Grow-Light Calculator

PPFD and DLI, explained simply

PPFD — photosynthetic photon flux density — is the amount of usable light hitting a square metre of plant every second, measured in micromoles per square metre per second. It is the number that matters for a grow light, and it falls quickly as you move the light away: doubling the distance roughly quarters the intensity, which is why hanging height matters so much. DLI — daily light integral — is the total light a plant receives over a whole day: PPFD multiplied by the hours the light is on, converted into moles per square metre per day. Two setups can deliver the same DLI in different ways — a strong light for 10 hours, or a weaker light for 16 hours — and the plant mostly cares about the total. Outdoors, a sunny summer day delivers a DLI of roughly 30 to 50; a dull winter day indoors near a window might deliver 2 to 5, which is why houseplants struggle in winter and why seedlings stretch toward the glass.

How much light do different plants need?

The targets in this calculator are the ranges commonly published in horticultural lighting guidance, and they are ranges because varieties, growth stage and temperature all shift the ideal. Seedlings and cuttings need modest light — about 8 to 12 mol/m²/day — because young leaves scorch easily. Most foliage houseplants are content with 5 to 10. Leafy greens and herbs grow fast at 12 to 16, flowering plants need 12 to 18 to bloom well, and fruiting vegetables such as tomatoes and peppers are the hungriest, at 20 to 30. If your plants are stretching tall with pale, widely spaced leaves, they are almost certainly short of light. If the leaves are bleaching, curling or developing brown patches on the side facing the lamp, the light is too close or too strong. Move the light in small steps and watch the plants for a week — they will tell you what they think of the change.

Distance, hours and the dark period

Most LED grow lights work best 12 to 24 inches above the plants, but the right height depends on the light’s power and the manufacturer’s guidance — always check the spec sheet, and raise the light as the plants grow. As a rule, run the light for 12 to 16 hours a day for seedlings and vegetables, and 10 to 14 hours for most houseplants. Almost all plants need a dark period: many processes, including flowering, depend on night length, and running lights around the clock stresses plants and wastes electricity. A simple timer plug solves this — set it once and the plants get a consistent day length, which matters more than most people realise. Keep the light on a schedule that matches the plants’ needs rather than your own: a light that is on all evening in the living room can disturb short-day plants that need long nights to flower.

Do you actually need a grow light?

If you are starting seeds indoors in late winter, a grow light is almost essential — a windowsill rarely gives enough light in February and March, and leggy seedlings never fully recover. For houseplants, a light is worth it if a plant that should be thriving is stretching or dropping leaves in a dim room; first check whether a brighter spot is available, since free daylight beats a lamp. For winter herbs and salads, a modest LED panel pays for itself in fresh leaves. When choosing a light, look for a full-spectrum LED with a published PPFD chart rather than a coloured “blurple” bulb with no data. To work out when to start the seeds your light will raise, use the seed-starting calculator, and to check the room’s other conditions, the humidity and temperature checker and the sunlight calculator. Full care details are in the houseplant care guides.