--- title: "DLI Calculator for Grow Lights: PPFD to DLI | Plant Porch" description: "Turn a grow light's PPFD and hours into a daily light integral (DLI), or find the hours you need, with target bands for houseplants, herbs and seedlings." url: https://plantporch.me/ --- Daily light integral # How much light do your plants get in a day? Enter your grow light's PPFD and how long it is on. The calculator adds it up into a daily light integral (DLI) and shows where that sits in the target band for your plants, or works out the hours of light you need. ## PPFD, µmol and DLI in plain English Plants count light as particles called photons. A **micromole (µmol)** is simply a fixed, enormous number of them: about 602 million billion. **PPFD** (photosynthetic photon flux density) is how many micromoles of the light plants use, wavelengths 400–700 nm, land on one square metre (10.8 sq ft) every second. It is a snapshot, like a car's speedometer. **DLI** (daily light integral) adds those snapshots up over a day, the way a rain gauge collects rain: PPFD × hours × 3,600 seconds ÷ 1,000,000 gives moles per square metre per day (mol/m²/day). A light giving 200 µmol/m²/s for 16 hours delivers 11.5 mol/m²/day. Houseplant bands come from Purdue's table and run from "good" to "high" quality; the paler amber on the gauge starts at the least that still gives acceptable plants. Edible crops use Virginia Tech's suggested ranges. Treat both as guides, not hard limits. ## Sources - Stallknecht E. [Calculating and Using Daily Light Integral (DLI): An Introductory Guide](https://www.pubs.ext.vt.edu/SPES/spes-720/spes-720.html). Virginia Cooperative Extension SPES-720NP, 2025. The DLI formula, the 200 µmol × 16 h example and Table 3, the bands for seedlings, cuttings, microgreens, lettuce, spinach, parsley, coriander, basil, tomato, cucumber and courgette. - Torres AP, Lopez RG. [Measuring Daily Light Integral in a Greenhouse](https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf). Purdue Extension HO-238-W. DLI as a daily total, and Table 2 (after J. E. Faust, *Ball Redbook*), the houseplant, poinsettia and chrysanthemum bands. - Apogee Instruments, [Conversion – PPFD to Lux](https://www.apogeeinstruments.com/conversion-ppfd-to-lux/): the micromole as a number of photons divided by the Avogadro constant (6.022 × 10²³ per mole). - Trinklein DH. [Lighting Indoor Houseplants](https://extension.missouri.edu/publications/g6515). University of Missouri Extension G6515: hours of light under lamps, and short-day plants. ## More calculators for the porch Light, feeding, humidity and sowing, for plants grown indoors. - [Lux to PPFD converter](https://plantporch.me/lux-to-ppfd-converter/): Turn a light-meter or phone reading in lux or foot-candles into PPFD for your light source. - [Grow light schedule](https://plantporch.me/grow-light-schedule/): Hours on and off for seedlings, microgreens and houseplants, with timer times and a DLI check. - [Fertiliser dilution calculator](https://plantporch.me/fertilizer-dilution-calculator/): Grams or millilitres per litre or gallon for a target ppm of nitrogen, from the N-P-K label. - [VPD calculator](https://plantporch.me/vpd-calculator/): Leaf vapour pressure deficit in kPa from temperature and humidity, with the dew point. - [Microgreens calculator](https://plantporch.me/microgreens-calculator/): Seed per tray for 18 crops, soak and blackout days, and a staggered sowing plan. ## Recommended Two free services for the online side of indoor gardening. 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[Try WebTraceCheck](https://webtracecheck.com/) ## Questions **Is a DLI of 12 enough for my plants?** It depends on the plant. In Purdue's table, maidenhair ferns, peace lilies and other low-light foliage plants do well from about 4 mol/m²/day, while croton and weeping fig want 12 or more. Virginia Tech suggests 12–17 for lettuce, 15–25 for basil and 20–30 for tomatoes. Pick your plant in the calculator to see its band. **How do I find my grow light's PPFD?** A quantum (PAR) meter held at leaf height is the reliable way. Many grow-light makers also publish a PPFD chart for set hanging heights. A lux meter or phone light-meter app gives a rough estimate once converted for the light source, which the lux to PPFD converter does. **Can a weak light just stay on for longer?** Up to a point, because DLI grows with the hours. Missouri Extension suggests 16–18 hours a day for plants that get no outdoor light, so past that, raise the PPFD instead. Short-day plants such as poinsettia and chrysanthemum flower only on short days, about 10 hours of light, so long days keep them from blooming. **Does window light count?** Yes: DLI is all the light a plant gets in a day. Daylight changes by the hour and the season, so a single reading says little. Purdue's guide describes sensors that log light all day and total the DLI for you.