DLI Calculator for Houseplants: Is Your Light Enough?
A DLI calculator multiplies light intensity by time: PPFD × hours × 0.0036 gives the daily light integral in moles per square meter per day. A grow light delivering 100 µmol/m²/s at leaf level for 12 hours adds up to about 4.3 mol, which Purdue Extension’s table rates as good quality for peace lilies, ferns and African violets.
I run grow lights in my darkest corners, and a light meter reading means little until it’s turned into a daily total and set against what the plant needs. Enter what you measured, the hours, and the plant.
- PPFD, lux or foot-candles to daily light integral
- Targets for 25 houseplants from Purdue Extension
- Hours or brightness needed to hit the target
DLI calculator for houseplants
Enter your light reading and how many hours it’s on. See the daily light integral and whether it’s enough for your plant.
1. What reading do you have?
Use the figure for the height your light actually hangs above the leaves.
Purdue Extension’s factor for sunlight: 1 foot-candle = 0.20 µmol/m²/s.
2. Hours of light per day
Timer hours for a grow light. For a window, a single midday reading held all day overstates what the plant gets.
3. Your plant
Snake plant, pothos, ZZ and spider plant aren’t in Purdue’s table.
Houseplant targets for the DLI calculator
Low-light foliage plants such as peace lily and ferns look good at about 4 mol/m²/day, while dracaena and Chinese evergreen want about 6 and weeping fig about 12. Each row gives the least for acceptable quality, the level for good quality, and the range for top quality.
| Plants | Minimum | Good | High quality |
|---|---|---|---|
| Ferns (brake, maidenhair), prayer plant, moth orchid, African violet, peace lily | about 2 | about 4 | 6 to 10 |
| Chinese evergreen, bromeliads, dumb cane, dracaena, Boston fern, Cape primrose | about 4 | about 6 | 8 to 14 |
| English ivy | about 4 | about 6 | 8 and up |
| Rieger begonia, gloxinia, Christmas cactus, cyclamen, kalanchoe | about 4 | 6 to 8 | 10 and up |
| Poinsettia | about 6 | 8 to 10 | 12 and up |
| Schefflera, zonal geranium, hibiscus, miniature rose | 6 to 8 | 10 to 12 | 14 and up |
| Weeping fig, croton | 8 to 10 | 12 to 16 | 18 and up |
I read these values off the bar chart in Purdue Extension’s guide to measuring daily light integral, whose columns step by 2 mol, so treat each figure as give or take 1. They are greenhouse production targets, the light that grows a saleable plant, not the least a plant can live on. Snake plant, pothos, ZZ and spider plant aren’t in the table, and I haven’t found a university figure for them, so they’re left out rather than guessed.
Purdue also sorts crops into broad classes: 3 to 6 mol/m²/day for low-light crops, 6 to 12 for medium, 12 to 18 for high and more than 18 for very high. If your plant isn’t listed, those bands are a starting point for the calculator’s custom target.
How many hours should a grow light be on?
Usually 12 to 14 hours for foliage plants, and fewer than that is enough only if the light is bright. The table shows the hours each intensity needs to reach a daily target; anything past 16 means the light needs to be brighter or closer, not on longer.
| PPFD at the leaves | 2 mol | 4 mol | 6 mol | 8 mol | 12 mol |
|---|---|---|---|---|---|
| 50 µmol/m²/s | 11 h | over 16 h | over 16 h | over 16 h | over 16 h |
| 100 µmol/m²/s | 5.6 h | 11 h | over 16 h | over 16 h | over 16 h |
| 150 µmol/m²/s | 3.7 h | 7.4 h | 11 h | 14.8 h | over 16 h |
| 200 µmol/m²/s | 2.8 h | 5.6 h | 8.3 h | 11 h | 16.7 h |
| 300 µmol/m²/s | 1.9 h | 3.7 h | 5.6 h | 7.4 h | 11 h |
The 16-hour line comes from Illinois Extension, which says not to give plants more than 16 hours of light because they need a rest period. University of Minnesota Extension sets timers at 12 to 14 hours for foliage houseplants and 14 to 16 for flowering ones. So a peace lily aiming for 4 mol on a 12-hour timer needs about 93 µmol/m²/s at its leaves, and a dracaena aiming for 6 needs about 139.
Distance changes all of this. Minnesota notes that PPFD goes down as plants get further from the light, and one grower measured how much on r/RareHouseplants:
“Under one of my grow lights, the closest plant (a silver Squill) gets 6.9 mol/day while the farthest one (a Philodendron tortum) gets 1.7 mol/day”
– r/RareHouseplants, July 2024 (5 upvotes)
That is a fourfold difference under one fixture. Take the reading where each plant’s leaves actually are, not under the center of the lamp. My guide to grow lights for indoor plants covers hanging height and fixture choice.
What a window adds up to
A north window falls short of every target in the table. Illinois Extension puts the spot directly in front of one at about 150 foot-candles, and even held steady for 12 hours that is only about 1.3 mol/m²/day, under the roughly 2 mol Purdue lists as the minimum for a peace lily.
A few feet back, at Illinois’ 75 foot-candles, it’s about 0.6.
Real daylight ramps up and down, so the true totals are lower still. That doesn’t mean a peace lily dies in a north window. Minnesota’s point is that a plant may tolerate lower light, but more light may be required for dense foliage and flowering, which is the gap the table measures. My list of plants for a north-facing window sticks to the ones that cope with it.
Converting lux and foot-candles to PPFD
Purdue’s factor for sunlight is 0.20 µmol/m²/s per foot-candle, and 0.13 for high-pressure sodium lamps; one foot-candle is 10.764 lux. A phone reading of 1,000 lux in daylight is about 93 foot-candles, or about 19 µmol/m²/s, and 12 hours of it comes to about 0.8 mol/m²/day.
Plenty of people meet these numbers the other way round, with a meter in hand and no idea what to compare it to. A post on r/houseplants urging people to buy a light meter before another grow light drew 164 upvotes:
“I have ordered 3 fairly expensive grow lights from Amazon. I test them with my light meter & some don’t test higher than 300 lux.”
– r/houseplants, March 2025 (164 upvotes)
Several of the top replies asked the obvious follow-up: what do you do with the number once you have it? Here is the answer. Run 300 lux through the daylight factor and you get about 5.6 µmol/m²/s, roughly 0.24 mol over 12 hours, about an eighth of the minimum for even the lowest-light plants in the table.
That figure is rough, because no single factor fits every LED. The conversion depends on the light’s spectrum, so for an LED the PPFD chart the maker publishes for your hanging height beats any lux conversion. For window light, Purdue’s method is to average readings taken through the whole day, divide by 24 and multiply the PPFD by 0.0864; its worked example averages 184 foot-candles, which becomes 36.8 µmol/m²/s and 3.2 mol/m²/day.
DLI questions
What is a good DLI for houseplants?
A good DLI for low-light houseplants such as peace lily, ferns and African violets is about 4 mol/m²/day, with about 2 as the minimum. Dracaena, Chinese evergreen and dumb cane look good at about 6, Christmas cactus and cyclamen at 6 to 8, and weeping fig at 12 to 16, based on Purdue Extension’s table.
How do you calculate DLI?
Multiply PPFD in µmol/m²/s by the hours of light and by 0.0036. The 0.0036 is the 3,600 seconds in an hour divided by 1,000,000 micromoles in a mole, so 150 µmol/m²/s for 12 hours gives about 6.5 mol/m²/day.
How do I convert lux to PPFD?
For daylight, divide lux by 10.764 to get foot-candles and multiply by 0.20, which works out to about 0.019 µmol/m²/s per lux. For high-pressure sodium the foot-candle factor is 0.13; LEDs vary with their spectrum, so use the maker’s PPFD figures.
What DLI do low-light plants need?
Low-light plants need roughly 2 to 8 mol/m²/day. Minnesota Extension’s low-light band of 50 to 150 µmol/m²/s, run for its recommended 12 to 14 hours, works out to about 2.2 to 7.6 mol, close to the 2 to 10 range Purdue gives for peace lilies and ferns. My peace lily care guide covers the rest of what a low-light plant needs.