Grow Light DLI & Timer Calculator
Turn your grow light’s PPFD and hours into daily light integral (DLI), or work backwards from a target DLI to the hours and switch-off time for your timer.
PPFD and DLI in plain terms
PPFD (photosynthetic photon flux density) is how intense the plant-usable light is at one moment, measured where it lands on the leaves. The unit is µmol/m²/s: micromoles of photons in the 400–700 nm range hitting one square meter every second. It drops quickly as a light is raised, so the number only means something at a stated distance.
DLI (daily light integral) is the total of that light over a whole day, in mol/m²/day. Two setups can deliver the same DLI in different ways: a dim light for a long day or a brighter light for a short one.
The PPFD to DLI formula
DLI = PPFD × hours × 0.0036
The 0.0036 is a unit conversion, not a fudge factor. An hour has 3,600 seconds, and a mole is 1,000,000 micromoles, so 3,600 ÷ 1,000,000 = 0.0036 turns µmol per second into mol per hour of light.
Worked example: a 150 PPFD grow light
| Runtime | Calculation | DLI |
|---|---|---|
| 10 hours | 150 × 10 × 0.0036 | 5.40 mol/m²/day |
| 12 hours | 150 × 12 × 0.0036 | 6.48 mol/m²/day |
| 14 hours | 150 × 14 × 0.0036 | 7.56 mol/m²/day |
Each extra two hours at 150 PPFD adds 1.08 mol/m²/day. Doubling the PPFD to 300 would double every figure in the table without changing the timer.
How many hours should a grow light run?
If you already have a target DLI from a care guide, a grower or an extension source, rearrange the formula to get the runtime:
hours = target DLI ÷ (PPFD × 0.0036)
For a target of 7.56 mol/m²/day under 150 PPFD: 7.56 ÷ (150 × 0.0036) = 7.56 ÷ 0.54 = 14 hours. Switch the light on at 07:00 and the timer should turn it off at 21:00. The calculator rounds any part-minute up, so the timer always reaches the target rather than falling just short.
When the answer comes out above 24 hours, the target is out of reach at that PPFD, and the calculator says so instead of quietly capping it at 24. Even a reachable target that leaves fewer than 8 hours of darkness is flagged: Iowa State Extension suggests keeping indoor-plant timers between 8 and 16 hours, and a brighter light or a closer hanging height is usually a better fix than an all-day timer.
If the plant also gets window light, a target is only met by the two together. Supplemental-lighting worksheets handle that by subtracting the natural light DLI from the target first, then running the lamp for the shortfall. You can do the same here: enter the target minus your measured window DLI as the target.
What this calculator does not do
- It only counts the grow light. It assumes the light runs at a constant output for the whole period. Sun through a window adds to the total and is not included.
- It doesn’t pick a target for your plant. It does the light math; it doesn’t diagnose what a particular species needs. Two plants given the same DLI can grow, color up or flower quite differently, and day length matters on its own for some flowering plants.
- It doesn’t convert lux to PPFD. Lux is weighted to human vision, so the ratio between lux and PPFD changes with the light’s spectrum. Any single conversion factor is only an approximation for one kind of lamp.
- It doesn’t treat phone readings as PPFD. Light-meter apps estimate from a phone camera or ambient sensor. They can show whether one spot is brighter than another, but they aren’t calibrated PPFD measurements.
- It doesn’t recommend 24-hour lighting. A full-day result is shown only as arithmetic, with a warning about the missing dark period.
Frequently asked questions
How do I convert PPFD to DLI?
Multiply PPFD (µmol/m²/s) by the hours the light is on and by 0.0036. A light giving 150 PPFD for 12 hours delivers 150 × 12 × 0.0036 = 6.48 mol/m²/day.
How many hours should my grow light run?
Divide your target DLI by PPFD × 0.0036. With 150 PPFD and a 7.56 mol/m²/day target, that is 14 hours. If the result is longer than about 16 hours, the light is probably too weak or too far away for that target.
Does this calculator include sunlight?
No. It calculates the grow light’s contribution only. If a plant also gets window light, subtract your estimate of that daylight DLI from the target before calculating runtime.
Can I convert lux directly to PPFD?
Not with one universal number. The lux-to-PPFD ratio depends on the light’s spectrum, so it differs between sunlight, white LEDs and other lamps. A PAR meter, or a PPFD chart from the light’s maker at your hanging height, is more reliable.
Can the same DLI be used for every plant?
No. Plants adapted to shade and plants adapted to sun need very different totals, and identical DLI can still give different growth or flowering between species. Look up your plant’s light needs first, then use this tool to plan the hours.
Keep going
- Grow lights for houseplants: choosing a light, hanging height and timers.
- What “bright indirect light” actually means: reading plant labels and windows.
- Low-light houseplants that actually survive and the low-light collection in the encyclopedia.
- Full-sun houseplants: the plants most likely to need a grow light indoors.
- Winter houseplant care: when shorter days make supplemental light worth it.
- Plant encyclopedia: light, water and care notes for 282 houseplants.
Sources
- Iowa State University Extension and Outreach, How to determine how much supplemental light to provide indoor plants (last reviewed December 2023).
- University of New Hampshire Extension, Supplemental lighting run time worksheet.
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