Grow Light Calculator: PAR to Lux and DLI

Two free calculators for anyone growing under lights. Convert a lux reading into PPFD (or a PPFD target into the lux to aim for), and work out your daily light integral (DLI) from PPFD and hours. Each one tells you which growth stage your number fits, using the targets from our grow light distance guide and light schedule guide.

Light targets by stage

Stage	PPFD	Lux, white LED (roughly)	DLI
Clones, until rooted	50 to 150	3,000 to 10,000	3 to 10 at 18/6
Seedlings, weeks 1 and 2	200 to 400	13,000 to 27,000	13 to 26 at 18/6
Seedlings, weeks 3 and 4	400 to 600	27,000 to 40,000	26 to 39 at 18/6
Vegetative growth	600 to 900	40,000 to 60,000	39 to 58 at 18/6
Flowering	600 to 900, more if the tops stay cool	40,000 to 60,000	26 to 39 at 12/12

The reasons behind each number, and how to hang a light to hit them evenly, are in our grow light distance guide. One thing worth knowing before you move a light to change these numbers: in a reflective grow tent, raising or lowering a light barely changes the average. It changes how evenly the light lands. Set the amount with the dimmer.

Where the conversion factors come from

Lux measures light the way a human eye sees it, which is mostly green; plants use red and blue too. So the number of lux in one unit of PPFD depends on the light's color mix, and every light type needs its own factor.

Light source	Lux per 1 PPFD	Source
Sunlight	54	Apogee, after Thimijan and Heins1
Cool white fluorescent	74	Apogee, after Thimijan and Heins1
High pressure sodium	82	Apogee, after Thimijan and Heins1
Double-ended HPS	77	Apogee1
Metal halide	71	Apogee, after Thimijan and Heins1
Ceramic metal halide, 4200 K and 3100 K	65 and 59	Apogee1
White LED	67 (about 15 PPFD per 1,000 lux)	Sharakshane2, a preprint
White LED with added deep red	52	Our estimate for a light getting a fifth to a third of its output from 660 nm red; no published factor exists

Treat any lux conversion as a good estimate, not a measurement. If you can borrow a quantum PAR meter once, read the same spot with both and you have a correction for your own light. Our guide covers meters, phone apps and lux meters.