| Pick the right LED size for your grow tent without trusting the marketing copy on Amazon. This tool tells you the actual PPFD and daily light integral your crop needs, then converts that to the LED wattage you should be looking for. No brand bias, no inflated “equivalent” wattage claims — just the science your plants actually respond to.
Works for tents, shelves, or any defined grow area. Output gives you target PPFD, daily light integral, and a recommended LED wattage range you can use to filter products. |
| PPFD, DLI and why watts barely matterWatts measure electricity, not light. Two LEDs pulling the same wattage can produce wildly different amounts of usable light depending on their efficiency. Modern quality LEDs put out around 2.5 μmol per watt-second (called "efficacy"). Cheap LEDs from generic Amazon listings often deliver 1.5 or less. Same wattage, 40% less plant-usable light. This is why you can't size a grow light by wattage alone.
PPFD (photosynthetic photon flux density) is what your plants actually receive at canopy level. Measured in micromoles per square metre per second (μmol/m²/s), it's the instantaneous light intensity. Different crops want different PPFD: lettuce thrives at 200–400, fruiting tomatoes want 600+, cannabis flower can use 700–1000.
DLI (daily light integral) is PPFD multiplied by photoperiod. It's the total amount of useable light a plant receives across a full day, measured in moles per square metre per day. This is the number that actually predicts growth — a plant getting 500 PPFD for 12 hours and 333 PPFD for 18 hours both reach a DLI of about 21.6, and they grow similarly. DLI is the bottom line; PPFD and photoperiod are just the two levers you adjust to reach it. |
| How this calculator worksThe wattage estimate uses three inputs: the PPFD your crop needs, the area you're lighting, and how efficiently modern LEDs convert electricity into photosynthetic light.
PPF needed (μmol/s) = PPFD × Area (m²) × 1.3
Wattage = PPF ÷ 2.5
The 1.3 multiplier accounts for real-world coverage — light doesn't distribute perfectly evenly across a tent, and edges get less than the centre. The 2.5 figure is the efficacy of a decent modern LED panel (Samsung LM301H boards, Mars Hydro FC series, Spider Farmer SE/EVO, HLG quantum boards all hit this comfortably).
For a 1.2m × 1.2m tent growing leafy greens, that works out to roughly 100–190W of real draw. A "1000W" cheap blurple light claiming to cover 4×4 with 60W actual draw is wildly underpowered. A quality 150W board is plenty. The math doesn't lie — wattage marketing usually does. |
PPFD and DLI targets by crop
| Crop |
PPFD (μmol/m²/s) |
DLI (mol/m²/day) |
Photoperiod |
| Microgreens / seedlings |
100–200 |
6–12 |
16 hr |
| Lettuce, leafy greens |
200–400 |
12–17 |
14–18 hr |
| Herbs (basil, mint) |
300–500 |
14–20 |
14–16 hr |
| Strawberries |
300–500 |
17–22 |
14–16 hr |
| Tomatoes / peppers (fruit) |
600–900 |
22–30 |
14–16 hr |
| Cannabis (vegetative) |
400–600 |
25–40 |
18 hr |
| Cannabis (flowering) |
700–1000 |
30–43 |
12 hr |
|
| Frequently asked questionsMy LED says it's "1000W equivalent" — is that real?
Almost certainly not. "Equivalent" wattage compares to outdated HPS lights and is a marketing term with no standard definition. Look at the actual power draw at the wall, usually buried in the spec sheet. A genuine 240W LED is excellent; a "1000W equivalent" pulling 100W is not.Can I just get a bigger light and run it dimmed?
Yes — and it's actually a smart move. A 240W board dimmed to 150W gives better coverage uniformity and runs cooler than a 150W board at 100%. Just make sure the light has a dimmer dial; cheap fixtures often don't.
How high should I hang the light?
Manufacturer charts vary, but a rough guide: seedlings 60–80cm, vegetative 40–60cm, flowering/fruiting 30–45cm. Most quality boards include a hang-height chart in the manual showing PPFD at different distances. If yours doesn't, that's a red flag about the brand.
Do I need a PAR meter to grow well?
No, not for home growing. Reputable LED brands publish PPFD maps showing intensity at different heights. Pick a light with a published map from a known brand and you'll get within 10% of target without measuring. Pro growers and commercial setups use proper meters; hobby growers don't need to.
Are full-spectrum white LEDs better than blurple panels?
For home growers, yes — by a long way. White-light LEDs (3000K + 5000K with red supplementation) deliver better efficacy, better colour rendering for inspecting plants, and the spectrum plants actually evolved to use. Blurple lights are a relic of LED grow tech from 2015. Don't buy them.
What about more light = more growth?
Up to a point, yes. Beyond about 1000 PPFD, plants can't use the extra without supplemental CO2 — they hit "light saturation" and just waste energy on heat dissipation. For home grows without CO2 enrichment, 600–800 PPFD is the practical ceiling for fruiting crops.
Can I use regular household LED bulbs?
For microgreens or a few herbs on a windowsill, yes — they'll grow. For anything that flowers or fruits, no. Household bulbs aren't designed for high PPFD output and you'd need so many that proper grow LEDs are cheaper. The exception is high-output T5 fluorescents, which work surprisingly well for seedlings and leafy greens at low cost. |
Lighting is one decision. There are about twenty more.
Start Your Hydroponics Journey guides you through every choice — system, lighting, nutrients, timing, troubleshooting — in the order they matter. Stop overspending on the wrong gear and start growing.
Get the ebook → |
