Calculate Gallons And Minutes For Watering Vegetable Garden One Inch

Photo of dancer, clouds, pumpkins digitally derived by Robert Kernodle
Photo of dancer, clouds, pumpkins digitally derived by Robert Kernodle

The often-recommended minimum amount of water for irrigating a vegetable garden is one inch of rain per week. But how do you decisively translate this measure into practical terms that you can apply? This article explains exactly how.

"One Inch Of Rain" – What It Means

Imagine an inch-thick, clear sheet of glass covering the patch of ground where you grow vegetables. This is the volumetric shape of your garden's "inch of rain". If you had a rectangular garden, then an inch of rain ideally would look like an inch-thick, flat sheet of water sitting on top of the ground there. If you had a circular garden, then an inch of rain would look like an inch-thick, flat disk of water. If you had any other garden shape, then an inch of rain would look like the inch-thick volume corresponding to that particular garden patch's shape.

As the size of any surface increases, its associated volume (at one inch of thickness) also increases. Consequently, an inch of rain in a bucket might amount to only a fraction of a gallon, while the same inch of rain in a nearby open field might amount to thousands of gallons. You can think of this "inch of rain" simply as the THICKNESS of an imaginary water container the size of your vegetable garden, where you have to know the OTHER dimensions. What you then want to determine is how many gallons (for USA readers) your inch-thick, garden-sized water container holds.

Keep in mind that the inch recommendation is a general suggestion. Hot or windy weather can easily call for more water. The method for determining an ideal inch of rain, therefore, is NOT a replacement for good judgment. Rather, this method is a supplement to good judgment for those gardeners who insist on converting that ideal inch into practice. I assume that readers will be using some sort of hose (drip, soaker, etc.) connected to a faucet.

Cubic foot to gallons photo derived by Robert Kernodle
Cubic foot to gallons photo derived by Robert Kernodle

To Calculate Gallons

For your vegetable garden, the most general formula for calculating gallons of water equal to an inch of rain looks like this:

G = A x 0.62084, where

.......... G = Gallons,
.......... A = Area for whatever shape your garden might be,
0.62084 = the result of multiplying 0.083 x 7.48, a constant figured from converting one inch to feet and multiplying by number of gallons per cubic foot.

For calculating gallons in RECTANGULAR gardens, use the formula:

G = L x W x 0.083 x 7.48, where

..... G = Gallons,
...... L = garden Length,
..... W = garden Width,
0.083 = the decimal measure of one twelfth foot, or one inch of rain,
. 7.48 = the number of gallons in one cubic foot of water.

For calculating gallons in CIRCULAR gardens, use the formula:

G = pi x R2 x 0.083 x 7.48, where

G = Gallons,
pi = 3.1416,
R = garden Radius.

For calculating gallons in OTHER garden shapes, use the appropriate ............... area formula for that other shape.


To Calculate Minutes

You probably will NOT collect, cart, and uniformly distribute your calculated gallons of water by hand. Instead, you probably will use an irrigation hose connected to a water faucet, and you will want to know how many minutes to run the water faucet to deliver this quantity of gallons.

The best way to determine total watering minutes is by connecting a simple, inexpensive water gauge to your water faucet, use a stopwatch to time the number of SECONDS for one gallon to flow, multiply the one-gallon time by your total gallons needed, and divide by 60.

P3 International P0550 Save A Drop Water Meter
P3 International P0550 Save A Drop Water Meter | Source

To figure total watering MINUTES that deliver one-inch worth of rain, use the formula,

TTOTAL = (TG x G)/60 , where

TTOTAL = total watering MINUTES,
...... TG = SECONDS for one gauged gallon of water to flow,
........ G = A x 0.62084 (YOUR garden's total gallons required),
....... 60 = 60 seconds per minute, which divides total watering time by seconds
............... into minutes.

In A Nutshell

To calculate total number of minutes to run your water faucet, use the formula

TTOTAL = (TG x A x 0.62084)/60 , where

TTOTAL = Total MINUTES of watering equivalent to an inch of rain
...... TG = gauged SECONDS for one gallon of hose-water flow
........ A = SQUARE-FEET AREA of your vegetable garden

In a sentence,

  • calculate your garden's gallons,
  • gauge your water faucet's gallon flow time in seconds,
  • multiply your one-gallon flow time by your garden gallons, and
  • divide by sixty.

Photo of garden watering time calculation for an inch of rain by Robert Kernodle
Photo of garden watering time calculation for an inch of rain by Robert Kernodle

Happy watering.

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