Grow box 8 min read

Grow Light Calculator (lux, PPFD, DLI)

Winter seedlings on a windowsill almost always suffer from too little light. But how many watts of lamp do you actually need, and how high should you hang it? This calculator converts lamp power into the numbers that matter to plants — lux, PPFD and DLI — and sizes power and height to your grow box.

Open this tool in the app →

From watts to DLI — the whole chain

Lamp wattage tells you little about how much light a plant receives. The calculator walks through each conversion, all grounded in physics and lamp data:

Watts → lumens (by lamp efficiency), lumens → lux (lumen per m² of area), lux → PPFD (a factor of ~0.017 for typical white LED), PPFD → DLI (PPFD × on-hours × 3600 ÷ 1,000,000). Lamp height is included too — intensity falls with the square of distance.

Lamp type	Efficacy	Use
Horticultural LED	~140 lm/W	most efficient, low heat
HPS (sodium)	~110 lm/W	high power, lots of heat
T5/T8 fluorescent	~90 lm/W	seedlings, low plants
CFL (compact)	~60 lm/W	small grows, top-up light

What DLI is and how much you need

DLI (Daily Light Integral) is the total amount of light a plant receives in a day, in mol/m²/day. It is the single best "do my plants have enough light" number. Seedlings and herbs are happy with a lower DLI; fruiting plants (tomato, pepper) need much more.

Lamp height matters enormously

Light intensity falls with the square of distance — hang the lamp twice as high and plants get roughly four times less light. So the calculator also works out a recommended lamp height to hit your target DLI without scorching the plants.

How to size a lamp

enter the grow box dimensions (width × depth) and the lamp hanging height
pick a lamp type (LED/HPS/CFL/fluorescent) or enter lumens from the label
read lux, PPFD and DLI plus a recommended power and height

To measure a spot’s natural sunlight instead, use the phone light meter.

Frequently asked questions

It depends on area and hanging height — which is why targeting DLI beats targeting watts. The calculator gives a recommended power once you enter the grow-box dimensions and lamp type.

PPFD is the instantaneous photosynthetic light level (µmol/m²/s); DLI is its daily total (mol/m²/day). DLI = PPFD × hours × 3600 ÷ 1,000,000.

For typical white LED a factor of about 0.017 is used (PPFD ≈ lux × 0.017). The value depends on the lamp spectrum — sunlight and horticultural LEDs use slightly different factors.

High enough to hit the target intensity — higher means weaker (inverse-square falloff). The calculator computes a recommended height for the chosen plant and lamp.