Step by step
- Install a free light meter (lux meter) app. Three good ones are listed below.
- Hold the phone at leaf height with the light sensor (usually next to the front camera) facing the window or lamp.
- Take three readings: morning, midday and afternoon. Use the average.
- Enter the average in the calculator, choose “A phone app or light meter reading,” pick the kind of light, and enter the hours of light.
Free light meter apps
- Photone iPhone and Android Made for plants. Free to download, with paid upgrades. App StoreGoogle Play
- Lux Light Meter Pro iPhone Simple lux and footcandle readings. Free, with optional purchases. App Store
- Light Meter - Lux Meter Android Free. Reads the phone’s light sensor in lux or footcandles. Google Play
Any “lux meter” app works. Choose one that uses the phone’s light sensor, and expect a rough reading. Names and prices can change.
How accurate is a phone?
Treat it as an estimate. Phone sensors differ between models, and lux is designed for human eyes, not plants. Michigan State University notes that the standard conversion only works well for sunlight; under electric light it changes with the type of lamp. For pink or purple grow lights, lux readings are misleading, so use the PPFD figure from the manufacturer.
A quantum (PAR) meter measures PPFD directly and is the most accurate choice if you grow many plants under lights.
Conversions the calculator uses
| Light source | PPFD from lux | Source |
|---|---|---|
| White or full-spectrum LED grow light | lux × 0.0185 | Mars Hydro (approximate) |
| Sunlight through a window | lux ÷ 54 | Michigan State |
| Fluorescent tube or bulb (T5, T8, CFL) | lux × 0.0135 | Mars Hydro (approximate) |
| Pink or purple (red/blue) LED grow light | Not reliable | Use the maker’s PPFD |
Footcandles convert to lux by multiplying by 10.764 (Iowa State University Extension).
Typical window light levels
University of Illinois Extension describes houseplant light levels in footcandles and where to find them near windows (Northern Hemisphere; in Australia, New Zealand and South Africa, swap north and south). The lux and PPFD columns are conversions for sunlight. No meter at all? The calculator can estimate your spot from this table: choose “No tools: describe the window”.
| Level | Footcandles | Lux | PPFD (µmol/m²/s) | Where you’ll find it |
|---|---|---|---|---|
| Low light | 75 | 807 | 15 | A few feet from a north window; 3–10 ft from an east or west window; 15–20 ft from a south window |
| Medium light | 150 | 1,615 | 30 | Directly in front of a north window; a few feet from an east or west window; 3–10 ft from a south window |
| High light | 300 | 3,229 | 60 | Directly in front of an east or west window; up to 5 ft from a south window |
| Direct sun | 1,500 | 16,146 | 299 | Directly in front of a south window |
Light falls off quickly as you move away from a window, and it’s much weaker in winter. A spot that reads well at noon in June may be dim at 4 p.m. in December, which is why averaging several readings matters.
Estimate your window’s DLI with the calculator
Sources
- Runkle, Michigan State University: Which light sensor should I use? (light units and sensors)
- University of Illinois Extension: Houseplants, lighting
- Iowa State University Extension: How to determine how much supplemental light to provide indoor plants
- Mars Hydro: How to convert lux to PPFD (manufacturer guide, approximate factors)