Enter your garden area, plant type, and local climate. Get the exact litres per week your irrigation system needs to deliver — for lawns, beds, vegetables, and containers. No app, no account.
From garden dimensions to a weekly water budget in under 5 seconds. No app, no account, no guesswork.
Select from lawn, garden beds, vegetable plot, or drip system. Each uses a different water demand model — so your calculation reflects the actual plants you're growing, not a generic average that misses the detail that matters.
Takes 5 secondsArea, plant type, climate, and season are the four variables that drive irrigation demand. Warm weather doubles water use compared to mild conditions. Mulch halves the frequency. Enter what applies to your garden and the calculator adjusts for all of it.
Takes 15 secondsReceive total litres per week, recommended number of sessions, and minutes per session based on your system type. Use this to programme a timer, plan your watering schedule, or size a water butt or irrigation pump for your setup.
Results in under 5 secondsNot just a total volume. A complete weekly irrigation plan — split across sessions, timed for efficiency.
The core number your irrigation system needs to deliver across the week. Use this to size a water butt, calculate a water bill, or set a controller timer on an automated system. It adjusts for your specific climate and plant type — not a one-size estimate.
Dividing the weekly total into multiple shorter sessions is almost always better than one long soak. Frequent shorter sessions keep the root zone consistently moist rather than creating a cycle of flood-and-drought that stresses most plants.
Based on your selected system type — rotary sprinklers, oscillating sprinklers, drip systems, and hose watering all have very different output rates. This gives you the actual time to run each session to hit your target volume.
Every result includes a practical note on adjusting for rainfall. 10mm of rainfall delivers approximately 10 litres per m² — so a 50 m² lawn that receives 10mm of rain in a week needs roughly 500 litres less from irrigation that week.
The "water for 20 minutes three times a week" advice you find on most gardening websites ignores the variables that actually determine what your garden needs.
A lawn in a 28°C heatwave loses water to evapotranspiration at roughly twice the rate of the same lawn during a mild 15°C week. Any irrigation schedule that doesn't account for actual temperature conditions will be consistently wrong — either underwatering in hot spells or wasting water in cool ones.
Tomatoes during fruiting need up to 15 litres per m² per week. A mature perennial border might need just 3–4 litres in the same week. Applying a flat watering rate to everything in the garden simultaneously either starves your tomatoes or drowns your drought-tolerant plants.
A 3-inch bark mulch layer reduces surface evaporation by 40–60% — meaning you can irrigate less often and with less water per session while maintaining the same soil moisture levels. Our calculator accounts for whether you've mulched your beds or not, so the output reflects your actual conditions.
A rotary sprinkler might apply 10mm of water per hour. An oscillating sprinkler on a medium setting applies closer to 6mm. A drip system running at 2 litres per emitter per hour applies water much more slowly than either. Without knowing your system's output rate, a time-based irrigation schedule is meaningless.
In areas with metered water supply, overwatering a medium-sized garden by just 20% adds up to a significant annual cost. Beyond the bill, overwatered lawns develop shallow root systems that make them more vulnerable to drought stress when watering is eventually reduced or interrupted.
Every irrigation planning app on the market either charges a monthly fee, requires smart hardware, or pushes you toward a connected system. This calculator gives you the same planning capability for free, in any browser, with no account and no ongoing cost.
Whether you're hand-watering with a hose or running a fully automated drip system, the calculator gives you accurate volumes and timings for your specific setup.
Based on real evapotranspiration rates and plant water-use data — not rough guesses.
The foundation of any irrigation calculation is evapotranspiration (ET) — the combined rate at which water evaporates from the soil surface and transpires through plant leaves. ET rate is primarily driven by temperature, solar radiation, wind, and the specific plants being grown.
For lawns, we use a base ET of approximately 15mm per week in warm conditions, scaled to your climate selection. For vegetable plots, we use crop-specific water demand coefficients — tomatoes require roughly twice the water of root vegetables per square metre because of their larger leaf canopy and higher transpiration rate.
The system run time is calculated by dividing the target volume by the output rate of your selected irrigation system. An oscillating sprinkler covering a 50 m² lawn at 7mm/hr needs 2.1 hours to deliver 15mm. Splitting that into three 42-minute sessions across the week is more effective than a single long run that risks surface run-off on compacted soil.
The primary driver — temperature and sun intensity determine how fast water leaves the soil
Different plants draw different volumes — tomatoes vs root veg vs lawn grass vary by 3×
Your sprinkler or drip rate converts the volume target into actual minutes of run time
3"+ mulch reduces evaporation by ~50% — applied automatically when selected
Use these baseline figures to cross-check your calculator results or estimate without measuring.
| Garden Type | Season | Litres / m² / week | Sessions / week | Key Variable |
|---|---|---|---|---|
| Cool-season lawn | Hot summer | 15–20 L | 3–4× | Temperature & sun intensity |
| Cool-season lawn | Mild spring | 8–12 L | 2× | Growth rate demand |
| Mixed perennial border | Summer | 6–10 L | 2× | Plant canopy & density |
| Perennial border (mulched) | Summer | 3–5 L | 1–2× | Mulch reduces evaporation |
| Tomatoes (fruiting) | Peak summer | 20–28 L | 4–5× | Fruiting demand is highest |
| Leafy greens | Summer | 12–15 L | 3–4× | Shallow roots — frequent |
| Root vegetables | Summer | 8–10 L | 2× | Deep infrequent watering |
| Container garden | Hot summer | Daily top-up | Daily | Containers dry out fast |
Getting the volume right is the foundation. These five practices make your irrigation more efficient and your garden healthier.
Early morning is the single most effective time to irrigate. Temperatures are lower, wind is calmer, and leaves dry quickly — reducing fungal disease risk. Evaporation losses are 30–40% lower than midday watering.
Wet foliage encourages fungal disease and is wasteful — the leaves don't absorb water, only the roots do. Drip systems and soaker hoses deliver water directly to the root zone, making them significantly more efficient than sprinklers for beds and veg plots.
A 3-inch mulch layer applied before the growing season begins can halve your irrigation frequency by dramatically reducing soil evaporation. It's the single highest-impact step a gardener can take to reduce water use while maintaining plant health.
After each irrigation session, push a finger or a soil probe 10–15cm into the ground. The moisture should reach that depth. If it doesn't, your session is too short. If it's still wet from the last session, your interval is too frequent. Adjust until the profile is right.
A basic rain gauge tells you how much rainfall your garden actually received this week. 15mm of rainfall on a 50 m² garden is 750 litres — which could replace an entire week of irrigation. Without measuring, you'll consistently either over or undercount what the rain delivers.
The most common questions we hear from gardeners setting up or optimising their irrigation.
From first-time gardeners setting a simple sprinkler schedule to allotment holders managing a full growing season water budget.
Planning how long to run a sprinkler, figuring out why the lawn looks stressed in July, or setting a controller timer for the first time — this calculator gives the numbers to do all of it properly.
Water management is one of the most important factors in vegetable yields. Consistent soil moisture during fruiting determines whether you get a good crop of tomatoes or a disappointing one — get the irrigation right and the results follow.
A timer is only as good as the schedule programmed into it. Use the calculator to get the correct run time and session frequency, then programme your timer accordingly — and update it as the seasons change.
Knowing exactly how much your garden needs prevents the enormous waste that comes from guessing. Many gardeners overwater by 50% or more simply because they water until it looks wet rather than until the root zone is reached.
Newly planted trees and shrubs need consistent moisture during their first growing season to establish. Use the calculator to plan a proper watering regime during establishment rather than relying on occasional spot watering.
A typical allotment plot has multiple zones with very different water requirements — the polytunnel, the open beds, the fruit cage. Calculate each zone separately to build a full weekly watering plan for the whole plot.
Calculating irrigation requirements for a new planting scheme, specifying a drip system for a client, or advising on water-efficient planting — this tool provides quick and reliable numbers without the overhead of specialist irrigation software.
