Plants do not respond to how bright a light looks, but to how much usable light they receive over a day. That is the daily light integral, or DLI. It is the total number of light particles in the plant growing range that land on a square meter each day. This grow light DLI calculator turns the light intensity at your plants, and the hours the light is on, into a DLI and compares it with typical targets for seedlings, greens, herbs and fruiting crops.
How to use the grow light DLI calculator
- Enter the light intensity at the plants. This is PPFD, in micromoles per square meter per second, measured with a PAR meter or read from the light maker’s chart at your hanging height.
- Enter the hours of light per day.
- Pick your crop type to compare the result with a target range.
The math behind it
DLI = PPFD × hours × 3,600 ÷ 1,000,000
Worked example: lettuce under a shelf light
A light delivering 200 PPFD for 14 hours gives 200 × 14 × 0.0036 = 10.1 mol per square meter per day. Lettuce does best at about 12 to 17, so the light is a bit low. At 200 PPFD you would need about 20 hours, so it is better to raise the intensity to around 275, which gives 13.9 over 14 hours.
Worked example: tomato seedlings
At 300 PPFD for 16 hours the DLI is 17.3, which suits seedlings (10 to 15, so slightly high) and is a good lead-in to the 20 or more that fruiting plants want later.
Typical DLI targets
- Seedlings and microgreens: about 10 to 15.
- Lettuce and leafy greens: about 12 to 17.
- Herbs such as basil: about 15 to 20.
- Fruiting crops such as tomatoes and peppers: about 20 to 30.
These are typical indoor growing guidelines. Varieties and temperatures change what a plant can use, so treat them as a starting range and watch the plants.
Getting more or less light
Raising the light closer to the plants increases intensity, so move it up in small steps. Adding hours also raises DLI, but most plants need a dark period, so 14 to 18 hours is usual for seedlings. Signs of too little light include tall, pale, leggy growth, and signs of too much include bleached leaves or curling.
Related tools
Estimate electricity costs with the grow light cost calculator, plan the whole indoor seed setup with the seed tray planner, and time the start with the seed starting calculator. If you use a heat mat, size it with the heat mat capacity calculator.
Frequently asked questions
What is DLI?
Daily light integral is the total amount of photosynthetically active light a plant receives over a day, measured in moles per square meter per day.
What is PPFD?
Photosynthetic photon flux density is the light intensity at the plant, in micromoles of light per square meter per second. It is what a PAR meter reads.
What DLI do seedlings need?
Typically about 10 to 15 mol per square meter per day. Too little makes them stretch, and too much can stress very young plants.
How do I measure PPFD without a meter?
Many grow light makers publish a chart of PPFD at different hanging heights. A phone app can give a rough guide, but a PAR meter is more accurate.