The Planters' Guild

Working tools · Grow lights

How much light is your plant getting?

Turn a grow light's intensity and the hours it's on into a daily total, or work backward from a plant's target to the setting you need. The formula and the targets are Iowa State's; the arithmetic runs in your browser.

What do you want to work out?

µmol·m⁻²·s⁻¹

Only have a lux or foot-candle reading? Convert it below.

hours

Convert a meter reading

Phone apps and most light meters read in foot-candles or lux, which weight light the way the human eye sees it. This turns a reading into an estimate of PPFD for the light source you measured.

What the numbers mean

PPFD, photosynthetic photon flux density, counts the light a plant can use that lands on a patch of leaf each second, in micromoles per square meter per second (µmol·m⁻²·s⁻¹). It's the intensity (Steil, 2023). DLI, the daily light integral, adds that up over a whole day into moles per square meter per day (mol·m⁻²·d⁻¹), so it captures how bright and how long in one number (Steil, 2023).

The two are tied together by one line of arithmetic. Iowa State's extension service puts it this way: multiply the PPFD by the hours the light is on and by 0.0036 (Steil, 2023). The 0.0036 just converts seconds to hours and micromoles to moles. That's the whole calculator; it runs the same line in whichever direction you need.

Where the targets come from

The four bands in the menu are Iowa State's DLI needs for indoor plants, with the plant groups it lists for each (Steil, 2023). They're broad ranges for broad groups, not tested numbers for any one plant, and the PPFD and hours this page gives you are our arithmetic on those ranges.

Getting a reading you can trust

Iowa State recommends measuring at the level of the foliage, and notes that a fixture closer to a plant gives more intensity over a smaller area (Steil, 2023). For a quantum meter, it suggests taking readings in several spots and averaging them (Steil, 2023).

Phone apps are adequate for the typical home gardener, according to Iowa State, as long as the app or meter is set for the kind of light you're measuring (Steil, 2023). That setting matters because the conversion from foot-candles to PPFD depends on the source. Virginia Tech's extension guide gives 5.01 foot-candles per unit of PPFD for sunlight, 6.87 for cool-white fluorescent, 6.60 for metal halide and 7.62 for high-pressure sodium (Stallknecht, 2025). One foot-candle is about 10.76 lux (Thompson & Taylor, 2008).

Virginia Tech's table doesn't include LEDs (Stallknecht, 2025), so the converter won't guess at one. For an LED grow light, use an app or meter set for LED light, or a meter that reads PPFD directly.

About the hours

Iowa State suggests running lights 12 to 14 hours a day indoors, and for home gardeners no fewer than 10 and no more than 16 (Steil, 2023). It also points out that plants respond to the length of the dark period, not just the light (Steil, 2023). So if the math says you need 20 hours, the better fix is more intensity: a stronger setting, or the light moved closer to the leaves.

Want the longer version?

Module II of the Apprenticeship, Reading Light, walks through foot-candles, lux, PPFD and DLI, what "bright indirect light" means as a number, and how to measure a window.

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