Irrigation time calculator
Choose how much water you want to put down, then give either the area and the flow of your hose or valve, or the precipitation rate of your sprinklers. The calculator returns the running time and the volume of water, allowing for the share lost to wind, runoff and evaporation.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the units and enter the depth of water you want to put down per watering.
- Pick what you know about the water supply: the flow rate (enter the area and the flow) or the precipitation rate of the sprinklers.
- Set the application efficiency and read the watering time.
How it works
1 mm of water over 1 m² is 1 litre; 1 inch over 1 ft² is 144 cubic inches, or 0.623 US gallons. The water to pump is the depth divided by the efficiency:
- from the flow rate: time = depth × area ÷ efficiency ÷ flow;
- from the precipitation rate: time = depth ÷ efficiency ÷ rate.
Example
A 500 ft² bed, half an inch of water, a hose at 3 gal/min and 80% efficiency:
- water: 0.5 × 500 × 0.623 = 155.8 gal; ÷ 0.8 = 195 gal;
- time: 195 ÷ 3 = 65 minutes.
With sprinklers that put down 0.5 in per hour, the same half inch at 80% takes 0.625 ÷ 0.5 = 1.25 hours, or 75 minutes.
Tips and common mistakes
- Water deeply and less often rather than a little every day; it encourages deeper roots.
- On clay or slopes, split a long run into two or three cycles so the water soaks in instead of running off.
- Water early in the morning, when there is less wind and evaporation.
Frequently asked questions
How much water does a lawn need?
It depends on the grass, the soil and the weather. About 1 inch (25 mm) a week, rain included, is a common rule of thumb in warm weather. The evapotranspiration calculator estimates the actual water use for your climate.
How do I find my sprinkler's precipitation rate?
Place a few straight-sided cans across the area, run the sprinklers for 15 minutes and measure the water depth in each. Multiply the average by 4 to get the rate per hour.
What efficiency should I enter?
It is the share of the water that ends up in the root zone. It is a rough figure that you set yourself: drip lines lose little, while sprinklers lose more to wind, overspray and evaporation. 100% means no losses.
How do I measure the flow rate of a hose?
Time how long it takes to fill a bucket of known size. A 10 L bucket filled in 50 seconds means 12 L/min; a 5 gallon bucket in 100 seconds means 3 gal/min.