Free tool

Grow Light Calculator

This LED grow light calculator turns your fixture’s watts or PPF into the average PPFD and DLI on your canopy. It also shows how many lights your crop needs.

I know my light’s

WattsPPF (µmol/s)

Watts per fixture

Wall-socket watts, not the “equivalent” on the box.

Number of fixtures

lights

Canopy in

FeetMetres

Common tent sizes (floor)

Canopy length

Canopy width

Target for

Lights on

h/day

More options

Efficacy (only with watts)

µmol/J

From the spec sheet. 2.5 is the current DLC minimum.

Light reaching the canopy

90% is a best case; open rooms with gaps catch less.

Average PPFD on the canopy

—

Daily light (DLI): —
Target DLI: —
PPFD for that target PPFD for that target PPFD is the light landing on each square metre every second (µmol/m²/s). The daily light integral (DLI) is that light added up over the hours the lights are on.: —
Fixtures for the target: —
Power per area: —

Next: Turn your PPFD into a daily light integral

How it works

A fixture’s PPF is the light it produces each second. Spread over the canopy, after taking off the share that misses it, that becomes the average PPFD. Over a day of lights-on hours it adds up to the DLI. This is an average: the middle of the canopy gets more than the edges.

Already measured your PPFD? Go straight to the DLI calculator.

Watts per square foot can mislead

Two lights of the same watts can give very different light: a 2.8 µmol/J LED gives about 40% more than a 2.0 µmol/J one. Compare PPF or PPFD, not watts.

Formula

PPF   = watts × efficacy        µmol/s
PPFD  = PPF × lights × share on canopy ÷ area (m²)
DLI   = PPFD × hours × 3,600 ÷ 1,000,000

LED grow light distance chart

Starting heights from two makers’ guides, from the top of the canopy to the light. Every fixture spreads its light differently, so use your own light’s chart first, then check with a PAR meter.

Suggested LED hanging heights by growth stage, in inches
Stage	Spider Farmer SF series	VIVOSUN guide
Seedlings and clones	16–22 in	12–24 in
Vegetative	14–16 in	18–24 in
Flowering	12–14 in	12–18 in

1 inch = 2.54 cm, so 12–24 in is about 30–60 cm. Too close: bleached or upturned leaf tips. Too far: stretched, leggy plants.

What an LED grow light calculator tells you

The watts on a grow light say how much electricity it uses, not how much light reaches your plants. This calculator starts from the light itself: watts times efficacy (µmol/J), or the PPF from the spec sheet. It spreads that over your canopy and gives the average PPFD, and the daily light integral (DLI) at your day length.

That lets you compare lights by what matters to the plant, and check a fixture against your crop’s target before you buy it. Both defaults, efficacy and the share of light on the canopy, can be changed under More options; your own spec sheet is better than either.

The share on the canopy matters as much as the light. The default 90% is a best case, measured in large greenhouses with narrow aisles. If a quarter of the light misses (75%), the 600 W example below drops from about 908 to about 757 µmol/m²/s.

Worked example: a 600 W LED in a 4×4 tent

Take one 600 W LED at 2.5 µmol/J over a 4×4 ft canopy (1.49 m², 16 ft²), with 90% of the light landing on it.

PPF: 600 × 2.5 = 1,500 µmol/s.
Average PPFD: about 908 µmol/m²/s.
DLI over 18 hours (veg): 58.9 mol/m²/day.
DLI over 12 hours (flower): 39.2 mol/m²/day.
Power per area: 37.5 W/ft².

Against the cannabis target of 30–40 mol/m²/day, the same light is right for a 12-hour flowering day and well above target for an 18-hour veg day. In veg you would dim it to about 51–68% of full power, then check again.

Grow light wattage for a 4×4 tent

Working backwards, these are the wall-socket watts a 2.5 µmol/J LED needs on a 4×4 ft canopy for the cannabis band, with 90% on the canopy:

12-hour flowering day: about 459–612 W, for 694–926 µmol/m²/s.
18-hour veg day: about 306–408 W, for 463–617 µmol/m²/s.

That is about 29–38 W/ft² in flower and 19–26 W/ft² in veg. Change the efficacy and those figures move: at 2.0 µmol/J the same 600 W gives about 727 µmol/m²/s, at 2.8 about 1,017. A watts-per-square-foot rule only holds for the efficacy it was written for.

How to act on the result

Below target: add a fixture or, in veg, more hours. The calculator shows how many fixtures reach the low end of the target, rounded up.
Above target: dim the light or, in veg, shorten the day. Photoperiod plants in flower need about 12 hours of dark, so change the intensity, not the hours.
Above about 1,500 µmol/m²/s: most plants need extra CO₂ to use the light. Plan it with the CO₂ calculator for grow rooms.
Other crops: pick them from the list. Lettuce wants 12–17 mol/m²/day, only 185–262 µmol/m²/s over 18 hours, so a light sized for cannabis is far more than it needs.
Bigger canopy: on a 4×8 ft canopy the same 600 W LED gives about 454 µmol/m²/s, so the calculator asks for two fixtures for either stage.
Before you buy: price the extra watts with the grow light electricity cost calculator.

Common mistakes, and the grow light distance chart

Typing the “equivalent” watts from the box instead of the wall-socket watts.
Reading the average as the figure under the light. The centre of the canopy gets more and the corners less.
Setting the canopy share to 100%. Some light always lands on walls and floor.

Many distance charts give one height per stage. The two makers in the chart below disagree: 14–16 inches for veg from one, 18–24 from the other. That is the honest picture. Height depends on the fixture, and the real check is PPFD at the canopy with a PAR meter.

FAQ

Questions growers ask

What PPFD does cannabis need?

About 694–926 µmol/m²/s over a 12-hour flowering day, or 463–617 over an 18-hour veg day. Both come from the 30–40 mol/m²/day DLI growers are traditionally advised. One study found yield still rising up to about 78 mol/m²/day, so treat the band as a starting point, not a ceiling.

How many watts of LED does a 4×4 tent need?

About 459–612 wall-socket watts for flower at 12 hours a day, and 306–408 W for veg at 18 hours. That assumes a 2.5 µmol/J LED with 90% of its light on the canopy. A more efficient fixture needs fewer watts, so enter your own light’s efficacy to check.

Is watts per square foot a good way to size grow lights?

Only as a rough check, because it ignores efficacy. On a 4×4 ft tent, 600 W gives about 727 µmol/m²/s at 2.0 µmol/J and 1,017 at 2.8, at the same 37.5 W/ft². Compare PPF or PPFD from the spec sheet instead, then use watts for the running cost.

What is the difference between PPFD and PAR?

PAR is the kind of light: the wavelengths plants use for photosynthesis. PPFD is how much of that light lands on a square metre each second, in µmol/m²/s. A so-called PAR meter reads PPFD. The DLI adds PPFD up over the hours the lights are on.

How many grow lights do I need?

Enough to reach the low end of your crop’s target, which the calculator rounds up to whole fixtures. One 600 W, 2.5 µmol/J LED covers a 4×4 ft canopy for cannabis. On a 4×8 ft canopy it gives about 454 µmol/m²/s, so a 12-hour flowering day needs two.

Why does my PAR meter read more in the middle?

Because the calculator gives the average over the whole canopy. Light is strongest under the fixture and falls off towards the walls, so the centre reads above the average and the corners below it. Measure several points across the canopy and average them before you compare.

Can a 600 W LED be too strong for veg?

Yes, at 18 hours a day on a 4×4 ft tent. At 2.5 µmol/J it gives about 908 µmol/m²/s, which adds up to 58.9 mol/m²/day, well above the 30–40 cannabis target. Dimming it to about 51–68% of full power brings it into the band.

Where do I find my light’s PPF or efficacy?

On the spec sheet: PPF in µmol/s, or efficacy (PPE) in µmol/J. Multiply efficacy by the wall-socket watts to get PPF. If you only know the watts, 2.5 µmol/J is the current minimum for a DesignLights Consortium listed fixture; good LEDs are 2–3 µmol/J.

Sources

The formulas and target ranges on this page come from these references.

DesignLights Consortium, Horticultural Technical Requirements v4.0 (effective April 2025): minimum photosynthetic photon efficacy 2.5 µmol/J. designlights.org/our-work/horticultural-lighting/technical-requirements/hort-v4-0/
Kusuma, Pattison & Bugbee (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7, 56 (typical fixtures 2–3 µmol/J). doi.org/10.1038/s41438-020-0283-7
Nelson & Bugbee (2014). Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS ONE 9(6), e99010 (canopy photon capture above 90% for large greenhouses with narrow aisles). doi.org/10.1371/journal.pone.0099010
Torres & Lopez, Purdue Extension HO-238-W: Measuring daily light integral in a greenhouse. www.extension.purdue.edu/extmedia/HO/HO-238-W.pdf
Mattson, e-GRO Edible Alert 2(5), 2017: daily light integral targets for greenhouse vegetables. www.e-gro.org/pdf/E205a.pdf
University of Kentucky, Center for Crop Diversification CCD-CP-63: Hydroponic lettuce. ccd.uky.edu/sites/default/files/2026-02/ccd-cp-63_hydrolettuce-updated_accessible.pdf
Cannabis Science and Technology 7(1), 2024: Cornell University research on hemp responses to light (“traditionally” 30–40 mol/m²/day). www.cannabissciencetech.com/view/cornell-university-research-on-hemp-responses-to-light-treatments-part-1
Rodriguez-Morrison, Llewellyn & Zheng (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12, 646020 (yield kept rising with DLI up to about 78). doi.org/10.3389/fpls.2021.646020
Park et al. (2023). Plants 12, 731, citing Ohio State: strawberry minimum DLI 10–12, optimum 20–25. doi.org/10.3390/plants12040731
Spider Farmer: SF series LED user manual, hanging recommendations (germination 22 in; seedling 16–22; veg 14–16; flower 12–14). cdn.shopify.com/s/files/1/0504/4713/files/Spider_Farmer_Led_user_manual.pdf
VIVOSUN (2026). How far away should grow lights be? (seedlings and clones 12–24 in; veg 18–24; flower 12–18). vivosun.com/growing_guide/how-far-away-should-lights-be/

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