PPFD and DLI Explained: Judge Plant Light Without Watts

The short answer: PPFD (photosynthetic photon flux density) is the amount of photosynthetically useful light landing on a surface each second, measured in µmol·m⁻²·s⁻¹. DLI (daily light integral) is the total of that light over a day, in mol·m⁻²·d⁻¹. To get DLI, multiply PPFD by the hours the light is on and by 0.0036 (Iowa State Extension). For example, 200 µmol·m⁻²·s⁻¹ for 16 hours gives about 11.5 mol·m⁻²·d⁻¹ (Virginia Tech, SPES-720). Watts tell you how much electricity a fixture uses, not how much usable light reaches your plants. Lumens and lux are weighted to human vision. PPFD at leaf height, together with the hours of light, is what to compare.

Why watts, lumens and lux mislead

Watts are "the amount of energy used by the fixture" (Iowa State Extension). Energy units don't directly reflect how much light a plant can use for photosynthesis (Virginia Tech).
Lumens measure light "as perceived by the human eye". Iowa State says lumens and PPF "are not equivalent". You can roughly estimate PPF by dividing lumens by 60, but the conversion rate for different light sources ranges from about 40 to 75 or 80 (Iowa State Extension).
Foot-candles and lux (lux is the metric equivalent) are photometric units, based on the visible light the human eye detects, which is mostly green. Each reading is also only a snapshot of light that keeps changing through the day (Purdue Extension, HO-238-W).

Virginia Tech's guide sums it up: it is "not recommended to use units of candela, lumen, foot-candles, lux, Watts, or Joules when measuring light levels for plants" (Virginia Tech). So when you use our grow-light sizing guide, compare fixtures by PPFD rather than wattage. The same applies to regular bulbs and ordinary LEDs: their wattage says little about the light your plants can use.

PAR, PPF and PPFD in plain language

Term	What it describes	Unit
PAR (photosynthetically active radiation)	Light between 400 and 700 nm, the range plants use for photosynthesis	a waveband, not a reading
PPF (photosynthetic photon flux)	The PAR given off by the light source	µmol/s
PPFD (photosynthetic photon flux density)	The PAR arriving on a surface, such as the top of your plant	µmol·m⁻²·s⁻¹
DLI (daily light integral)	All the PAR a surface receives over 24 hours	mol·m⁻²·d⁻¹

Sources: Iowa State Extension for PPF, PPFD and DLI; Purdue Extension and Virginia Tech for the 400–700 nm range.

Purdue explains PPFD as the number of photons used in photosynthesis that fall on a square metre (10.8 square feet) every second (Purdue Extension). PPF describes the fixture. PPFD describes what reaches the plant, so it depends on distance. Moving lights closer to plants "will significantly increase the PPFD" (Virginia Tech).

DLI: the plant's daily rain gauge

Purdue compares DLI to a rain gauge. Instead of the intensity of light at one moment, it "collects" all the photosynthetic light a plant receives through the day (Purdue Extension, HO-238-B-W). PPFD is the rainfall rate, and DLI is the amount in the gauge at the end of the day. A dim light left on for a long time and a bright light on for a short time can give the same DLI. Virginia Tech notes that too little light can slow growth, while too much can waste energy or even harm sensitive plants (Virginia Tech).

The formula

DLI = PPFD × hours of light × 0.0036 (Iowa State Extension). The 0.0036 combines two steps: multiplying by 3,600 seconds per hour and dividing by 1,000,000 to turn micromoles into moles. Virginia Tech writes it as PPFD × 3,600 × hours ÷ 1,000,000 (Virginia Tech).

Worked example

An LED gives 200 µmol·m⁻²·s⁻¹ at the top of the plants and runs for 16 hours: 200 × 16 × 0.0036 = 11.5 mol·m⁻²·d⁻¹. This is Virginia Tech's own example (Virginia Tech). Here is the same formula across a few combinations:

PPFD (µmol·m⁻²·s⁻¹)	8 hours	12 hours	16 hours
100	2.9	4.3	5.8
200	5.8	8.6	11.5
400	11.5	17.3	23.0

DLI values in mol·m⁻²·d⁻¹, calculated with the formula above and rounded to one decimal place.

Working backwards from a target DLI

Turn the formula round to find the PPFD you need: PPFD = DLI ÷ (hours × 0.0036) (Iowa State Extension). For example, to reach 10 mol·m⁻²·d⁻¹ with a 14-hour light period: 10 ÷ (14 × 0.0036) ≈ 198, so you need roughly 200 µmol·m⁻²·s⁻¹ at leaf height. For houseplants, keep the light period within Iowa State's range of no less than 8 and no more than 16 hours (Iowa State Extension). See why lights shouldn't run around the clock.

Reading a grow-light spec sheet

PPF (µmol/s): the fixture's total PAR output (Iowa State Extension). It's useful for comparing fixtures, but it doesn't tell you how much light reaches a plant at a particular distance.
PPFD at a stated height, or a PPFD map: this is the number that relates to your plant. Check the height it was measured at. Lights closer to plants raise PPFD but spread it less evenly, so fewer plants are well lit (Virginia Tech).
Watts: the energy the fixture uses (Iowa State Extension). Divide PPF (µmol per second) by watts (joules per second) and you get micromoles per joule, which tells you how efficiently the fixture turns electricity into PAR. Compare that figure, not raw wattage.
Lumens: treat them with caution. They aren't equivalent to PPF, and the conversion varies by light source (Iowa State Extension).
Dimming: LED output is linearly related to PPFD, so halving the power roughly halves the PPFD (Virginia Tech). That makes dimmable LEDs easy to tune to a target.

Virginia Tech notes that very high PPFD can damage some plant leaves, and suggests raising lights if plants grow too close to them (Virginia Tech). More on setup: how to choose a grow light, types of grow lights, how to hang grow lights, how far grow lights should be from seedlings, can grow lights burn plants and purple vs white grow lights.

Measuring at home

Quantum sensors and phone apps

Quantum sensors, also called PAR sensors or meters, measure PPFD directly in µmol·m⁻²·s⁻¹. Some phone apps use the phone's internal photodiode to estimate PPFD and are usually the cheapest option (Virginia Tech). Purdue's tips for using a sensor apply to either (Purdue Extension):

Measure at plant height, and move the sensor up as the plant grows.
Use a level to keep the sensor flat.
Keep the sensor clean.
If possible, send dedicated sensors back to the manufacturer every few years to be recalibrated.

If your meter reads PPFD, make sure it is calibrated for your type of light, such as LED or fluorescent (Iowa State Extension).

Converting lux or foot-candles to PPFD

You can convert foot-candles or lux to PPFD, but the conversion factor depends on the light source. No single number works for every light. Virginia Tech's factors show how many foot-candles correspond to 1 µmol·m⁻²·s⁻¹ (Virginia Tech):

Light source	Foot-candles per 1 µmol·m⁻²·s⁻¹	Example: 500 fc ≈ PPFD
Sunlight	5.01	about 100
Metal halide	6.60	about 76
Cool-white fluorescent	6.87	about 73
High-pressure sodium	7.62	about 66

To estimate PPFD, divide the foot-candle reading by the factor for your light source. Purdue expresses the same relationship as a multiplier: multiply the foot-candle reading by 0.20 for sunlight or 0.13 for high-pressure sodium (Purdue Extension). For lux, convert to foot-candles first by dividing by 10.764 (Iowa State Extension). For sunlight, that works out to roughly lux ÷ 54. Neither source gives one factor for LEDs, whose colour mix varies between products (Virginia Tech). Iowa State says to get the factor from the manufacturer or a published table or calculator (Iowa State Extension).

Rough target ranges, and where they come from

There is no single correct DLI. Virginia Tech says the right DLI "depends on the type of plant, its age, and the production goals" (Virginia Tech). Most published DLI targets come from greenhouse and crop research, not houseplant care:

Greenhouse crop classes (Purdue): low-light crops 3–6, medium-light 6–12, high-light 12–18, and very high-light more than 18 mol·m⁻²·d⁻¹ (Purdue Extension).
Example crops (Virginia Tech): seedlings and cuttings 5–10, lettuce 12–17, basil 15–25, tomato 20–30 mol·m⁻²·d⁻¹. These are general reference values that may need adjusting (Virginia Tech).
Houseplants: Purdue's DLI chart for greenhouse crops includes several genera sold as houseplants, such as Spathiphyllum, Aglaonema, Dracaena and Phalaenopsis. Those values describe commercial production, not a living room (Purdue Extension). Extension advice for houseplants is usually given in foot-candles instead. For example, University of Maryland Extension calls 100–500 foot-candles medium-bright light (UMD Extension). Using Virginia Tech's sunlight factor of 5.01, that is very roughly 20–100 µmol·m⁻²·s⁻¹.

For herbs and seedlings, see growing herbs under grow lights, starting seeds under grow lights and grow lights for seedlings.

Why window light varies by season

Outdoor DLI across the United States ranges from about 5 to 60 mol·m⁻²·d⁻¹, depending on latitude, time of year, day length and cloud cover (Purdue Extension). Virginia Tech, using maps from Faust and Logan (2018), gives Virginia's average as 40–45 mol·m⁻²·d⁻¹ in July and 15–20 in December. It notes that these figures are for open ground, and that DLI in a home "could be far lower in most cases" (Virginia Tech). In northern winters, greenhouse glazing, structures and hanging baskets can cut DLI to as little as 1 to 5 mol·m⁻²·d⁻¹ (Purdue Extension). In winter, short days, a low sun and overcast weather cut indoor light further (Iowa State Extension). Iowa State also notes that many houseplants benefit from supplemental light in winter or all year. See grow light vs sunlight and grow lights for indoor plants. Our grow-light DLI calculator does the arithmetic for you.

Frequently asked questions

Do I need a 600 W grow light for a monstera?

Wattage won't tell you. Watts measure the energy a fixture uses, not the light reaching your plant (Iowa State Extension). Compare the PPFD at the height you'll hang the light, over the area your plant covers. Our guide to grow lights for indoor plants answers this question in more detail.

Can I convert lux to PPFD?

Only approximately, and the factor depends on the light source. For sunlight, Virginia Tech's factor of 5.01 foot-candles per µmol·m⁻²·s⁻¹ works out to roughly lux ÷ 54. Fluorescent and high-pressure sodium lamps use different factors. For an LED, get the factor from the manufacturer.

Is a phone app good enough to measure PPFD?

It can give useful readings for home use. Virginia Tech notes that some apps use the phone's internal photodiode to measure PPFD, that they are typically the most cost-effective option, and that a phone app can give homeowners useful information. Measure at leaf height with the phone level, and if your app or meter reads PPFD, check that it is calibrated for your type of light.

What DLI do houseplants need?

There is no single figure. Virginia Tech says the right DLI depends on the plant, its age and your goals. Purdue classes greenhouse crops as low-light at 3–6 mol·m⁻²·d⁻¹ and medium-light at 6–12, but those are production values. Houseplant guidance is usually given in foot-candles instead.

How many hours a day should grow lights be on for houseplants?

Iowa State Extension suggests no less than 8 and no more than 16 hours, and about 14 hours for general setups. University of Maryland Extension also advises no more than 16 hours of light a day in total, because most plants need a dark period.

What is the difference between PPF and PPFD?

PPF is the photosynthetic light a fixture gives off, in µmol per second. PPFD is the photosynthetic light arriving on a surface, in µmol per square metre per second (Iowa State Extension). PPFD depends on how far the plant is from the light.