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PPFD and DLI calculator

Find the light target for each stage, work out the daily light your plants actually get, and estimate what a fixture delivers in your tent. Members here run about 150 to 250 PPFD for seedlings, 400 to 600 in veg, and up to about 1000 in flower.

Checked Oct 20, 2026 Source: THCFarmer member consensus (threads 155972, 155349, 104635, 143081)
Light targets by stage PPFD targets by stage from member consensus: seedlings about 150 to 250, veg 400 to 600, flower up to about 1000 micromoles per square metre per second. Daily light integral equals PPFD times hours times 0.0036, giving about 10 to 16 for seedlings and 26 to 39 for veg at 18 hours, and up to about 43 for flower at 12 hours. FIG. 03 · LIGHT BY STAGE PHOTOPERIOD, 18/6 AND 12/12 Target light, by stage STAGE PPFD, µmol/m²/s DLI, mol/m²/day 0 200 400 600 800 1000 Seedling 18 HRS 150 TO 250 10 to 16 Veg 18 HRS 400 TO 600 26 to 39 Flower 12 HRS UP TO ~1000 up to ~43 DLI = PPFD × HOURS × 0.0036 RANGES: MEMBER CONSENSUS
01

Your daily light

From a meter or app reading

µmol/m²/s, measured at the top of the plants
Changes with the stage. Edit it to match your timer.
DLI 32.4 mol/m²/day
Target for this stage 400–600 PPFD · 25.9–38.9 DLI at 18 hours
Verdict In range for veg.
02

What your light delivers

An estimate when you have no meter

Actual draw, not the "equivalent" number on the box.
Average PPFD 765 across the canopy
DLI 33.1 mol/m²/day
Watts per sq ft 35.6
Good for Early flower (weeks 1 to 3).
03

Light targets by stage

Measured at the top of the canopy

Stage PPFD Typical hours DLI
Seedlings (first two weeks) 150 to 250 18 9.7 to 16.2
Clones and mother plants 200 to 400 18 13.0 to 25.9
Veg 400 to 600 18 25.9 to 38.9
Early flower (weeks 1 to 3) 600 to 800 12 25.9 to 34.6
Mid to late flower 800 to 1,000 12 34.6 to 43.2
Flower with added CO2 (about 1000 to 1200 ppm) 1,000 to 1,200 12 43.2 to 51.8

Autoflowers usually run 18 to 20 hours all the way through, so the same PPFD gives them a higher DLI than a photoperiod plant in flower. Type your own hours into the calculator above.

04

The plant is the real meter

Numbers get you close. The leaves tell you the rest. Flat, level, green leaves are happy. Edges curling up, leaves folding like a taco, or bleached tops mean too much light, so raise or dim the fixture. Leaves still reaching up an hour before lights-out can take more.

Change one thing at a time, by about 10 percent, and give the plants a few days before you judge.

05

What growers here actually run

From the THCFarmer archive

  • Aims for 200 to 250 PPFD for the first two weeks after sprout, then 400 to 500 until flower.

    Grower1974 Read the post →

  • Likes about 150 to 200 on seedlings, enough to stop them stretching. If they stretch, drop the light an inch.

    Mikedin Read the post →

  • Runs about 1000 PPFD at the canopy with 1000 ppm CO2 through flower. Much past 1000 seems to go unused.

    Homesteader Read the post →

  • Flat, level, green leaves are happy. Edge curl, taco-ing and bleaching mean too much light. Leaves still praying an hour before lights-out want more.

    ArtfulCodger Read the post →

  • Raises intensity until the plants complain, then backs off. For most LEDs, about 18 inches is a good minimum distance.

    LoveGrowingIt Read the post →
06

Threads worth reading

Discussions this page draws on

07

Go deeper

Learn guides on THCFarmer

08

Common questions

What is the difference between PAR, PPFD and DLI?

PAR is the band of light plants use for photosynthesis (400 to 700 nm). PPFD is how much of that light lands on one spot each second, in µmol/m²/s. That is what a light meter reads. DLI is the total that spot receives over a whole day, in mol/m²/day. It rises with both intensity and hours, so 600 PPFD for 18 hours delivers more light than 800 PPFD for 12.

How do I work out DLI?

Multiply PPFD by the hours your lights are on, then by 0.0036. For example, 600 PPFD for 12 hours gives 600 x 12 x 0.0036 = 25.9 mol/m²/day.

Can I measure PPFD with a phone app?

Apps like Photone use the phone camera and give a rough reading. Members use them to get in the right range, then watch the plants. A real quantum (PAR) meter is far more accurate, especially under purple or very white LEDs.

If I double the light height, does PPFD drop to a quarter?

Not with a wide LED in a reflective tent. The inverse-square rule assumes a single point of light in open space. A member who raised their light 1.9 times saw PPFD fall to about 39 percent, not the 28 percent the rule predicts. Measure after every move.

Should I hang my light at the manufacturer's recommended height?

Use it as a starting point. Members find the charts often hang lights too low for real tents and real plants. Start higher or dimmer, then raise the intensity a little at a time while you watch the leaves.

How accurate is the fixture estimate?

It is an average for the whole canopy, assuming about 80 percent of the light lands on the plants in a reflective tent. The center will read higher and the corners lower. Use your fixture's measured µmol/J from its spec sheet if you have it, and the watts it actually draws from the wall, not the equivalent wattage on the box.

Still not sure? Ask the growers.

Post your light, its height and dimming, your tent size and a photo of the canopy. Members will tell you if it's too much or not enough.

09

More grow tools

These numbers are starting points, not rules. Strains, water, medium and light all change what a plant wants, so watch your plants and adjust.

THCFarmer is not affiliated with any nutrient or equipment brand named on this page.

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