🥬 How much water does lettuce need?
There is no single answer, and a page that gives you one is guessing at where you live. Lettuce has a fixed thirst relative to the weather — its crop coefficient — and the weather is yours. Pick your climate and this works out the litres.
Lettuce roots to about 35 cm. In loam / medium soil that root zone holds roughly 49 mm of usable water, so watering when about half of it is gone means roughly 25 L per m² every 6 days rather than a little each day.
Deep and infrequent beats shallow and daily for almost everything. Light daily watering keeps the roots in the top few centimetres, which is the layer that dries first the moment you miss a day.
Set your plot and its crops once and the app costs the water you actually put on, warns when a critical stage is coming, and raises the job on the day rather than leaving it to the weather to remind you. Free while in early access.
Create your free accountThe weeks that decide the crop
Water stress is not evenly costly across a season. For lettuce the stage that matters is the whole crop — it is shallow-rooted and never far from stress — stress there does damage that watering afterwards cannot undo.
What it looks like when it goes wrong: Bitterness and bolting. Once it bolts nothing brings it back.
The corollary is that the rest of the season has more slack than most people assume. If water is short, protect that window and economise around it, rather than spreading the shortage evenly and damaging the crop at the one point it cannot recover from.
Why the number changes with where you are
A crop does not decide how much water it uses; the weather does, and the crop takes a fixed share of it. That share is the crop coefficient — lettuce sits at about 1 at mid-season — and the weather is reference evaporation, which runs near 2.5 mm a day in a cool maritime summer and near 9 in a desert.
That is the whole reason a figure copied from one country misleads in another. Same crop, same coefficient, three times the water. It is also why "an inch a week" survives as advice: it is about right for a temperate summer, and it is quietly wrong everywhere hotter or cooler.
Rain counts, and most people forget to subtract it
The figures above are total demand, not the amount to irrigate. Rainfall in the week comes off the top — and only rain that actually soaked in counts. A brief shower on dry ground mostly evaporates before it reaches the roots, and on heavy soil a downpour runs off. A rain gauge pays for itself in a season, mostly by stopping you watering when you did not need to.
Drip or sprinkler for lettuce
At this crop's demand, sprinkler is usually the better fit. Sprinkler suits a crop grown densely across a whole area, where running drip line to every plant costs more than the water it saves. Either way, water early in the morning: midday irrigation in a hot wind can lose a quarter of what you apply before it reaches the soil.
→ When to plant lettuce ← All farm calculators
Method: weekly need = Kc × ET0 × 7, where Kc is the FAO-56 mid-season crop coefficient and ET0 is reference evaporation for the climate band you picked. One millimetre of depth over one square metre is one litre. Rooting depths are FAO-56 maximum effective values. These are planning figures: your own soil, shelter and season will move them.