How much light do your cuttings get in a day?
Cuttings stall in dim rooms because they get too little usable light over the day, not because the fixture looks bright to your eyes. Iowa State: light is often the most limiting indoor factor. University of Minnesota: lack of light is a major cause of elongated, skinny stems; a windowsill is often a poor seed-start site. Virginia Tech (SPES-720): eyes are poor judges of plant light; prefer PPFD and daily light integral (DLI).
How to measure PPFD
Options from Iowa State and Virginia Tech notes:
- A quantum or PAR meter at cutting or seedling height. Iowa State: measure at foliage level.
- A manufacturer PPFD chart for that fixture at a stated distance.
- A smartphone app if it is calibrated for the lamp type (Iowa State: adequate for many home gardeners when calibrated).
Do not guess a µmol number from fixture type or hanging inches. Fluorescent and LED distance notes from Iowa State, plus tighter seedling gaps from UMN, Clemson, and Wisconsin, live on best grow lights for indoor propagation. Use that page for inches. Type a measured or charted PPFD here.
How the calculation works
Daily light integral (DLI) is the total usable light your cuttings or seedlings get in one day. DLI = PPFD × hours × 0.0036. Enter PPFD in µmol·m⁻²·s⁻¹. The answer is in mol·m⁻²·d⁻¹.
PPFD is a rate per second. Virginia Tech’s DLI guide multiplies by 3,600 to turn seconds into hours, and a mole is 1,000,000 micromoles, so the two steps together give the 0.0036 factor. Iowa State Extension prints the same formula.
The reading has to come from where the leaves are: the top of the canopy in Virginia Tech’s words, the foliage level in Iowa State’s. In Virginia Tech’s example, 200 µmol·m⁻²·s⁻¹ for 16 hours gives a DLI of about 12 mol·m⁻²·d⁻¹.
The formula also runs backward. If you know the DLI you want and how long the lamp runs, Iowa State’s version gives the PPFD to aim for: PPFD = DLI ÷ (hours × 0.0036). This calculator only goes forward, so do that division by hand.
How to use this calculator
- Measure PPFD at leaf height with a quantum or PAR meter, a smartphone app calibrated for your lamp type, or the fixture’s PPFD chart at your hanging height.
- Enter that PPFD and the hours the lamp is on each day.
- Read your DLI and the note under it, which compares the number with the cutting and seedling ranges below.
- If the number is low, move the light closer and measure again, or add lamp hours. Iowa State’s home range runs from as little as 10 to no more than 16 hours a day. Keep a dark period each night, as University of Minnesota Extension advises.
Enter PPFD and hours to see your daily light integral.
What the number means
Compare your result to these published bands. They are crop examples, not a single perfect number for every jar.
| Source | DLI (mol·m⁻²·d⁻¹) | Named use |
|---|---|---|
| Virginia Tech (SPES-720) | 5 to 10 | Seedlings and cuttings |
| Iowa State medium | 6 to 10 | Vegetative cuttings; starting seedlings on the high side of this band |
| Iowa State high | 12 to 16 | Some seedling starts on the low side of this band |
Iowa State also prints a low band (3 to 6, foliage houseplants) and a very high band (18 to 30, many fruiting crops). Those are not the cutting and seedling targets this tool highlights. The calculator note under your result uses the Virginia Tech 5 to 10 band and the Iowa State medium and high cutting/seedling rows above.
Water jars and soil pots use the same light math
A water jar and a pot of mix use the same formula. Neither Iowa State nor Virginia Tech gives a separate PPFD for jars, so once you have a reading at the leaf, the math is identical. Iowa State stem-tip cuttings: 4 to 6 hours of bright, indirect light; too little light slows root formation and cuttings may rot or dry before roots form; too much can burn foliage, especially under a bag or dome. Air about 65°F to 75°F on that page.
Jar steps live on water propagation. Stick-in-mix steps live on soil propagation. This tool only converts the light you measured. If the jar water itself looks wrong, check it with the water change calculator.
When a window is not enough
PPFD falls off as you raise the fixture. Use best grow lights for indoor propagation for hang height and hours from university sources, then come back here with a PPFD reading. Species recipes live on plant propagation.
People also ask
What is DLI?
Daily light integral is the total usable light your cuttings or seedlings get in one day. Virginia Tech (SPES-720) and Iowa State prefer plant units such as PPFD and DLI. Eyes are a poor judge of plant light (Virginia Tech). This tool uses DLI = PPFD × hours × 0.0036.
How do I measure PPFD without guessing from the fixture?
Measure at the leaf with a quantum or PAR meter, or use a manufacturer PPFD chart for that fixture at a stated height. Iowa State: measure at foliage level; smartphone apps can be adequate for home gardeners if calibrated for the lamp type. Use a measured or charted µmol number. Do not guess from fixture type or hanging inches.
What DLI should cuttings and seedlings sit in?
Virginia Tech (SPES-720): about 5 to 10 mol·m⁻²·d⁻¹ for cuttings and for seedlings. Iowa State medium: 6 to 10 for vegetative cuttings, with starting seedlings on the high side of that band. Iowa State high: 12 to 16, with some seedling starts on the low side. Those are published crop examples, not one perfect number for every jar.
Do water jars and soil pots need different light?
Neither Iowa State nor Virginia Tech gives a separate PPFD for water jars. Use the same formula once you have a reading at the leaf. Iowa State stem-tip cuttings want 4 to 6 hours of bright indirect light whether you root in media or water. Too little light slows rooting; too much can burn foliage. Method steps live on water propagation and soil propagation.
How many hours should the lamp run?
This tool needs your hours to compute DLI. University of Minnesota: 12 to 16 hours and a dark period; do not leave lights on continuously. Iowa State home range is about 10 to 16 hours, often 12 to 14. Fixture distance and hour tables live on best grow lights for indoor propagation.
Sources
- Calculating and Using Daily Light Integral (DLI): An Introductory Guide, Virginia Cooperative Extension SPES-720 (Stallknecht, 2025)
- Important Considerations for Providing Supplemental Light to Indoor Plants, Iowa State University Extension
- How to Determine How Much Supplemental Light to Provide for Indoor Plants, Iowa State University Extension
- How to Propagate Houseplants by Stem Tip Cuttings, Iowa State University Extension
- Starting seeds indoors, University of Minnesota Extension
Cite this tool
Propagate How-To. “Propagation light calculator (DLI).” https://www.propagatehowto.com/tools/propagation-light-dli. Formula and ranges from Virginia Tech (SPES-720) and Iowa State University Extension.
<a href="https://www.propagatehowto.com/tools/propagation-light-dli">Propagation light calculator (DLI)</a>