One millimetre over one square metre is one litre
That identity is the whole calculation, and knowing it changes how you think about watering. Rainfall and irrigation are both measured in depth, and depth times area is volume. A bed of 10 m² needing 25 mm of water this week needs 250 litres — about twenty-eight full watering cans.
Most people water far less than that and believe they are watering thoroughly. A nine-litre can over a 10 m² bed is not quite one millimetre of rain. It wets the surface, looks convincing, and reaches almost nothing.
This is also why "an inch a week" is the traditional rule of thumb: 25 mm across the growing area, from rain and hose combined. The calculator subtracts the rain you have already had, because watering on top of a wet week is worse than not watering at all.
| Bed size | 25 mm of water | Watering cans (9 L) |
|---|---|---|
| 1 m² · 11 ft² | 25 L | 3 |
| 5 m² · 54 ft² | 125 L | 14 |
| 10 m² · 108 ft² | 250 L | 28 |
| 20 m² · 215 ft² | 500 L | 56 |
| 50 m² · 538 ft² | 1,250 L | 139 |
Soak deeply, and rarely
The most common watering mistake is not the amount but the frequency. A light daily sprinkle keeps the top few centimetres damp and the soil below it dry, and roots grow where the water is. The result is a shallow root system that cannot survive a week without attention.
A heavy soak once or twice a week wets the full root zone. Roots follow the moisture down, and a deep-rooted plant copes with heat and neglect in a way a shallow-rooted one never will. The same total volume applied in two soakings rather than seven sprinkles produces a visibly tougher plant.
Timing matters less than people claim, but it is not nothing. Watering in the evening or early morning loses less to evaporation than midday. Watering the soil rather than the foliage matters considerably more, because wet leaves overnight are how blight and mildew establish.
The crop factor, and what it does not cover
Different crops use water at different rates, and the multiplier in this calculator is the crop coefficient — an established agronomic figure describing water use relative to a reference grass surface. Sweetcorn at 1.20 and radish at 0.90 are genuinely different propositions in a dry week.
What the coefficient does not capture is growth stage. A seedling with a tiny root system uses very little water in total but is far more vulnerable to the soil surface drying, because that is where all of its roots are. A mature plant uses much more water but can reach it. Peak demand for most crops comes at flowering and fruit set, which is exactly when a dry spell does the most damage.
Nor does it capture soil. Sand holds roughly half the available water of a loam for the same depth, so a sandy bed needs the same weekly total delivered in smaller, more frequent doses — the one case where more frequent watering is correct. Clay holds more but releases it slowly, and can be wet at depth while the surface looks parched.
Treat the number as a starting point and check the soil. Push a finger in to the second knuckle: if it is dry at that depth, water; if it is damp, wait.
Common questions
How much water does a vegetable garden need per week?
About 25 mm — one inch — across the growing area, from rain and watering combined. For a 10 m² bed that is 250 litres, or roughly 28 nine-litre watering cans.
Is it better to water a little every day?
No. A deep soak once or twice a week drives roots downward and produces plants that cope with heat. Daily sprinkling keeps roots shallow and makes plants dependent on you.
Should I water in the morning or evening?
Either is better than midday. What matters more is watering the soil rather than the leaves — foliage that stays wet overnight is how blight and mildew get established.