Garden watering calculator
Enter the area and how deep you want to water. 25 mm (1 in) a week is a common guide for lawns and beds in dry weather. You get the volume and, with your hose or sprinkler flow rate, how many minutes to run it.
Area and depth
About the garden watering calculator
Enter the area and how deep you want to water. 25 mm (1 in) a week is a common guide for lawns and beds in dry weather. You get the volume and, with your hose or sprinkler flow rate, how many minutes to run it.
Part of SmartCalcy’s garden tools. It runs in your browser with no account, uses your own currency and units (change them at the top of the page), and gives estimates for information only, so check official guidance before you rely on a result.
How to use the garden watering calculator
What it calculates and when to use it
Converts a garden area and a depth of water into litres or gallons, and works out how long to run a hose or sprinkler.
How to use it, step by step
- Enter the area to water.
- Enter the depth of water, such as 25 mm (1 in).
- Measure your flow rate by timing how long a bucket takes to fill.
- Read the volume and the running time.
What each input means
- Area – Starts at 40 m².
- Depth – Starts at 25 mm.
- Flow rate – Starts at 15 litres a minute.
How to read the results
- Water needed – Litres or US gallons.
- Running time – Volume ÷ flow rate.
- Per unit area – 1 mm over 1 m² is 1 litre.
The formula
Litres = area (m²) × depth (mm). In US units, gallons = square feet × inches × 0.623. Time = volume ÷ flow rate.
A worked example
With the starting entries: 40 m² × 25 mm = 1,000 litres; at 15 litres a minute that is about 1 hour 7 minutes.
Assumptions and limits
- All the water soaks in evenly.
- Sprinklers lose some water to wind and evaporation.
Common mistakes
- Watering little and often, which keeps roots shallow.
- Watering at midday in hot weather.
Frequently asked questions
How much water does a lawn need?
About 25 mm (1 in) a week in dry weather is a common guide.
How do I measure flow rate?
Time how long a bucket of known size takes to fill.
Related tools
- Per m²
- 25 litres1 mm over 1 m² is 1 litre
- Watering cans (10 L)
- 100