What a daily light integral actually is

Plants do not respond to how bright a light looks to you. They respond to how many usable photons land on a leaf over the course of a whole day.

The daily light integral is that total, counted between lights-on and lights-off and expressed in moles per square metre per day. Two numbers produce it: how intense the light is, and how long it runs.

That is why a modest light left on for eighteen hours and a powerful one run for eight can deliver the same crop. It is also why “I upgraded my light and nothing changed” is such a common complaint — if the photoperiod came down at the same time, the daily total may not have moved at all.

The formula, in full

DLI = (PPFD × 3,600 × hours) ÷ 1,000,000

PPFD is measured in micromoles per square metre per second. Multiplying by 3,600 turns seconds into hours. Dividing by a million turns micromoles into moles. There is nothing else to it — no fudge factor, no brand adjustment, no secret coefficient.

Worked through: a canopy sitting at 400 PPFD with the light on 16 hours a day receives (400 × 3,600 × 16) ÷ 1,000,000 = 23.0 mol/m²/day.

Getting an honest PPFD number

This is where most DLI calculations go wrong, and it is worth slowing down over.

With a meter. A PAR or quantum meter held at the top of the canopy gives you the real figure. Take readings in the centre, at each corner, and midway along each edge, then average them. The spread between centre and corner is routinely two to one. If you only ever measure the bright spot, you will overestimate your DLI for most of your growing area.

From the spec sheet. If your fixture publishes a PPF figure in µmol/s, dividing it by your floor area in square metres gives an average PPFD across that footprint. It is a reasonable planning number and a poor operating number, because it tells you nothing about where the light actually falls.

From wattage. Do not. Efficacy varies by more than threefold across fixtures on sale right now, and “equivalent to a 1,000 W HPS” is a marketing claim rather than a measurement. Any calculator that converts watts straight to PPFD is guessing on your behalf, and this one will not do it.

Published targets, by crop

These ranges come from Virginia Cooperative Extension publication SPES-720, Table 3. They are typical production ranges — a starting point to aim at, not a promise about your particular variety, container or climate.

• Seedlings and cuttings — 5 to 10 mol/m²/day
• Micro-greens — 9 to 12
• Parsley — 10 to 15
• Lettuce — 12 to 17
• Spinach — 14 to 20
• Cilantro — 15 to 20
• Basil — 15 to 25
• Impatiens — 8 to 12
• Begonia and geranium — 12 to 19
• Petunia — 20 to 25
• Tomato, cucumber and zucchini — 20 to 30

Notice how wide the span is. A tray of cuttings and a fruiting tomato differ by a factor of five. Running one light schedule across a mixed tent means something in it is always in the wrong place.

When you are under target

Work through the cheap fixes before buying anything.

• Add hours first. Photoperiod costs you electricity and nothing else. The calculator above tells you exactly how many hours would close the gap at your current intensity. If that number comes back above about eighteen, stop — most crops need a genuine dark period, so you have an intensity problem rather than a schedule problem.
• Then close the distance. Lowering the fixture raises intensity quickly. Every light has a minimum hanging height in its manual, and that figure exists for a reason: going below it creates a hot centre and scorched tips while the corners stay dim.
• Then check the boring things. A dimmer knob someone turned down. A diffuser coated in dust. Plants that have spread past the edge of the footprint and are now growing in shade. These account for more light deficits than worn-out fixtures do.
• Only then consider more light. If the arithmetic says you need substantially more photons per second and the fixture is already as low as it safely goes, the fixture is the limit.

When you are over target

Past the top of the published range, extra light stops buying growth and starts buying an electricity bill. Beyond that it does damage — bleached upper leaves, curled edges, and a plant spending energy on protecting itself instead of producing.

Cutting hours is the better lever here, because it is the one that also cuts your running cost. Raising the fixture or dimming it works too, but you keep paying for the same day length.

Light is only half of it

A plant can only use the light it receives if the rest of its environment lets it. Stomata close when the air is too dry or too wet, and a closed stoma is a plant that has stopped taking in carbon dioxide no matter how good the DLI looks on paper.

If your light is on target and growth still is not what you expect, the environment is the next place to look. Our VPD calculator works out where your temperature and humidity are putting you.

Questions

Does light from a window count?

Yes, and this calculator does not include it. If your space gets meaningful daylight, your real DLI is higher than the figure here. A meter reading taken with both the daylight and the fixture on is the only reliable way to capture it.

Do reflective walls change my DLI?

They change the distribution more than the total. Reflective walls mostly recover light that would otherwise have been lost past the edges, which evens out the corners rather than lifting the centre. The effect shows up in meter readings and not in any calculation.

Can I just run the light 24 hours a day?

For a few crops you can, and lettuce is one where continuous light has been used commercially. Most plants need a dark period to run processes that only happen at night, and several will show damage under continuous light. Decide this on the crop, not on the DLI arithmetic.

My DLI is right but the plants look pale.

DLI is a photon count, not a nutrition report. Pale new growth with correct light usually points at feeding, root zone pH, or root health rather than at the fixture.

Should I match the target exactly?

Aim for the middle of the published range and stop tuning. The ranges are wide because the underlying biology is forgiving. Chasing a single number to one decimal place is a good way to change three things at once and learn nothing.

How to read these numbers. Your DLI is calculated from your inputs using a published formula — it is arithmetic, and it is exact. The crop target ranges are general guidelines drawn from extension research, and real requirements vary by variety, container and climate. Anything to do with your fixture’s PPF output, its minimum hanging height or its dimming behaviour is manufacturer-specific: follow your own light’s documentation over anything shown here. Nothing on this page replaces watching your own plants.

Sources. DLI formula and crop target ranges: Virginia Cooperative Extension, Calculating and Using Daily Light Integral (DLI): An Introductory Guide, publication SPES-720, Table 3. Last reviewed 16 September 2026.