Garden Watering Calculator
By Santaji GadepatilinUpdated October 27, 2026
Plants need a certain depth of water each week. Knowing your garden area and flow rate tells you how long to run sprinklers or hoses. This free garden watering calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the garden watering calculator
Enter the area, the water needed per week, this week’s rainfall and your hose or sprinkler flow rate. The water to apply and the watering time are calculated.
Example
50 m² lawn needing 25 mm, 5 mm of rain, 12 L/min hose:
- Water = 50 × 20 = 1,000 L
- Time = 83 minutes per week (for example, two sessions of about 40 minutes)
Water needs by plant type (per week)
| Plants | Typical water need |
|---|---|
| Established lawn (cool climate) | 25 mm |
| Lawn (hot, dry weather) | 30–50 mm |
| Vegetable garden | 25–40 mm |
| Established shrubs | 10–25 mm |
| Drought-tolerant plants | 0–10 mm |
1 mm of water on 1 m² equals 1 litre.
Measuring your sprinkler
Place a few straight-sided containers (such as tuna cans) across the area, run the sprinkler for 15 minutes and measure the average depth. If it collects 6 mm, the sprinkler applies 24 mm per hour.
Best time to water
- Early morning: less evaporation and wind, leaves dry quickly.
- Avoid evening watering where fungal disease is a problem.
- Water deeply and less often to encourage deep roots.
Drip vs sprinkler
| Drip irrigation | Sprinkler | |
|---|---|---|
| Efficiency | 85–95% | 60–80% |
| Best for | Beds, trees, rows | Lawns |
| Weed growth | Less | More |
Rainwater
Use collected rainwater to cut mains water use — see the rainwater harvesting calculator.
Frequently asked questions
How much water does a lawn need?
About 25 mm (1 inch) per week in warm weather, including rainfall.
When is the best time to water?
Early morning, when evaporation is lowest and leaves dry during the day.
How do I measure sprinkler output?
Place a few straight-sided cans on the lawn and time how long it takes to collect 25 mm.
How long should I water my lawn?
Long enough to apply about 25 mm a week. A sprinkler applying 24 mm/hour needs about an hour a week, split into one or two sessions.
How many litres is 1 mm of water on 100 m²?
100 litres.