How much water do greenhouse containers need?
A container crop loses water in two ways: the plant transpires it, and it evaporates from the growing medium. Together that is evapotranspiration (ET), which is measured as a depth of water, the same way rain is. Multiply that depth by the ground area each container occupies and you have the volume each container uses per day.
Growers usually apply somewhat more than the crop uses so that some water drains from the bottom of the pot. The share that drains is the leaching fraction. This calculator adds that extra water, then converts the total into a volume for the whole greenhouse and, if you know your delivery rate, a run time for emitters or a boom.
Check local rules and your water source. Some regions regulate irrigation runoff and drainage from nurseries and greenhouses. Where those rules apply, they come before any calculated amount.
How this greenhouse irrigation calculator works
The calculator works in daily depths, applies your crop factor and leaching fraction, then converts to a volume using each container's area.
1. Daily crop water use
Net depth per day
Net depth = ET per day × Kc
If you enter ET per week, it is divided by 7 first. Kc is your crop factor; it is 1 when your ET already applies to your crop. Reference ET describes a standard grass surface (FAO-56), so it normally needs a crop factor.
2. Add the leaching fraction
Gross depth per day
Gross depth = Net depth ÷ (1 − LF)
LF is the leaching fraction as a decimal: drained water ÷ applied water. If crop use is 8 units and LF is 0.2, apply 10 units so 2 drain. LF must be below 1.
3. Area and volume per container
Area per container (pot diameter option)
US: Area (sq ft) = Diameter (in)² ÷ 144
Metric: Area (m²) = (Diameter (cm) ÷ 100)²
Volume per container per day
US: Gallons = Area (sq ft) × Gross depth (in) × 0.623
Metric: Liters = Area (m²) × Gross depth (mm)
The US factor is 144 ÷ 231, the number of cubic inches in one inch over one square foot divided by cubic inches per gallon. There are 128 fluid ounces in a US gallon, and 1 mm over 1 m² is exactly 1 L. Total water is the per-container volume times the number of containers.
4. Run time
Emitters
Minutes = Volume per container ÷ (Flow per emitter × Emitters per container) × 60
Volume in gallons with flow in gph (US), or liters with L/h (metric).
Boom
Minutes = Gross depth ÷ Application rate × 60
Rate in in/hr (US) or mm/hr (metric).
Depths and rates are shown to 2 decimals (US) or 1 decimal (metric). Totals are rounded because the inputs are estimates. Conversions are checked both ways, so switching units returns the same physical answer apart from display rounding. For a plain depth-to-volume conversion, use the inches of water to gallons calculator.
When this calculator isn't the right tool
- Working out the ET itself. See the evapotranspiration calculator and the crop coefficient calculator.
- Outdoor containers, raised beds and patio pots. See the container plant watering calculator.
- Drip layouts and emitter selection. See the drip emitter calculator.
- Field crops and open-field irrigation. See the agriculture calculators.
Greenhouse irrigation calculation examples
500 pots with an 8 in top diameter, touching. ET is 0.20 in per day, Kc is 1, the leaching fraction is 15%, and each pot has one 0.5 gph emitter.
- Area per container: 8² ÷ 144 = 0.4444 sq ft.
- Net depth: 0.20 × 1 = 0.20 in.
- Gross depth: 0.20 ÷ (1 − 0.15) = 0.2353 in.
- Per container: 0.4444 × 0.2353 × 0.6234 = 0.0652 gal, or about 8.3 fl oz.
- Whole greenhouse: 0.0652 × 500 = 32.6 gal, shown as about 33 gallons per day.
- Emitter run time: 0.0652 ÷ (0.5 × 1) × 60 = 7.8 min, shown as about 8 min.
The same calculation in metric
1,000 pots, 20 cm diameter, touching. ET is 5 mm per day, Kc is 1, the leaching fraction is 20%, and a boom applies 15 mm per hour.
- Area per container: (20 ÷ 100)² = 0.04 m².
- Net and gross depth: 5 × 1 = 5 mm; 5 ÷ (1 − 0.20) = 6.25 mm.
- Per container: 0.04 × 6.25 = 0.25 L, or about 250 mL.
- Whole greenhouse: 0.25 × 1,000 = about 250 liters per day.
- Boom run time: 6.25 ÷ 15 × 60 = about 25 min.
How to measure the inputs and apply the result
Measure pot diameter at the rim, and if pots are spaced, measure the bench area divided by the number of pots to get area per container. Measure the leaching fraction directly: collect the drainage from a few containers and divide it by the water applied to them. Check emitter flow with a bucket test, and boom rate with catch cans, since the manufacturer's figure applies only at the rated pressure. The catch-can test calculator does the boom arithmetic.
The result is a daily amount. If you irrigate several times a day, divide the run time by the number of events. A common mistake is entering an outdoor weather ET for a shaded or heated house without checking it against how quickly your containers actually dry; confirm the estimate by weighing a few pots or watching drainage.
Factors that change the result
| Factor | Effect | What to adjust |
|---|---|---|
| Light and temperature | Stronger light and heat raise ET. | Update the ET estimate as conditions change. |
| Crop and canopy size | Larger plants use more water per container. | Change the crop factor as the crop grows. |
| Container spacing | Wider spacing gives each pot more area. | Use area per container. |
| Leaching fraction | Higher leaching raises water applied and drainage. | Set the fraction you actually measure. |
| Delivery uniformity | Uneven emitters or boom passes leave some pots dry. | Check flows, and see the emission uniformity calculator. |
Frequently asked questions
How much water does a greenhouse container need per day?
It depends on how much water the crop uses and how much space each container occupies. This calculator multiplies your daily ET estimate and crop factor by the ground area per container, then adds extra for the leaching fraction you choose. Enter an ET figure from your own records, a greenhouse weather station or another source you trust.
What is a leaching fraction?
The leaching fraction is the share of applied water that drains out of the container. If you apply 10 liters and 2 liters drain, the leaching fraction is 0.2, or 20%. The calculator divides crop water use by one minus the leaching fraction to get the gross amount to apply.
What ET value should I enter?
Enter the evapotranspiration (ET) you measured or estimated for your greenhouse, as a depth per day or per week. Reference ET describes a standard grass surface, so it usually needs a crop factor (Kc) to represent your crop. If your ET figure already applies to your crop, leave the crop factor at 1.
See the crop evapotranspiration calculator.
How is container size used in the calculation?
Water use follows the ground area a container occupies, not the volume of the pot. With pots touching, the calculator assumes each pot takes a square whose sides equal its top diameter. If pots are spaced apart, choose the area per container option instead.
How do I find my emitter or boom rate?
Emitter flow is usually printed on the emitter or in the manufacturer's data, and a bucket test measures it directly. For a boom, run it over several straight-sided cans, then divide the average depth collected by the run time to get a rate per hour.
See the bucket test calculator.
Does this calculator include fertilizer, salts or runoff?
No. It converts a water demand into volume and run time only. Fertilizer injection, salinity management and drainage collection need separate calculations, and any chemical you inject should be used as its label directs.
See the fertigation calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Both worked examples traced by hand through the same formulas the calculator uses.
- Unit conversions checked in both directions for depth, area, pot diameter and emitter flow.
- Leaching formula checked against its definition: drained water divided by applied water.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998). Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. Food and Agriculture Organization of the United Nations. [VERIFY URL]
- Ayers, R.S. and Westcot, D.W. (1985). Water quality for agriculture. FAO Irrigation and Drainage Paper 29, Rev. 1. Food and Agriculture Organization of the United Nations. [VERIFY URL]