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Sprout & Spectrum

Coverage and Sizing Math

One division sizes a grow light: photons per second, divided by the area you are lighting. Everything else on this page is that sum, applied.

An LED grow light mounted above a measured rectangular tray of plants on a rack

One division sizes a grow light

Sizing a grow light looks like a hard problem and is not. It is one division, and then one multiplication to check it against a published target. The reason it feels hard is that most product pages do not give you the number you need to divide.

PPFD (µmol/m²/s) = PPF (µmol/s) ÷ area (m²) × uniformity
DLI (mol/m²/day) = PPFD × hours × 3600 ÷ 1,000,000

PPF is what the fixture emits. Divide it by the area you are lighting and you have the average intensity at the canopy. Multiply that by your photoperiod and you have the daily total, which is the figure every published plant target is expressed in.

Worked example: a 2 × 2 ft shelf

The Mars Hydro TS 1000 publishes 343 µmol/s. A 2 × 2 ft area is 4 sq ft, or 0.372 m².

343 ÷ 0.372 = 922 µmol/m²/s at perfect uniformity. Applying a 0.8 uniformity factor: about 738 µmol/m²/s average.

At 14 hours: DLI = 738 × 14 × 3600 ÷ 1,000,000 = 37 mol/m²/day. That is fruiting-crop territory — roughly three times what a seed tray wants and ten times a low-light houseplant. The right response is to spread it over more area, raise it, or dim it.

What the uniformity factor is doing

A fixture does not deliver its average everywhere. The center is brighter than the corners, and the falloff at the perimeter is steep. A uniformity factor of 0.8 is a reasonable working assumption for one fixture over a small area; using 1.0 gives you the theoretical ceiling, which no real shelf achieves. It is a modeling assumption rather than a measurement, and we label it as such wherever it appears.

How this category divides

  • Shape matching. A square panel over a long shelf wastes its edges. A long bar over a square block leaves the corners dark. Match the fixture footprint to the growing footprint before you consider anything else.
  • Height. The same fixture at two heights is two different fixtures. Higher spreads the light over more area at lower intensity, which is often what a houseplant grower actually wants.
  • Count. Two smaller fixtures over one area give far better uniformity than one larger one. For a shelf, more bars beats a brighter bar.

The mistake nearly everyone makes

Buying by wattage. Watts measure electricity consumed, not light produced, and the conversion between them varies by nearly a factor of two across the fixtures on this site — from 1.39 to 2.7 µmol/J. A 100 W fixture at 2.7 µmol/J produces more usable light than a 150 W fixture at 1.6. Wattage tells you what it costs to run, and nothing about what it delivers.

Run your own numbers in the coverage calculator, then check the result against PPFD targets by plant type.

Picks in this category

Each of these names a top pick and links straight out to it. Prices come from the live price layer, which is why they read “check price” rather than showing a figure we cannot verify at the moment you load the page.

Tools

Put your own numbers in. The formula and a worked example sit under each one.

Everything else here

Questions people actually ask

+How do I work out what size grow light I need?
Divide the fixture's photon output (PPF, in µmol/s) by the area you are lighting in square meters, then multiply by a uniformity factor of about 0.8. That gives average PPFD at the canopy. Multiply by your daily hours and 3600, divide by a million, and you have DLI to compare against a published plant target.
+Can I size a grow light by watts?
Not reliably. Watts measure electricity consumed rather than light produced, and efficiency across common fixtures varies from about 1.39 to 2.7 µmol per joule. A 100 W fixture at the efficient end out-produces a 150 W fixture at the inefficient end.
+What is a uniformity factor and what value should I use?
It accounts for the fact that a fixture's center is brighter than its edges, so the average across the area is lower than the ideal division suggests. Use 0.8 for a single fixture over a small area. It is a modeling assumption rather than a measurement, and we label it that way everywhere it appears.

Sources