Lawn watering calculator: litres per week and sprinkler minutes
Enter your lawn size, soil and grass to estimate how much water it needs this week, how often to water and how long to run your sprinkler.
Your lawn this week (estimate)
| Water needed per week | 2,079 litres |
|---|---|
| Waterings per week | 2 waterings |
| Sprinkler time per watering | about 62 minutes |
| Depth per watering | 10.4 mm |
Spread 2 waterings evenly across the week, ideally in the early morning.
The RHS says established lawns recover after rain and generally need no watering, so treat this as the most you would need to keep the grass green in a dry spell.
Hosepipe ban? Temporary use bans under section 76 of the Water Industry Act 1991 include sprinklers, so you could not use this plan. Watering cans and rainwater are normally still allowed. Check your water company's notice.
How this calculator works
We use the FAO-56 method for crop water need. It starts with reference evapotranspiration (ET0), the water a short grass sheet loses on a typical day, taken from the table below or from your temperature. Multiplying by a crop coefficient for your grass (0.90 for hard-wearing ryegrass, 0.85 for fine fescue and bent, 0.75 for a shade mix) gives the daily need. Over seven days that is your weekly need in millimetres, and one millimetre over one square metre is one litre.
| Month | ET0 |
|---|---|
| April | 1.8 |
| May | 2.7 |
| June | 3.3 |
| July | 3.3 |
| August | 2.8 |
| September | 1.8 |
| October | 0.9 |
These monthly values are our own [estimate], a synthesis of UK MORECS and Silsoe data, and the temperature option uses 0.2 × (temperature − 3), kept between 0.5 and 4.5 mm per day.
Rain in the last seven days counts at 80%, because some runs off or evaporates. What remains is split into as few waterings as possible, never more than the soil can hold: about 10 mm on sandy soil, 20 mm on loam and 25 mm on clay. Watering thoroughly and less often encourages deeper roots, a point made in the RHS advice on caring for lawns in drought. Waterwise has more ways to save water in the garden.
Sprinkler minutes are the depth per watering divided by your sprinkler rate. Clay absorbs only about 3 mm an hour, as the FAO guidance on infiltration explains, so a faster sprinkler runs in short cycles with pauses to let the water soak in.
Giving the fence or shed a coat while the weather is dry? Work out the litres with our paint calculator.
Frequently asked questions
How much water does a lawn need each week?
It depends on the weather, the grass and the soil. In a typical UK July, a hard-wearing lawn on loam loses about 21 mm a week, which is roughly 21 litres per square metre, before rain is counted. In April or October the figure is far lower. The calculator works out your own figure from your inputs.
Do I need to water an established lawn at all?
Usually not. The Royal Horticultural Society says established lawns recover after rain and generally need no watering, even if they turn brown in a dry spell. Watering matters most for new turf and seed, which have shallow roots.
When is the best time of day to water?
Early morning is best because less water evaporates and the grass dries during the day. Evening watering is the next best option, though a lawn that stays wet overnight can be more prone to disease.
How do I measure my sprinkler rate?
Put several flat-bottomed tins, such as tuna cans, on the lawn within the sprinkler area and run it for 30 minutes. Measure the depth of water in each tin in millimetres, average them and double the result to get millimetres per hour. Enter that under advanced options.
What is cycle-and-soak, and why does clay need it?
Clay soil absorbs water slowly, at only about 3 mm an hour, so a typical sprinkler delivers water faster than the ground can take it and the excess runs off or pools. Cycle-and-soak means running the sprinkler in several short bursts with a pause between each so the water soaks in.
Can I use a sprinkler during a hosepipe ban?
No. A temporary use ban under section 76 of the Water Industry Act 1991 covers hosepipes, and that includes sprinklers attached to them. Watering cans and rainwater from a water butt are normally still allowed. Check your water company's notice for the exact rules.