Free greenhouse shading calculator: enter roof length, width and shade percent to get roof area, cloth to buy, light passing through and shade density tips.
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Enter the greenhouse roof length and width in metres and the shade cloth density as a percent. The calculator returns the roof area, the cloth to buy with a 10 percent allowance for hems, fixing and overhang, and the share of light that passes through versus the share blocked. Shade percent is the share of light blocked: 50 percent cloth blocks half the light and passes half. Use the density guide on this page to match the percent to your crop and climate.
Shade cloth is graded by the percent of light it blocks, so a 50 percent cloth blocks half the incoming light and passes the other half. The roof area is simply length multiplied by width, and the cloth to buy is the roof area plus a 10 percent allowance, because real installations need hems, fixing points and an overhang at the edges. Worked example: a 10 m by 6 m roof is 60 m2, so buy 66 m2 of 50 percent cloth, which passes 50 percent of light and blocks 50 percent. A second example: an 8 m by 5 m roof is 40 m2, so buy 44 m2 of 30 percent cloth, which passes 70 percent of light and blocks 30 percent. Competitor calculators sometimes multiply area by the shade percent and call that the cloth size, which understates the material badly; you must cover the whole roof, not just the shaded fraction.
Match density to the crop, not to a single rule. Vegetables and most flowers do well at 30 to 50 percent: tomatoes, peppers and cucumbers at 30 to 40 percent, lettuce, spinach and herbs at 40 to 50 percent, and bedding plants around 45 percent. Shade loving plants need more: orchids, ferns and philodendrons at 60 to 70 percent, and palms and Chinese evergreen up to 75 percent. In temperate zones 50 to 60 percent suits most greenhouses, while hot southern climates often need 70 to 80 percent on the roof. Too much shade is a real failure mode: a 90 percent cloth over tomatoes builds a dark room, with leggy growth and poor fruit set.
White vs black shade cloth is a common choice: white or light coloured cloth reflects more sunlight and usually cools better than black cloth, which absorbs heat and can warm the air under it. Black cloth remains the workhorse for vegetables and livestock shade because it is cheap and durable, but for cooling a greenhouse, white wins. Install cloth on the outside wherever possible: outside shading is about 40 percent more effective than inside shading, because it stops heat before it enters. Suspend the cloth 4 to 9 inches above the glazing so air can flow underneath, and make panels at least 4 feet wider than the greenhouse so they hang down 2 feet on each side.
As a rough guide to shade cloth temperature reduction, 40 percent cloth cuts greenhouse temperature by about 10 to 15 degrees Fahrenheit, 50 percent by about 15 to 20 degrees, and 70 percent by about 20 to 30 degrees, depending on sun, ventilation and glazing. Glass greenhouses trap more heat than polycarbonate ones, so glass houses often need 70 percent while polycarbonate houses manage with 60 percent. Shade is only one cooling tool: ventilation, wet walls and opening vents on sunny days matter just as much, and a shaded house with closed vents can still overheat. Use a light meter to check real light levels under the cloth rather than trusting the label alone.
Alternatives to shade cloth include a knitted shade net, shade netting, bamboo screens, and liquid shading compounds or whitewash painted on the outside, which can be layered for more density and washed off later. Doubling cloth is possible, but two layers of 50 percent do not give 100 percent shade: the second layer blocks half of what remains, so the combination blocks about 75 percent. Join panels with clips, ties or sewing, and choose UV stabilised knitted polyethylene for a 5 to 10 year life, or finished hemmed panels with grommets every 2 feet for 8 to 15 years. Rain passes through most shade cloth, so watering and ventilation planning stay unchanged.
Measure roof length and width, multiply to get the area, then add about 10 percent for hems and overhang. The shade percent is chosen separately from the density guide, it is not multiplied into the area.
Most vegetables do well at 30 to 50 percent, temperate greenhouses at 50 to 60 percent, and hot climates at 70 to 80 percent on the roof. Match the percent to your crop and climate.
Yes, for most uses. It blocks half the light, suits many vegetables and flowers, and cuts temperature by roughly 15 to 20 degrees Fahrenheit, depending on ventilation.
Outside is better, about 40 percent more effective, because it blocks heat before it enters. Hang it 4 to 9 inches above the glazing for airflow.
White reflects more sunlight and cools better. Black absorbs heat but is cheaper and widely used for vegetables and livestock shade.
No, in hot sunny climates 30 to 40 percent suits tomatoes well and helps prevent sunscald and heat stress. In mild climates, tomatoes may need no shade at all.
Yes, but the effect multiplies, not adds. Two 50 percent layers block about 75 percent in total, so check that the result still leaves enough light for your crop.
Shade netting, bamboo screens, or liquid shading compound and whitewash on the outside. Whitewash is the cheapest temporary option and can be layered or washed off.