Why this page exists
Most grow-light calculators hide their math behind a black-box "verdict" and call it a day. We don't. The formulas, constants, and assumptions are below — copy them, argue with them, find a better number. If you can show one of our constants is wrong, we'll change it.
The calculator only earns trust if its working is shown. So here it is.
The PPFD math
The calculator runs four small formulas in order. None of them are secret — they're the standard horticultural conversions any grow-light spec sheet uses.
area_m² = length_m × width_m
target_ppf = target_ppfd × area_m²
recommended_watts = target_ppf / 2.5
dli = target_ppfd × hours × 0.0036The constant 2.5 is a typical efficacy figure for a modern LED in µmol per joule of input power. Older fixtures or budget lights may run closer to 1.8 µmol/J; premium boards top out near 2.9. We use the middle so the wattage estimate isn't optimistic.
The constant 0.0036 is just the conversion that turns "µmol/m²/s × hours" into "mol/m²/day": 0.0036 = 3600 / 1,000,000.
The PPFD targets per plant
The targets are conservative ranges drawn from horticulture extension publications and grow-light manufacturer guidance. The minimum is roughly the threshold below which plants stretch or fruit poorly; the maximum is roughly where additional light stops helping without CO₂ supplementation.
- Microgreens: 100–200 µmol/m²/s
- Leafy greens & herbs: 200–400 µmol/m²/s
- Most houseplants: 100–300 µmol/m²/s
- Flowering / fruiting: 600–900 µmol/m²/s
- Tomatoes / peppers: 400–700 µmol/m²/s
The coverage badge
If you enter a light's claimed PPF, we compare what it can cover at your plant's midpoint PPFD against your tent area:
coverage_ratio = (light_PPF / midpoint_PPFD) / area_m²Then we badge it:
- Green (ratio ≥ 1.20) — plenty of headroom.
- Amber (0.85–1.20) — adequate, expect some falloff at the edges.
- Red (< 0.85) — undersized; plants on the edges will likely stretch.
Caveat: claimed PPF is a manufacturer figure, not an independently measured one. Real-world output is typically 5–15% lower than the spec sheet.
How we pick the lights we recommend
The Recommended Lights section is hand-curated, not algorithmically ranked. We look for:
- Honest specs. Brands that publish PPF and PPFD-at-distance numbers, not just "1000 watts equivalent" marketing.
- Track record. Lights that have been in the market long enough for grower communities to confirm the spec sheet against measured reality.
- Range coverage. A small set spanning countertop herbs through to a 3×3 ft flowering setup, so most readers find one in their tier.
- Direct affiliate availability. See below.
Why no Amazon links
Amazon's affiliate program (Associates) requires three sales in the first 180 days or your account is closed. That creates a chicken-and-egg for new sites and pushes a lot of cheap, low-quality listings into the ranking pool. More importantly, the same SKU on Amazon is often a few dollars cheaper direct from the manufacturer with a longer warranty.
So we link to manufacturers and to ShareASale-network grow-light brands. We get a small commission if you buy through these links, but the price is the same as if you went direct — and you get the manufacturer's warranty, not Amazon's restocking-fee policy.
Limits we're honest about
- The wattage figure is a recommended system wattage at modern LED efficacy. If you're shopping HID/CMH or older LEDs, expect to need more.
- The coverage badge assumes the light is hung at the manufacturer's recommended height. Hang it too high and the badge will overestimate coverage.
- We don't model spectral effects. PPFD is a count of photons in 400–700 nm; some plants respond differently to red-heavy vs. blue-heavy mixes within that band.
- None of this replaces an actual quantum sensor. If you're growing commercially, buy one.