# Why don't lumens matter for grow lights?

> Lumens don't matter for grow lights because lumens, lux and foot-candles measure light as the human eye sees it, weighted heavily to green-yellow, while plants use photons from 400–700 nm about equally. The plant-relevant numbers are PPF (µmol/s from the fixture), PPFD (µmol/m²/s at the canopy), DLI (mol/m²/day) and efficacy (µmol/J). Lux converts to PPFD only roughly, and only for a known spectrum.

- Canonical: https://genetixbay.com/knowledge/answers/lumens-vs-ppfd.html
- Topic: Environment · Type: Definition
- Evidence: Established · Status: Draft: awaiting expert review
- Full article: [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- Also asked as: Lumens vs PAR for grow lights | How much lux do plants need? | How many lumens per square foot for cannabis? | Is lux the same as PPFD?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Lumens, lux and [foot-candles](https://genetixbay.com/knowledge/glossary/foot-candle.md) measure light **as the human eye sees it**, weighted heavily to green-yellow. Plants use photons from **400–700 nm** about equally. So the numbers that matter are **[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md)** (µmol/m²/s at the canopy), **[DLI](https://genetixbay.com/knowledge/glossary/dli.md)** (mol/m²/day) and fixture **[efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md)** (µmol/J). *Established*.

## Why

Photometric units (lumens, lux) and quantum units (µmol) describe different things. Converting between them needs a factor that depends on the light source's spectrum; published factors differ from lamp to lamp [1].

A red-heavy grow light [looks](https://genetixbay.com/knowledge/glossary/bag-appeal.md) dim to us and scores low lumens, yet delivers plenty of [photosynthetic](https://genetixbay.com/knowledge/glossary/photosynthesis.md) photons. A cool white household lamp can score high lumens and deliver fewer photons per watt than a horticultural fixture. Lumens rank these lights in the wrong order.

Grow lights are therefore rated in [PPF](https://genetixbay.com/knowledge/glossary/ppf.md) (µmol/s) and efficacy (µmol/J). Good horticultural [LEDs](https://genetixbay.com/knowledge/glossary/led.md) exceed 2.5–3 µmol/J [2].

PPFD at the canopy is the number that predicts yield: dry flower yield rose linearly from 120 to 1,800 µmol/m²/s in one controlled cannabis trial [3].

## The numbers

| Unit | Measures | Use for plants? |
|---|---|---|
| Lumen (lm) | total visible light, eye-weighted | no |
| Lux (lm/m²) | eye-weighted light per m² | rough only, known spectrum |
| Foot-candle | lux per ft² (1 fc ≈ 10.76 lux) | rough only |
| PPF (µmol/s) | fixture's 400–700 nm photon output | yes |
| PPFD (µmol/m²/s) | photons reaching the canopy | yes: the setpoint |
| DLI (mol/m²/day) | PPFD × hours × 0.0036 | yes |
| Efficacy (µmol/J) | PPF ÷ watts | yes: running cost |

**Rough sunlight conversion:** about 54 lux per µmol/m²/s [1]. Other spectra differ.

## What to do

1. Ignore lumen and "equivalent watt" claims on grow lights.
2. Ask for PPF, µmol/J and a PPFD map at stated heights.
3. Set the light by PPFD: 150–300 [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md), 300–700 veg, 800–1,200 flower.
4. Use [lux meters](https://genetixbay.com/knowledge/glossary/lux.md) only to compare spots under the same light.

## Common mistakes

- Choosing a grow light by lumens per square foot.
- Applying the sunlight lux factor to LEDs.
- Assuming a brighter-looking light grows better.

## Related

- Full article: [PPFD and DLI by stage](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- [How can I measure PPFD without a PAR meter?](https://genetixbay.com/knowledge/answers/measure-ppfd-without-a-par-meter.md)
- [Can I grow with household LED or CFL bulbs?](https://genetixbay.com/knowledge/answers/can-i-grow-with-household-bulbs.md)
- Terms: [lux](https://genetixbay.com/knowledge/glossary/lux.md) · [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) · [DLI](https://genetixbay.com/knowledge/glossary/dli.md) · [foot-candle](https://genetixbay.com/knowledge/glossary/foot-candle.md)

## Follow-up questions

**What is the difference between lux and PPFD?** Lux measures how bright light looks to a human per square metre, weighted to the eye's peak sensitivity in green-yellow. PPFD counts the photosynthetic photons between 400 and 700 nm landing on a square metre each second, in µmol/m²/s, which is what drives photosynthesis. A red-heavy lamp can have low lux but high PPFD; the two don't convert without knowing the spectrum.

**How many lumens does cannabis need?** Cannabis needs are not set in lumens: lumens measure brightness to the human eye, not plant-usable light. Use PPFD instead: about 150–300 µmol/m²/s for seedlings, 300–700 in veg and 800–1,200 in flower at ambient CO₂. As a rough sunlight-only guide, 1,000 µmol/m²/s is about 54,000 lux, but lamps with other spectra convert differently.

**What do PPF, PPFD and DLI mean?** PPF (photosynthetic photon flux) is a fixture's total output of 400–700 nm photons, in µmol/s. PPFD (photosynthetic photon flux density) is how many of those land on a square metre of canopy each second, in µmol/m²/s. DLI (daily light integral) is the day's total, in mol/m²/day: PPFD × hours × 0.0036. Efficacy, in µmol/J, is PPF divided by watts.

## Sources

1. Thimijan RW, Heins RD, 1983. Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. HortScience 18(6):818–822. https://doi.org/10.21273/HORTSCI.18.6.818
2. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
3. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020