Greenhouse Rainwater Calculator
See how much rainwater your greenhouse roof can collect for your plants, and how many rain barrels you need to catch it.
How it works
How much rainwater can a greenhouse roof collect?
Every inch of rain on a square foot of roof gives about 0.623 gallons of water. We use the flat footprint of the roof, because a slope does not catch more rain, and allow 20% for splashing, evaporation and overflow.
Gallons = Roof length × Width (sq ft) × Rain (in) × 0.623 × 0.8
A greenhouse in summer uses roughly 0.1 gallons per square foot of floor a day, more for thirsty crops such as tomatoes. Rainwater is soft and free of chlorine, which most plants prefer.
Questions
Rainwater FAQ
How much water can I collect from a greenhouse roof?
About 0.5 gallons per square foot of roof for every inch of rain, after losses. A 10 x 12 ft greenhouse collects about 60 gallons from a 1 inch rain, or around 2,700 gallons a year with 45 inches of rain.
Is rainwater good for greenhouse plants?
Yes. Rainwater has no chlorine and is naturally soft, so most plants grow well with it. Keep barrels covered and clean, and use a screen to keep leaves out.
How many rain barrels do I need for a greenhouse?
Enough to hold one good storm. For a 10 x 12 ft greenhouse, two 55-gallon barrels will catch most of a 1 inch rain.
Do greenhouses come with gutters?
Many do. Royal Victorian and Junior Victorian greenhouses come with large gutters and downspouts, and MONT greenhouses have built-in rain gutters.
More tools
Other greenhouse calculators
- Heater Size CalculatorHow many BTUs it takes to keep your greenhouse warm on the coldest night.
- Heating Cost CalculatorWhat a winter of heating will cost with electric, propane or natural gas.
- Fan and Ventilation CalculatorThe exhaust fan CFM and vent openings that keep summer heat down.
- Snow Load CalculatorHow much the snow on your roof weighs and when to brush it off.
- Foundation CalculatorConcrete, gravel, pavers and lumber for your greenhouse base.
- Greenhouse Size CalculatorHow big a greenhouse you need for the plants you want to grow.
- Shade Cloth CalculatorThe right shade percentage and cloth size for your greenhouse.
- Grow Light CalculatorHow many grow lights it takes to give your plants enough light in winter.
Want a second opinion?
Tell us about your greenhouse and where you live, and we will help you get it right.
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