Garden Irrigation Calculator
Watering by the clock without knowing how much you are applying wastes water and can drown or starve plants. This calculator turns a target watering depth and your garden area into the volume of water required, then divides by your irrigation system's flow rate to give the run time. Target depth and flow rate are editable, because the right amount of water depends on your plants, soil, and climate, and your system's output is specific to your setup. The result is the time and volume to apply your chosen depth.
Garden irrigation formula
volume (cubic feet) = area * (depth inches / 12)
gallons = volume (cubic feet) * 7.48052
run time (minutes) = gallons / flow rate (gpm)
run time (hours) = run time (minutes) / 60
One cubic foot holds about 7.48052 U.S. gallons of water. Depth in inches is divided by 12 to convert to feet so the area and depth multiply to cubic feet.
Irrigation facts
- A common guideline is about 1 inch of water per week, but it varies by plant and climate.
- One cubic foot of water is about 7.48 gallons.
- Watering early in the day reduces losses to evaporation.
- Subtract recent rainfall from your target depth to avoid overwatering.
- Deep, infrequent watering encourages deeper, more drought-tolerant roots.
Garden irrigation: frequently asked questions
How do I calculate irrigation run time?
Find the volume of water you want to apply (area times target depth), then divide by your system's flow rate. Keeping units consistent, area in square feet times depth in inches divided by 12 gives cubic feet, which convert to gallons. Dividing gallons by your flow rate in gallons per minute gives the run time in minutes.
How much water does a garden need?
A common guideline is around 1 inch of water per week for many lawns and gardens, but it depends heavily on plant type, soil, climate, and season. Because the right depth varies so much, this calculator takes your target watering depth as an input rather than assuming one figure.
What is my system's flow rate?
Flow rate is how fast your irrigation delivers water, in gallons per minute. You can measure it by timing how long it takes to fill a known container, or read it from the specifications of your sprinklers or drip emitters. Enter that value so the run time matches your actual system.
Why convert to gallons?
Water volume in the United States is commonly measured in gallons, and irrigation flow rates are quoted in gallons per minute. One cubic foot holds about 7.48 gallons. The calculator converts the applied volume from cubic feet to gallons so it can divide by your flow rate to find the time.
Does this account for rainfall or evaporation?
No. The calculation gives the volume and time to apply your target depth across the area, before any natural rainfall or losses to evaporation and runoff. Subtract recent rainfall from your target depth, and water early in the day to reduce evaporation for the most efficient use.
Official sources
- U.S. Environmental Protection Agency (WaterSense): Outdoor watering tips.
- National Institute of Standards and Technology (NIST): Volume unit conversion (gallons and cubic feet).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.