PPFD & DLI Calculator: Size the Right LED Grow Light for Any Crop
Most grow light spec sheets are written to win an argument, not to help you. A “100W” panel and a “100W” panel can differ by more than a factor of two in photons actually delivered to your plants. The only numbers that answer the real question — is this enough light? — are PPFD (intensity) and DLI (total light per day).
Enter four numbers below and this page will tell you what a fixture delivers to your growing area, what that means for a crop, and how much PPF you would need to hit a specific target.
Total photons the fixture emits per second. A spec sheet value, not wall-power watts.
Growing area m² ft² Both boxes stay in sync. Type in either unit.
16–18 h for leafy greens and herbs, 12–14 h for many houseplants.
Modern quantum-BOARD fixtures: 2.4–3.2. Older or budget panels: 1.2–1.8.
- Wall-power equivalent
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- PPF needed, DLI 12 @ 16 h
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- PPF needed, DLI 20 @ 16 h
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Everything runs in your browser. Nothing is sent anywhere — no email gate, no tracking of your inputs.
The three formulas
Everything above is three equations. There is no trick to them.
1. Average PPFD — spread the fixture’s total photon output over the floor area it covers:
Average PPFD (µmol/m²/s) = PPF (µmol/s) ÷ Area (m²)
2. DLI (Daily Light Integral) — how many moles of photons land on one square metre in a full day:
DLI (mol/m²/day) = Average PPFD (µmol/m²/s) × Hours of light × 0.0036
The 0.0036 comes from 3,600 seconds in an hour times 1 µmol = 0.000001 mol. If you prefer imperial units, DLI in mol/ft²/day equals the m² value times 10.7639.
3. Required PPF — run the DLI formula backwards to size a light for a target:
Required PPF (µmol/s) = Target DLI × Area (m²) ÷ (Hours × 0.0036)
What the numbers mean
| Metric | What it measures | Useful range |
|---|---|---|
| PPF (µmol/s) | Total photons the fixture emits per second | Scale size, not quality |
| Efficacy (µmol/J) | Photons per watt of electricity | 2.4–3.2 modern, 1.2–1.8 old |
| PPFD (µmol/m²/s) | Intensity reaching the canopy | 200–600 for most indoor crops |
| DLI (mol/m²/day) | Total daily light the canopy received | 12–20 for most crops |
Note that PPF and PPFD are only as good as the area you divide by. Manufacturers quote PPF at the fixture, and PPFD at a specific mounting distance over a specific footprint. If a vendor quotes a single PPFD number, ask them for the map.
Target DLI by crop
Use the calculator, then compare the DLI it gives you against the range for your crop.
| Crop stage | Target DLI (mol/m²/day) | Typical photoperiod |
|---|---|---|
| Seedlings, cuttings | 4–8 | 16–18 h |
| Microgreens | 8–12 | 14–16 h |
| Lettuce, leafy greens | 12–17 | 14–16 h |
| Basil, mint, parsley, other herbs | 14–20 | 14–16 h |
| Fruiting — pepper, tomato, eggplant | 20–30 | 16–18 h |
| Root crops — radish, beet, carrot | 10–16 | 14–16 h |
| Flowering photoperiod crops | 20–30 | 12 h (controlled) |
The important word in that table is range. Plants tolerate a wide band and punish extremes much harder than they punish the middle.
Worked example: lettuce in a 2 × 4 ft tray
Say you have a 2 × 4 ft tray, which is 8 ft² or about 0.74 m², and you run a 16-hour day.
- Target: lettuce peaks around DLI 16, so the light must deliver 16 mol/m²/day.
- Required PPF: 16 × 0.74 ÷ (16 × 0.0036) ≈ 206 µmol/s.
- Average PPFD that implies: 206 ÷ 0.74 ≈ 278 µmol/m²/s.
- Check it against a real fixture: our 45W quantum-BOARD panel is rated near 110 µmol/s, so one panel gives 110 ÷ 0.74 ≈ 149 µmol/m²/s and a DLI of 149 × 16 × 0.0036 ≈ 8.6 — enough for seedlings and microgreens, not full lettuce. Two panels in a daisy chain, or a bar-style layout, is the realistic answer for this footprint.
That last step is the one sellers skip. The number on the box is not the number on your plants.
Average hides everything at the edges
The calculator gives you an average across the footprint. Real grows are not averages. A single 30W spot deep inside a canopy can read 600 µmol/m²/s while the corners of the same tray sit at 120. The average might be a healthy 300, and half your tray is still in the dark.
Two cheap ways to find out before you buy:
- Map it. Lay out a grid over the footprint and measure PPFD at every point with a quantum sensor or a calibrated phone app. Anything within about ±15% of the mean is a good map; anything worse means you need a second light or a reflector.
- Watch the plants. Within a week of a change in light, the pattern is visible. Pale, small leaves or stretching internodes are pointing at the darkest part of your footprint.
The plants are the cheapest sensor you own, but only if you look at the weak corner rather than the good one.
How to size PPFD yourself if you have no PAR meter
You do not need to buy a meter to get a useful answer. You need a spec sheet, a tape measure, and a calculator.
- Take the fixture’s PPF from its spec sheet, or estimate it as
wattage × efficacy in µmol/J— a 45W panel at 2.5 µmol/J is about 113 µmol/s. - Measure the floor area the light actually covers at your chosen mounting height, not the full tent.
- Divide. That is your average PPFD, which is exactly what the calculator above does.
For uniformity you really do want a sensor. But the average is where the sizing decisions happen, and the average is arithmetic.
Why we built this instead of gating it
Most PPFD calculators on the web ask for an email address before showing a result. We are not going to do that, for the same reason we publish our test data on the quantum board panel: a number a grower can verify is worth more to us than a lead we have to beg for.
If the calculator tells you the light you are considering is under-powered, that is useful information whether or not you buy from us. And if it says you need a bigger fixture than you planned, send us the numbers and we will tell you which of our models actually hits that target — including if the answer is a smaller, cheaper one.
Related reading
- Key factors influencing a customer’s choice of grow lights — the non-luminous factors that decide a purchase.
- How many watts for desktop plants — wattage, for when you only have a desk.
- Sizing panels from footprint up — the same math at room scale.
- The complete indoor basil guide — a full crop walked through end to end.
- Our 45W–50W panel range — the fixtures these numbers refer to.
- Grow light FAQ — the shorter answers to the questions we are asked most.
FAQ
How do I convert PPFD to DLI?
Multiply PPFD by the number of hours of light per day, then by 0.0036. A canopy at 300 µmol/m²/s under 16 hours of light receives 17.3 mol/m²/day, which is in the right band for lettuce and herbs.
What DLI do leafy greens need?
Lettuce, kale, rocket and most other leafy greens peak between 12 and 17 mol/m²/day. Basil, mint and parsley tolerate and reward up to about 20. Below 10 you will get growth but not full harvests, which is the most common disappointment in a home setup.
Can I run a grow light 24 hours a day?
You can, and at a 24-hour photoperiod each 1 mol/m²/day of DLI needs only about 7.7 µmol/m²/s of PPFD. But plants respire around the clock and consume sugars whether or not they are photosynthesising, so a 24-hour photoperiod usually underperforms the same DLI delivered over 16 hours. It also doubles your electricity bill and your heat load.
Is a grow light’s wattage a useful number?
Only as a proxy for cost and heat. Compare efficacy in µmol/J instead. A 50W fixture at 2.8 µmol/J delivers more usable light than a 90W fixture at 1.4 µmol/J while drawing less than half the power, and it puts half as much heat into the room.
How much does grow light distance change PPFD?
A lot — PPFD falls off roughly with the square of distance. Doubling the distance from a point source cuts intensity to about a quarter. Mount height is therefore one of the biggest levers you have, and it is free. If a fixture is dimming itself because of an over-temperature sensor, check the distance before you conclude the light is faulty.