Grow Light Calculator
Work out how much light your plants need and what it takes to deliver it. Choose the plant and photoperiod, enter your grow area and fixture, and get the target PPFD and DLI, the wattage, fixture count and running cost. Or convert a measured PPFD into DLI.
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Numbers are target DLI ranges in mol/m²/day.
From a quantum (PAR) meter or the fixture's PPFD map.
85–95% in a reflective tent, 60–75% on an open shelf or room.
Efficacy = photosynthetic photon efficacy (PPE) from the spec sheet.
Actual wall watts, not the 'equivalent' number on the box.
How to use the Grow Light Calculator
- Pick the plant type or stage, or enter your own target DLI.
- Enter the light hours per day you plan to run the lights.
- Enter the grow area and how much light stays on the plants (higher in a reflective tent).
- Choose the light type and the real wattage of one fixture.
- Read the wattage and fixture count, or switch to DLI mode to check a PPFD reading.
Grow light formulas
- DLI from PPFD: DLI = PPFD × hours × 3600 ÷ 1,000,000 = PPFD × hours × 0.0036
- PPFD for a target DLI: PPFD = DLI ÷ (hours × 0.0036)
- Photons needed: PPF (µmol/s) = PPFD × area (m²) ÷ share of light landing on the canopy
- Wattage: watts = PPF ÷ efficacy (µmol/J)
Example: lettuce at 15 mol/m²/day with 16 hours of light needs 15 ÷ (16 × 0.0036) = 260 µmol/m²/s. A 4 × 2 ft shelf is 0.743 m², so with 85% of the light reaching the leaves the lights must emit 260 × 0.743 ÷ 0.85 = 227 µmol/s. A 2.3 µmol/J LED needs 227 ÷ 2.3 ≈ 99 W.
Target DLI and PPFD by plant
| Plants | Target DLI (mol/m²/day) | PPFD at 12 h | PPFD at 16 h |
|---|---|---|---|
| Low-light houseplants (pothos, snake plant) | 2–5 | 45–115 | 35–85 |
| Medium-light houseplants | 5–10 | 115–230 | 85–175 |
| Seedlings, microgreens | 6–12 | 140–280 | 105–210 |
| Lettuce, spinach, leafy greens | 12–17 | 280–395 | 210–295 |
| Herbs (basil, parsley) | 12–18 | 280–415 | 210–315 |
| Succulents, cacti | 10–20 | 230–465 | 175–345 |
| Strawberries | 17–25 | 395–580 | 295–435 |
| Tomatoes, peppers (fruiting) | 22–35 | 510–810 | 380–610 |
Ranges are typical horticulture guidance; varieties differ. For comparison, full summer sun outdoors gives roughly 40–60 mol/m²/day, while a bright indoor windowsill often gives only 2–8.
Tips for sizing grow lights
- Use the fixture's real wall wattage and its PPE from the spec sheet. "1000 W equivalent" labels are marketing, not power draw.
- Buy a little more light than you need and dim it. Dimmable LEDs let you start seedlings low and raise intensity as plants grow.
- A longer photoperiod with lower intensity reaches the same DLI with smaller, cheaper fixtures, but check whether your plant is sensitive to day length before running 18–20 hours.
- Reflective walls (tent, white paint, Mylar) keep more light on the plants and can cut the wattage you need by 15–25% compared with an open room.
Frequently asked questions
What is the difference between PPFD and DLI?
PPFD (photosynthetic photon flux density) is the light intensity hitting the plants at one moment, in µmol/m²/s. DLI (daily light integral) is the total light received over a day, in mol/m²/day. DLI = PPFD × hours × 0.0036, so the same DLI can come from strong light for a short day or weaker light for a long day.
How do I convert PPFD to DLI?
Multiply PPFD by the number of light hours and by 0.0036 (3600 seconds per hour divided by 1,000,000 µmol per mol). For example, 300 µmol/m²/s for 16 hours gives 300 × 16 × 0.0036 = 17.3 mol/m²/day.
How many watts of LED grow light do I need per square foot?
It depends on the plant and the fixture efficacy. Leafy greens at about 15 mol/m²/day over 16 hours need around 260 µmol/m²/s, which is roughly 11–13 W per sq ft with a 2.3 µmol/J LED. Fruiting tomatoes at about 28 mol/m²/day over 14 hours need roughly 22–30 W per sq ft depending on the fixture.
What DLI do lettuce and herbs need?
Lettuce and most leafy greens grow well at about 12–17 mol/m²/day, and herbs such as basil at about 12–18. Very high light on lettuce can cause tip burn, so there is little benefit in going far above 17.
What does µmol/J mean on a grow light?
It is the photosynthetic photon efficacy (PPE): how many micromoles of usable photons (400–700 nm) the light produces per joule of electricity. Modern LED fixtures are about 2.3–2.9 µmol/J, HPS about 1.6–1.9 and fluorescent tubes about 1.0. Higher means more light for the same electricity.
Why is my calculated PPFD different from the fixture's PPFD map?
This calculator gives the average across the whole area. Real lights are brightest in the middle and dimmer at the edges and corners, and PPFD drops as you raise the light. Use a PAR meter or the manufacturer's map at your hanging height to fine-tune height and dimming.
Can I use lux or a phone light meter instead?
Lux meters weight light for human eyes, not plants, so the conversion depends on the light's spectrum. For sunlight, 1 µmol/m²/s is roughly 54 lux; for white LEDs it is often 65–80 lux. Treat phone readings as rough estimates.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.