How the calculation works

PPFD (photosynthetic photon flux density) is the amount of light plants can use that lands on each square metre every second, in micromoles per square metre per second (µmol/m²/s). The daily light integral (DLI) adds that up over a whole day, in moles per square metre per day (mol/m²/day). Purdue Extension compares DLI to a rain gauge: it collects all the light that falls in a day, not just how bright it is at one moment.

DLI = PPFD × hours of light × 3,600 ÷ 1,000,000. There are 3,600 seconds in an hour and a million micromoles in a mole, so this is the same as PPFD × hours × 0.0036, the formula Iowa State Extension gives.
PPFD needed = target DLI ÷ (hours × 0.0036).
Hours needed = target DLI ÷ (PPFD × 0.0036).
Running cost: watts × hours a day × 30 days ÷ 1,000 gives kWh a month. Multiply by your price per kWh.

Worked example

An LED panel gives 200 µmol/m²/s at the top of a tray of lettuce and runs 16 hours a day: 200 × 16 × 0.0036 =11.52 mol/m²/day. That is just below the 12–17 range for lettuce, whose middle is 14.5.

To reach 14.5 you could make the light brighter at the plants: 14.5 ÷ (16 × 0.0036) = about 252 µmol/m²/s, for example by hanging it closer if the maker’s PPFD chart shows that gets you there, or by using a stronger light. Or you could keep 200 µmol/m²/s and leave it on longer: 14.5 ÷ (200 × 0.0036) = 20.1 hours a day. That is more than the 16 hours Iowa State suggests as a maximum, so here a brighter light is the better fix.

How much light do plants need?

The plant ranges in the calculator come from university extension services and research:

Plants	Target DLI (mol/m²/day)	Where it comes from
Vegetable seedlings and transplants	8–14	UNH Extension suggests 8–14 for seedlings, even of sun-loving crops, and about 12 for transplants.
Microgreens	9–12	Cornell trials tested 3–12. Mizuna and mustard microgreens grew more with each step up to 12, the highest level tested, so aim for the top of that range.
Lettuce and leafy greens	12–17	University of Kentucky Extension gives 12–17 as the best range for lettuce. Cornell research found more than 17 caused tipburn, and Cornell suggests 13–14 if no fan moves air over the plants.
Leafy herbs such as basil	12–16	Iowa State Extension puts herbs and vegetables grown for their leaves, such as basil, at 12–16.
Fruiting crops: tomatoes, peppers	18–30	Iowa State Extension gives 18–30. Purdue's crop table shows acceptable tomatoes and peppers from about 10–12, good from 14 and the best from 22 up.
Houseplants: low light (ferns, peace lily, African violet)	3–6	Iowa State Extension's range for low-light foliage houseplants. UNH Extension suggests 4–6 for most houseplants.
Houseplants: medium light (Chinese evergreen, dracaena)	6–10	Iowa State Extension's range for medium-light foliage and flowering houseplants. Purdue groups medium-light crops at 6–12.
Houseplants: high light (succulents, flowering houseplants)	12–16	Iowa State Extension's range for succulents and flowering houseplants. Purdue groups high-light crops at 12–18.

The sources don’t always agree, so a few ranges need a word of explanation:

Lettuce: University of Kentucky Extension gives 12–17 as the best range and about 6.5–9.7 as the minimum, where lettuce takes longer to reach harvest. Iowa State lists leafy vegetables such as lettuce in both its 12–16 and 18–30 groups, but Cornell found that more than 17 caused tipburn, so the calculator treats 17 as a ceiling. Cornell uses 17 only with fans blowing air down over the plants, and suggests 13–14 without good airflow.
Tomatoes and peppers: Iowa State gives 18–30 for fruiting crops. Purdue’s crop table shows good quality from 14 and the best from 22 up, and UNH Extension suggests 14–20 for full-sun plants. The calculator uses Iowa State’s 18–30.
Microgreens: Cornell’s trials only tested 3–12. Mizuna and mustard kept growing more right up to 12, so the calculator uses the top of the tested range, 9–12.
Houseplants: the example plants come from Purdue’s crop table, which puts ferns, African violets and peace lilies among the lowest-light crops and Chinese evergreens and dracaenas one step up.

Longer hours or a brighter light?

For DLI, brightness and hours trade off evenly: halve the PPFD and you need twice the hours. Iowa State Extension suggests 12–14 hours a day for most indoor plants, and between 10 and 16 for home growers. It also warns that changing day length can change when photoperiodic plants flower, so for those, change the brightness rather than the hours. UNH Extension notes that tomatoes need about 6 hours of darkness to fruit, while peppers don’t.

Measuring PPFD, and why lux is only a guess

The number to use is the PPFD at the top of your plants, from the grow light maker’s PPFD chart at your hanging height or from a quantum (PAR) sensor. Purdue Extension explains that foot-candles (and lux, their metric counterpart) are based on light as the human eye sees it, so they are not a good measure of the light plants use for photosynthesis.

If a lux meter or phone app is all you have, the converter in the calculator multiplies your reading by Apogee Instruments’ factor for your type of light. The factor is different for each type of light, and Apogee’s table has none for LED grow lights. Purdue’s foot-candle factors, 0.20 µmol/m²/s per foot-candle for sunlight and 0.13 for high-pressure sodium, agree with Apogee’s.

Type of light	Multiply lux by	Multiply foot-candles by
Sunlight	0.0185	0.199
Cool white fluorescent	0.0135	0.145
High-pressure sodium (HPS)	0.0122	0.131
Metal halide	0.0141	0.152
Ceramic metal halide, 4200 K	0.0154	0.166
Ceramic metal halide, 3100 K (red-shifted)	0.017	0.183

Common questions

What DLI do seedlings need?: UNH Extension suggests 8–14 mol/m²/day for seedlings, even of sun-loving crops, and about 12 for transplants. A light giving 250 µmol/m²/s at seedling height reaches 12 mol/m²/day in about 13.3 hours a day (12 ÷ (250 × 0.0036)).
Is it better to run a grow light longer or make it brighter?: For DLI it makes no difference: 300 µmol/m²/s for 12 hours and 240 µmol/m²/s for 15 hours both give 12.96 mol/m²/day. Iowa State Extension suggests 12–14 hours a day for most indoor plants and no more than 16 for home growers. It also warns that changing the day length can change when photoperiodic plants flower, so for those plants change the brightness instead.
Can I use a lux meter or phone app instead of a PAR meter?: Only for a rough estimate. Apogee Instruments gives a factor of 0.0185 µmol/m²/s per lux for sunlight, so 10,000 lux of sunlight is about 185 µmol/m²/s. The factor is different for every type of lamp and Apogee lists none for LED grow lights, so for LEDs use the maker’s PPFD chart or a quantum (PAR) sensor.
Why are my lettuce leaf edges turning brown under strong light?: It may be tipburn, a calcium shortage in fast-growing leaves. Cornell research found lettuce got tipburn when it was given more than 17 mol/m²/day, and that fans moving air over the plants delayed it. Cornell’s CEA program uses 17 only with fans blowing air down over the plants, and suggests a lower target of 13–14 without good airflow.
How much does a grow light cost to run?: Multiply the light’s watts by the hours it runs each day and by 30 days, then divide by 1,000 to get kWh a month. A 100-watt light on for 16 hours a day uses 48 kWh a month; at 0.15 per kWh that is 7.20 a month in the same currency. Use the watts the light actually draws, from its label or spec sheet.

Sources

Last checked and updated: October 31, 2026.