Free greenhouse watering calculator: enter plants, litres per plant per watering and waterings per week to get litres per session, per week and per month.
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Enter the number of plants, the litres each plant gets at one watering, and how many waterings you do per week. The calculator returns litres per watering, litres per week, litres per month, and gallons per week, so you can size a tank, a drip schedule, or a holiday watering plan. Use the crop and season tables below to pick a sensible litres-per-plant figure: tomatoes and cucumbers in full growth need far more than seedlings or salad leaves. Treat the result as a starting plan and adjust from soil checks, not as a fixed rule.
The calculation has three steps. Litres per watering = plants × litres per plant. Litres per week = litres per watering × waterings per week. Litres per month = litres per week × 52 / 12, which converts a weekly figure to a calendar month. Gallons = litres / 3.78541.
Worked example: 50 plants at 2 litres per watering need 100 litres per watering. At 3 waterings per week that is 300 litres per week, 1,300 litres per month, and 79.25 gallons per week. A second example: 20 tomato plants at 2.5 litres, watered daily, need 50 litres per watering and 350 litres per week. Compare that with a published guide example of 50 tomato plants at 2.5 gallons per day, which is 125 gallons per day: per-plant figures vary widely by crop, season and system, which is why entering your own figure matters.
Count the plants you water together, or split beds with very different needs into separate runs and add the weekly totals. Enter the litres one plant gets at a single watering, not the daily total: if you water twice a day, enter the per-visit amount and set waterings per week to 14. Read litres per watering for your tank or watering-can planning, litres per week for drip runtime and reservoir sizing, and litres per month for water bills and holiday cover.
For a holiday plan, multiply litres per week by the weeks away and add a safety margin: one guide illustrates 6 litres per day over 14 days as 84 litres, plus 3 days of margin as 102 litres. Check the soil before you trust any schedule: stick a finger about 5 cm into the soil, and if it is still moist, wait another day.
Peak summer water use per square metre per day, from a commercial greenhouse guide, runs about 6 to 9 litres for tomatoes on low-volume systems (up to 12 in heat), 7 to 10 litres for cucumbers (up to 14 in heat), 5 to 8 litres for peppers (up to 10 in heat), and 3 to 5 litres for salad on NFT (up to 7 in heat). Spring and autumn values run 2 to 3 times lower, around 2 to 4 litres per square metre per day.
| Season | Suggested frequency | Notes | |---|---|---| | Spring, March to May | Every 2 to 3 days, mornings | Growth accelerating, watch night cold | | Summer, June to August | Daily, pots and grow bags often twice in hot spells | Highest demand, mulch to cut evaporation | | Autumn, September to October | Every 2 to 3 days, tapering | Light falling, growth slowing | | Winter, November to February | Every week or two for anything still growing | Only water what is actively growing |
Fully grown tomatoes and cucumbers in a greenhouse need roughly 2 to 4 litres a day each in full growth, while a mature tomato in open ground needs about 4 to 6 litres per week in warm weather and up to 8 litres in hot, dry conditions. Containers dry faster and usually need daily watering in summer.
To calculate greenhouse water use for a crop, start from its water requirement: the greenhouse water requirement for tomatoes and the greenhouse water requirement for cucumbers both rise with heat, and greenhouse cucumber water needs track evapotranspiration closely. Seedlings vs mature plants differ sharply: watering frequency for seedlings vs mature plants should start daily for seedlings and ease to deep waterings for mature plants.
Soil type changes both how much and how often you should water. Sandy soil drains fast and needs lighter, more frequent waterings. Loam holds moisture evenly and suits a deep soak every 3 to 4 days. Clay holds water longer, so a slower, deeper soak every 4 to 7 days is better. Raised bed mix dries faster than ground soil, and containers have the smallest volume, so they often need daily or every-other-day watering in summer.
Hydroponic growers plan a drainage fraction on top of plant use: target drainage of 25 to 35 percent for mineral wool and 20 to 30 percent for coconut substrate. To deliver 6 litres per square metre consumed at 30 percent drainage, supply 6 / (1 - 0.30) = 8.57 litres per square metre, of which 2.57 litres drains away. A university rule of thumb puts peak greenhouse use at 0.3 to 0.4 gallons per square foot of growing area per day: a 2,400 square foot bench area needs 720 to 960 gallons on the warmest day.
Water deeply two or three times a week rather than a little every day. Deep watering trains roots downward and makes plants more drought-tolerant, while a daily sprinkle trains shallow roots and wastes water to evaporation. Water the soil, not the leaves: wet foliage encourages disease, and watering in the heat of midday loses much of the water before it reaches the roots.
Morning is best. Watering early lets foliage dry through the day, which reduces fungal disease, and gives plants moisture before the midday heat. Evening watering is the second choice: it reduces evaporation but leaves leaves wet overnight, which can encourage mildew. Mulching around the base reduces evaporation and evens out moisture, and grouping plants with similar needs makes one schedule work for the whole bed.
Watering a little every day is the most common mistake: it trains shallow roots and wastes water. Watering in the heat of midday, ignoring soil type, forgetting to subtract rainfall during wet weeks, and watering leaves instead of soil are all frequent errors. Letting containers dry out completely between waterings stresses roots, while keeping compost constantly soggy causes yellow lower leaves, a sour smell, and fungal growth.
Wilting when the compost is wet usually means root damage or disease rather than thirst, so do not add more water blindly. Check with the finger test, check pot weight, and watch morning wilt versus afternoon flagging: plants that recover by evening were flagging from heat, not from dry soil.
It depends on crop, season and system. As a starting point, fully grown tomatoes and cucumbers in a greenhouse need roughly 2 to 4 litres a day each in full growth, and peak summer use runs about 6 to 9 litres per square metre per day for tomatoes and 7 to 10 for cucumbers.
In summer, daily, with pots and grow bags often twice in hot spells. In spring and autumn, every 2 to 3 days. In winter, every week or two for anything still growing. Always check the top 5 cm of soil first: if it is still moist, wait another day.
A light sprinkle every day is bad, because it trains shallow roots. A deep watering every day can be right for containers and mature fruiting plants in hot weather. The rule is water deeply, not often, and let the soil tell you when.
Early morning is best: foliage dries through the day, which reduces fungal disease, and plants have moisture before the midday heat. Evening is the second choice, but wet leaves overnight can encourage mildew.
Look for yellow lower leaves, soggy medium, a sour or rotten smell, and fungal growth. If puddles remain an hour after watering, you applied too much. Water a pot until it runs from the base, then stop.
Yes. Sandy soil drains fast and needs lighter, more frequent waterings. Clay holds water longer and needs a slower, deeper soak less often. Loam sits in between. Containers dry fastest of all and usually need daily watering in summer.
A cordon tomato in full crop typically takes 1 to 2 litres a day in warm weather, split across two visits for grow-bag plants. Mature plants in open ground need about 4 to 6 litres per week in warm weather and up to 8 litres in hot, dry conditions.
Yes, easily. Constantly wet compost suffocates roots, causes yellow leaves and rot, and invites fungus. Water until it drains from the base, then wait until the top 5 cm of soil dries before watering again.
A university rule of thumb puts peak use at 0.3 to 0.4 gallons per square foot of growing area per day on the warmest day. A 2,400 square foot bench area therefore needs 720 to 960 gallons on that day.
Multiply litres per week from this calculator by the weeks away and add a few days of margin. For example, 6 litres per day over 14 days is 84 litres, plus 3 days of margin is 102 litres. A timer on drip irrigation or a self-watering reservoir covers most absences.