DLI Calculator (Daily Light Integral)
This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.
Light intensity on its own does not tell you if a plant is getting enough light. What counts is the total light over a whole day, called the daily light integral, or DLI. This calculator turns a PPFD reading from a light meter and your hours of light into a DLI figure, then tells you what kind of plant that light level suits. It works for grow lights and for measured daylight.
Daily light integral:
This light level suits:
These bands are a general guide. Each plant species has its own preferred range.
How this works
DLI is light intensity multiplied by time. The formula is PPFD times the hours of light times 0.0036, which gives moles of light per square metre per day. The 0.0036 turns the per-second, per-micromole reading into a per-day, per-mole total. A dim light left on for a long time can add up to the same DLI as a bright light on for a short time.
Understanding your result
A rough guide from greenhouse research: under about 5 is too low for steady growth in most plants. Around 5 to 10 suits low-light foliage houseplants. Around 10 to 20 covers most houseplants and many leafy vegetables. Over 20 suits fruiting crops and sun-loving plants. More light is not always better, since every plant has a point above which extra light is wasted or even harmful.
Important limitations
This assumes the light stays at the intensity you entered for the whole time it is on, which is true for a grow light but not for changing daylight. For daylight, measure at the plant and take the reading around midday, and treat the result as a snapshot. The bands are general, since the right DLI varies a lot between species and even varieties.
Scientific basis
The DLI formula and light-level targets behind this tool come from Michigan State University Extension floriculture research, part of the land-grant university system:
- Michigan State University Extension — Low Daily Light Integrals in Northern Latitudes
- Michigan State University Extension — Maximizing Supplemental Lighting
Related reading
My guide on why plants need sunlight explains what that light does inside the leaf, and my article on grow lights for indoor plants covers choosing a light to hit the DLI you need.
This calculator is for educational purposes and general guidance only. It is not professional horticultural advice. Light needs vary by plant species, so check the needs of your specific plant.