PPFD and DLI for Cannabis: Light Targets by Growth Stage, With the Math

Nov 6

5 min read

Updated: 4 days ago

Two numbers decide most lighting conversations in a cannabis room: PPFD, how much light lands on the canopy every second, and DLI, how much light the canopy collects over a whole day. Get them right and fixture count, hang height and power budget follow. Get them wrong and you either leave yield on the table or pay to light a room past what your climate can support.

This post covers what each number means, the formula that links them, worked values for veg and flower photoperiods, and what the research says about how much light flowering cannabis can use.

PPF, PPFD and DLI in one minute

PPF (µmol/s) is a fixture's total photon output in the 400–700 nm range. It's on the spec sheet. The AG-140 bar, for example, puts out 455 µmol/s in R80 Red and 420 µmol/s in 3K Full Phase.
PPFD (µmol/m²/s) is how many of those photons land on one square metre of canopy each second. It depends on fixture output, layout, hang height and how much light is lost off the edges of the canopy. You measure it with a quantum sensor or model it in a light plan.
DLI (mol/m²/day) is the total light the canopy receives in a day. It is PPFD multiplied by the hours the lights are on.

The DLI formula

DLI = PPFD × hours of light × 3,600 ÷ 1,000,000

Or, more simply: DLI = PPFD × hours × 0.0036.

PPFD (µmol/m²/s)	18 h (veg) DLI	12 h (flower) DLI
300	19.4	13.0
400	25.9	17.3
600	38.9	25.9
900	58.3	38.9
1,200	77.8	51.8
1,500	97.2	64.8
1,800	116.6	77.8

Two things stand out. First, because flower runs on 12 hours instead of 18, you need 1.5 times the PPFD in flower to deliver the same DLI as veg. Second, small changes in PPFD add up: going from 900 to 1,200 µmol/m²/s in flower adds about 13 mol/m²/day.

Common starting points by stage

There is no single correct target. The ranges below are common starting points in commercial rooms, which you then adjust for cultivar, CO₂, temperature and how well your HVAC handles the load.

Clones and propagation: roughly 100–300 µmol/m²/s. Cuttings have no roots yet and can't use much light.
Vegetative: roughly 300–600 µmol/m²/s on 18/6, which is about 19–39 mol/m²/day.
Flower without added CO₂: roughly 700–1,000 µmol/m²/s on 12/12, or about 30–43 mol/m²/day.
Flower with added CO₂ and strong climate control: many growers push beyond 1,000 µmol/m²/s.

Ramp up over the first days of a stage rather than jumping to the target, and watch the plants. Bleaching at the top of the canopy or upturned leaf edges are signs you've gone past what the room can support.

What the research says about high light in flower

The most cited study on this question is Rodriguez-Morrison, Llewellyn and Zheng at the University of Guelph, published in Frontiers in Plant Science in 2021. They flowered cannabis for 12 weeks on 12/12 at canopy PPFDs from 120 to 1,800 µmol/m²/s under LEDs. Dry flower yield rose linearly across the whole range, from 116 g/m² to 519 g/m², about 4.5 times as much. Flower density and harvest index rose with light too, while cannabinoid potency did not change with light level.

Two caveats matter. The leaves' photosynthesis saturated well below 1,800, yet whole-plant yield kept climbing, so leaf measurements alone are a poor guide to the right target. And the right number for your room is an economic one: every extra µmol costs power and HVAC capacity, so the authors frame their models as a tool to find the best return, not as a rule to run 1,800.

Top-of-canopy PPFD isn't the whole story

PPFD is usually measured at the top of the canopy, but much of a dense flower canopy sits below that line, and light from above falls off quickly as it passes through leaves. A 2025 study in Plants found that adding light from below and within the canopy raised dry flower yield by 24.58% and 29.95% (two under canopy setups) over top lighting alone. We cover it in Does Under Canopy Lighting Increase Yield?. If your top PPFD is already high and the climate is maxed out, the lower canopy is often the cheaper place to add light. See under canopy lights.

From PPF to PPFD: a rough check

You can sanity-check a layout with simple division. Divide the total PPF of the fixtures over an area by that area in square metres to get the maximum average PPFD, before any light is lost off the edges or absorbed by walls.

Example: one fixture with a PPF of 2,354 µmol/s, such as the ROI-F720, spread over 5 × 5 ft (2.32 m²) gives at most about 1,014 µmol/m²/s on average. Real rooms deliver less, and the edges get less than the centre. That gap, and how even the light is across the bench, is exactly what a photometric plan measures.

How to plan a room around a target

Pick a DLI or PPFD target for each room and stage. Be honest about your CO₂ and cooling.
Model the layout, including fixture count, spacing and hang height, and check the average, minimum and uniformity across the canopy, not only the centre.
Check power and circuits for the fixture count the model gives you.
Add dimming control so you can ramp up through each stage. A Master Controller runs up to 100 AG-140 bars directly, or up to 1,000 per group using the bar's built-in signal booster.
Verify with a quantum sensor after install and adjust.

Our free light plan does steps 1 to 3 on paper before you buy: a PPFD map, average and uniformity figures, DLI at your photoperiod, power draw, and a check for utility rebates. Request a free light plan.

Adding light to the lower canopy? Start with the AG-140 Under Canopy LED Kit or browse all under canopy lights.

Sources

Rodriguez-Morrison V., Llewellyn D., Zheng Y. (2021). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
Garrido J. et al. (2025). Peer-reviewed supplemental lighting study. Plants 14(10):1469. https://doi.org/10.3390/plants14101469

FAQ

How do you convert PPFD to DLI?

Multiply PPFD by the hours of light and by 0.0036. For example, 900 µmol/m²/s for 12 hours is 900 × 12 × 0.0036 = 38.9 mol/m²/day.

What PPFD does cannabis need in flower?

Common starting points are roughly 700–1,000 µmol/m²/s without added CO₂, and higher with CO₂ and strong climate control. In a 2021 University of Guelph study, yield kept rising linearly up to 1,800 µmol/m²/s, but the best level for your room depends on power and HVAC costs.

What DLI should I target for cannabis?

On 12/12 in flower, 700–1,000 µmol/m²/s works out to about 30–43 mol/m²/day. In veg on 18/6, 300–600 µmol/m²/s is about 19–39 mol/m²/day.

Does more light lower THC?

In the Guelph study, light level did not change cannabinoid potency, but yield rose with light, so total cannabinoid output per square metre went up.

How do I know what PPFD my fixtures will deliver?

Model the room before you buy. A photometric light plan shows average, minimum and maximum PPFD across the canopy for a given layout and hang height. Ours is free.