How much water do vegetable gardens need per week?

A common rule of thumb is about 25 millimetres, or one inch, of water per week from rainfall and irrigation combined. Hot, windy, or sandy conditions push that higher, while cool weather and clay soils lower it. Adjust the requirement to match your crops and climate.

How is the deficit turned into a volume?

The deficit depth is the requirement minus measured rainfall, floored at zero. In metric, one millimetre of water over one square metre is one litre, so volume equals deficit in millimetres times area in square metres. In imperial, one inch over one square foot is 0.623 US gallons.

How is run time calculated?

Run time equals the water volume to apply divided by your flow rate. Measure your flow rate by timing how long a known container takes to fill from your hose, sprinkler, or drip system, then enter it so the tool converts the deficit into minutes of watering.

What if rainfall exceeded the requirement?

Then the deficit is zero and no irrigation is needed that week. The tool flags a surplus and recommends skipping watering, which prevents overwatering, waterlogged roots, and wasted water after a wet week.

Should I water a little every day?

Usually no. Deep, infrequent watering encourages roots to grow down and makes plants more drought resilient, whereas daily light sprinkling keeps roots shallow. Apply the weekly deficit in one or two longer sessions, ideally in the early morning to reduce evaporation.

What is the Garden Irrigation Deficit Calculator?

Enter your plants' weekly water requirement, measured rainfall, and bed area to compute the irrigation deficit, the litres or gallons to apply, and the run time at a known flow rate. For vegetable gardeners. It runs free in your browser on Gera Tools, with nothing uploaded.

Garden Irrigation Deficit Calculator

Name: Garden Irrigation Deficit Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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After a rainy stretch it is hard to know whether your garden still needs watering or has already had enough. This calculator settles it: enter how much water your plants need each week, how much rain actually fell, and the bed size, and it returns the exact deficit to make up, the volume to apply, and how long to run your irrigation.

How it works

The shortfall is the requirement minus what nature provided, converted to a real volume using the relationship between depth and area:

deficit depth = max(0, weekly requirement − rainfall)
metric:   volume(L)   = deficit(mm) × area(m²)          (1 mm over 1 m² = 1 L)
imperial: volume(gal) = deficit(in) × area(ft²) × 0.623
run time (min) = volume / flow rate

The deficit is floored at zero, so a wet week that meets or beats the requirement returns no irrigation — the tool simply tells you to skip watering that week.

Worked example

A 10 m² raised vegetable bed needs 25 mm of water per week. The rain gauge shows only 8 mm fell this week.

Deficit: 25 − 8 = 17 mm
Volume to apply: 17 mm × 10 m² = 170 litres
Hose flow rate: 8 litres per minute
Run time: 170 ÷ 8 = about 21 minutes

Apply those 170 litres in one session, preferably in the early morning. The same bed in imperial units: 1 inch target, 0.31 inch rainfall, 107 sq ft area → 0.69 in deficit × 107 × 0.623 ≈ 46 US gallons.

Typical weekly water requirements by crop

Crop	Approximate weekly need
Tomatoes	25–38 mm (1–1.5 in)
Lettuce and spinach	20–25 mm (0.75–1 in)
Courgette / squash	25–38 mm (1–1.5 in)
Beans	20–25 mm (0.75–1 in)
Established herbs	12–20 mm (0.5–0.75 in)
New transplants	38–50 mm (1.5–2 in) for first 2 weeks

These are approximate and increase in hot, windy, or sandy conditions. Adjust the requirement field to match your crops and local climate.

Measuring your flow rate accurately

Manufacturers’ flow rates for hoses and drip systems are often optimistic. The most reliable method: hold the hose into a bucket with a known capacity and time how long it takes to fill. For example, 10 litres in 75 seconds = 8 litres per minute. Drip emitters are rated in litres per hour (divide by 60 to get litres per minute). Enter your measured rate to get a realistic run-time figure.

When to skip the calculation

If the soil is already visibly wet 5–10 cm down when you push a finger or trowel into it, irrigation is not needed regardless of what the rain gauge shows. Heavy clay soils hold water much longer than sandy soils, so the same rainfall may satisfy a clay bed for two weeks but only four or five days in sandy ground.