 gallons per 100 square feet. University of Minnesota Extension suggests half an inch twice a week on sandy soil.

Let the planner lay out the beds

Once you know what each bed drinks, plan what goes in it. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick your crops, and it places each plant at square-foot spacing and builds a planting schedule from your frost dates.

Related: Best Water Timer for Drip Irrigation · Raised Bed with Drip Irrigation · How to Set Up a Rain Barrel · How to Winterize a Drip Irrigation System · Square Foot Gardening Chart · Raised Bed Soil Calculator