Light and grow lights
Plant Light Calculator: Lux, PPFD, DLI and Foot-Candles
Choose the measurement you have and get an instant lux, foot-candle, PPFD, or DLI result. The calculator labels fixed unit conversions separately from spectrum-dependent estimates.
Calculator
Choose the light measurement you have
The result updates as you type. Spectrum-dependent results are clearly marked as estimates.
Before you calculate
What this estimate can tell you
Lux weights light according to human visual sensitivity, PPFD counts photosynthetic photons reaching a surface, and DLI totals those photons over a day. Because they answer different questions, some conversions are fixed while others require a photoperiod or information about the light spectrum.
Which conversions are exact?
Lux and foot-candles have a fixed unit relationship: 1 fc = 10.76391 lx. Under constant electric light, DLI = PPFD × photoperiod in hours × 0.0036. Lumens divided by square meters gives ideal average lux for evenly distributed light. Lux-to-PPFD results depend on spectrum and are estimates based on the selected source.
Assumptions and limits
- The PPFD-to-DLI calculation assumes PPFD remains reasonably constant throughout the entered photoperiod.
- Lumens divided by area assumes the luminous flux is distributed evenly over the target plane; a lamp's rated lumens are not the same as light reaching the plant.
- Lux-to-PPFD factors apply only to the selected source type, are not universal factors for every LED or spectrum, and do not make the source list a grow-light recommendation.
Worked example
Example: a 12-hour grow-light schedule
If PPFD at plant level stays at 200 µmol·m⁻²·s⁻¹ for 12 hours each day, the calculated DLI is 8.64 mol·m⁻²·d⁻¹. A single PPFD reading cannot represent the full day when light intensity changes over time.
FAQ
Common questions
Are lux and PPFD the same measurement?
No. Lux is weighted for human vision, while PPFD counts photons from 400 to 700 nm that reach a surface. They can be estimated from one another when the spectrum or source type is known, but there is no single exact factor for every lamp.
Can grow-light watts be converted directly to lux or PPFD?
No. Watts describe electrical power use. Light output at plant level also depends on fixture efficiency, spectrum, beam distribution, distance, and coverage. Use a manufacturer's PPFD map or a suitable meter when you need surface-level light data.
How does a longer photoperiod change DLI?
When PPFD stays constant, DLI increases in direct proportion to the photoperiod. At the same PPFD, changing the schedule from 8 hours to 12 hours raises the calculated DLI by 50%.
Can I multiply one outdoor PPFD reading by daylight hours?
Usually not. Sunlight changes with time, weather, and shade, so one instant reading does not represent the day. Outdoor DLI requires measurements that are logged or integrated over time.
Calculation references
- NIST: Illuminance Conversion FactorsUsed to verify the fixed conversion between foot-candles and lux.
- Apogee Instruments: Converting PPFD to LuxUsed for source-specific lux-to-PPFD factors and the spectrum-dependence of the estimate.
- Apogee Instruments: Daily Light IntegralUsed for the constant-light PPFD-to-DLI calculation and the limit of single readings under variable light.