Shade Coverage Calculator — Greenhouse Shade Cloth

For a mixed UK greenhouse, 40-50% is the usual all-round choice. Heat-loving fruiting crops (tomatoes, peppers, aubergines) prefer a lighter 30-40% so they still ripen, while cool leafy salads such as lettuce want 50-70% in midsummer to stop bolting. Seedlings, ferns and orchids being propagated like 60-70%. The shade percentage is the fraction of light the cloth blocks, so 50% cloth lets through half the light.

The usual install drapes one sheet over the ridge so it covers the whole roof and hangs about two-thirds of the way down both long side walls, where the strongest sun strikes. This calculator sizes that single sheet: the length runs the full greenhouse length and the width spans the roof plus the drape down each side, with a 30 cm overlap added on every edge for fixing. Buy to the next standard roll size up.

No — they multiply, not add. Two 50% nets do NOT give 100% shade. Each net passes 50% of the light, so two together pass 0.5 × 0.5 = 25%, which is 75% total shade. The formula is combined shade = 1 − (1 − s₁)(1 − s₂). Enter an existing layer under Advanced Options and the calculator shows the real combined figure, so you do not accidentally plunge the crop into deep shade.

Outside is far more effective because it stops the sun before it heats the glass and the air inside. External cloth can cut greenhouse temperatures by several degrees more than internal shading. Internal cloth or blinds are easier to fit and adjust but mainly protect leaves from scorch rather than controlling heat. Either way, leave vents clear so air can still move.

In most of England and Wales, shading goes up from late May or June and comes off in September as light levels drop. Southern England usually needs it earlier than Scotland. Take it down promptly in autumn — shade that stays on too long cuts the light fruiting crops need to finish ripening. Good cloth lasts several seasons if folded and stored dry.