Last updated: June 19, 2026
Daily Light Integral Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The Daily Light Integral (DLI) Calculator converts a PPFD reading (µmol/m²/s) and a photoperiod (hours) into the daily total of photosynthetic photons in mol/m²/day, using DLI = PPFD × hours × 3,600 ÷ 1,000,000. For example, 500 µmol/m²/s × 14 h = 25.2 mol/m²/day. Most leafy greens want 12–17 mol/m²/day, fruiting crops 20–30, and cannabis flower 35–45; above about 45 mol/m²/day plants saturate and extra light is wasted. The calculator also returns weekly and monthly totals and a five-tier plant-suitability rating.
Daily Light Integral equals PPFD in micromoles per square metre per second times the photoperiod in hours times 3,600, divided by 1,000,000. For example, 500 micromoles times 14 hours gives 25.2 moles per square metre per day — a very-high DLI suitable for tomatoes and peppers. Lettuce thrives at 12 to 17, and cannabis flowering at 35 to 45 mol per square metre per day.
Key Takeaways
- DLI = PPFD × photoperiod × 3,600 ÷ 1,000,000 — the daily total of photosynthetic photons in mol/m²/day
- Most leafy greens thrive at 12–17 mol/m²/day; fruiting crops 20–30; cannabis flower 35–45
- Above ~45 mol/m²/day plants saturate — extra light becomes electricity and heat
- PPFD says 'how bright now', DLI says 'how much total light today' — DLI is what predicts growth
- Lux/foot-candle readings are calibrated for the human eye; use a quantum meter for real PPFD
- Outdoor DLI swings 5× between summer and winter at mid-latitudes — supplement to keep growth on track
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
DLI = PPFD × Photoperiod × 3,600 ÷ 1,000,000
Where:
- DLI=Daily Light Integral(mol/m²/day)
- PPFD=Photosynthetic Photon Flux Density(µmol/m²/s)
- h=Photoperiod (hours of light per day)(hours)
Worked Examples
Greenhouse tomatoes — 500 µmol/m²/s × 14 h
A typical heated greenhouse on a sunny day with supplemental lighting at the canopy.
- 1DLI = 500 × 14 × 3,600 ÷ 1,000,000
- 2 = 25,200,000 ÷ 1,000,000
- 3 = 25.2 mol/m²/day
- 4Tier 4 — 'very high', great for fruiting crops
Cannabis flowering room — 900 µmol/m²/s × 12 h
Indoor LED flowering setup with intense canopy light on a 12/12 photoperiod.
- 1DLI = 900 × 12 × 3,600 ÷ 1,000,000
- 2 = 38.88 mol/m²/day
- 3Tier 4 — near the upper end of 'very high' (25–45)
- 4Above ~45 mol/m²/day adds heat without extra growth
Indoor lettuce / leafy greens — 250 µmol/m²/s × 16 h
A vertical-farm or grow-tent setup for lettuce, kale, basil and other leafy crops.
- 1DLI = 250 × 16 × 3,600 ÷ 1,000,000
- 2 = 14.4 mol/m²/day
- 3Tier 3 — 'high', a sweet spot for leafy greens
Low-light houseplant — 50 µmol/m²/s × 12 h
A pothos or peace lily near a window with moderate ambient light.
- 1DLI = 50 × 12 × 3,600 ÷ 1,000,000
- 2 = 2.16 mol/m²/day
- 3Tier 1 — 'low', acceptable for shade plants only
Introduction
The Daily Light Integral (DLI) Calculator turns a light-intensity reading and a day length into the single number that actually drives plant growth: moles of photosynthetic photons hitting one square metre per day (mol/m²/day). DLI is the time-integrated form of PPFD (photosynthetic photon flux density) and is the metric researchers and commercial growers use to predict yield, time flowering, and tune supplemental lighting. Enter a PPFD reading from a quantum meter and your photoperiod, and the calculator returns the daily DLI, weekly and monthly totals, and a plant-suitability tier. For related growing tools, see our CO₂ grow room calculator, compost calculator, and fertilizer calculator.
How the DLI Formula Works
DLI is just PPFD multiplied by the number of seconds the light is on, then converted from micromoles to moles. The formula collapses unit conversion into one number you can compare across crops and conditions.
DLI (mol/m²/day) = PPFD (µmol/m²/s) × photoperiod (h) × 3,600 ÷ 1,000,000
3,600 converts hours → seconds (60 × 60)
Dividing by 1,000,000 converts µmol → mol
Weekly total = DLI × 7 · Monthly total = DLI × 30
Outdoor DLI in summer (mid-latitudes) reaches ~50–60 mol/m²/day; winter ~5–15
How to Use This Calculator (Step by Step)
Two numbers in, four numbers out. PPFD must come from an actual sensor — phone-camera lux apps don't give photosynthetic units.
Measure PPFD at canopy height with a quantum/PAR meter (in µmol/m²/s). For variable light, take readings at multiple points and average.
Enter the photoperiod: 16–18 h for vegetative growth under lights, 12 h for flowering, 14–16 h for long-day field/greenhouse crops.
Read the DLI in mol/m²/day, the weekly/monthly totals, and the suitability tier (1 low → 5 excessive).
Cross-check against the species DLI table below — adjust PPFD or photoperiod to land in the right band.
If you only have lux readings, multiply by ~0.0185 to estimate PPFD for daylight (rough — a quantum meter is far better).
If your DLI lands in tier 5 ('excessive') for the crop you're growing, you're paying for electricity that does nothing — back off PPFD or shorten the photoperiod.
PPFD vs DLI vs Lux — Why DLI Wins
Indoor-growing literature throws around several light units. Knowing what each measures keeps you from over- or under-lighting.
| Unit | What it measures | When to use |
|---|---|---|
| Lux (lx) | Light intensity weighted to **human eye** sensitivity (550 nm peak). | Almost never for plants — the eye and chlorophyll see different colours. |
| PAR (W/m²) | Total radiant energy 400–700 nm. | Engineering and HVAC sizing — not directly tied to photon counts. |
| PPFD (µmol/m²/s) | Photons of photosynthetically active light per second per square metre, **right now**. | Instantaneous canopy intensity; what your quantum meter reads. |
| DLI (mol/m²/day) | Total photosynthetically active photons per square metre **over a full day**. | Predicting growth, comparing crops, scheduling supplemental light. |
DLI is the time integral of PPFD. A high PPFD for a few hours and a moderate PPFD for many hours can produce the same DLI — and similar growth.
Recommended DLI by Plant Type
Target DLI ranges based on horticultural research and university extension data (Cornell, Michigan State, Oregon State).
| Plant type | Recommended DLI (mol/m²/day) | Tier |
|---|---|---|
| Low-light houseplants (pothos, peace lily, ferns) | 1–4 | Low |
| Microgreens, seedlings | 6–12 | Moderate |
| Lettuce, leafy greens, herbs | 12–17 | High |
| Strawberries, dwarf citrus | 12–20 | High |
| Tomatoes, peppers, cucumbers | 20–30 | Very high |
| Cannabis (vegetative) | 25–35 | Very high |
| Cannabis (flowering) | 35–45 | Very high |
| Light saturation (most C3 crops) | >45 | Excessive |
Outdoor DLI — Sunlight Through the Year
Even on a sunny day, DLI varies dramatically by latitude and season. Greenhouse growers add supplemental lighting to bring winter DLI up to crop-specific targets.
- Tropical full sun:
~50–65 mol/m²/day year-round
- Mid-latitudes (35–45° N/S) summer:
40–50 mol/m²/day on clear days
- Mid-latitudes winter:
5–15 mol/m²/day — often below crop minimums
Cloudy day cuts DLI by 50–80% versus a clear day at the same date
Greenhouse glazing transmits ~70% of available light, so indoor DLI ≈ 0.7 × outdoor
How Much Supplemental Light Do You Need?
A quick way to budget supplemental lighting: figure out the DLI gap, then back-calculate PPFD for your fixtures' on-time.
Find your target DLI from the plant table (e.g. tomatoes = 25 mol/m²/day)
Measure or estimate the natural DLI at canopy (e.g. 12 mol/m²/day in winter)
Gap = target − natural (e.g. 25 − 12 = 13 mol/m²/day)
Pick a fixture on-time (e.g. 12 h/day at canopy)
Required PPFD = Gap × 1,000,000 ÷ (on-time × 3,600). Example: 13 × 10⁶ ÷ (12 × 3,600) ≈ 301 µmol/m²/s of supplemental PPFD
Pair this with our CO₂ grow room calculator — high DLI without enough CO₂ wastes electricity
Common Mistakes to Avoid
Most DLI errors trace back to a handful of repeat issues:
Using lux/foot-candle apps instead of a quantum meter — they're calibrated for the human eye, not chlorophyll
Reading PPFD at the fixture instead of at the canopy — PPFD falls off as 1/distance²
Sampling only the centre of the canopy — edges often see 30–50% less light
Forgetting that 12 h × high PPFD beats 24 h × low PPFD for many crops because of dark-period requirements
Pushing DLI well past the crop's saturation point and paying for heat instead of growth
Comparing DLI between crops without matching the photoperiod (cannabis flower at 12 h is not the same as lettuce at 16 h)
DLI Glossary
Quick definitions for the lighting terms used in this calculator and on horticultural meters:
| Term | Definition |
|---|---|
| DLI | Daily Light Integral — total photosynthetic photons per m² over 24 h, in mol/m²/day. |
| PPFD | Photosynthetic Photon Flux Density — photons (400–700 nm) per m² per second, in µmol/m²/s. |
| PAR | Photosynthetically Active Radiation — light in the 400–700 nm range usable by chlorophyll. |
| Photoperiod | Hours of light per day a plant receives. Drives flowering in many species. |
| Light saturation | PPFD above which photosynthesis no longer increases — extra light just adds heat. |
| Quantum meter | Sensor calibrated in µmol/m²/s for the PAR band, the right tool for PPFD. |
| Lux (≠ PAR) | Light measured for the human eye; converting to PPFD is approximate at best. |
| C3 plant | Most cool-season crops (lettuce, wheat, soy) — saturates around 30–45 mol/m²/day DLI. |
Quick Reference Card
DLI — Quick Reference
Quick reference • Daily Light Integral Calculator
DLI = PPFD × hours × 3,600 ÷ 1,000,000 • Weekly = DLI × 7 • Monthly = DLI × 30Valid range: Plants saturate around 30–45 mol/m²/day; outdoor mid-latitude range 5–50
Common Values
⚠ Watch Out
- •Use a quantum/PAR meter for PPFD — lux apps don't read photosynthetic units correctly
- •Measure PPFD at canopy height, not at the fixture — light falls off as 1/distance²
- •Above ~45 mol/m²/day plants saturate; extra light just adds heat and cost
- •Don't compare DLI across crops without matching the photoperiod (12 h vs 18 h matter)
Pro Tips
- →Take 5+ PPFD readings across the canopy and average — edges can be 30–50% dimmer than the centre
- →Pair high DLI with CO₂ enrichment to lift the saturation point — see the CO₂ grow-room calculator
- →In winter, use the supplemental-light formula in the guide to size your fixtures to a target DLI
- →Track weekly DLI on a logger to catch under-lit weeks before yield drops
FAQs
What is a good Daily Light Integral for plants?
It depends on the crop. Low-light houseplants thrive at 1–4 mol/m²/day; lettuce and herbs at 12–17; tomatoes, peppers, and cucumbers at 20–30; cannabis flower at 35–45. Above ~45 mol/m²/day most plants stop responding.
How do I calculate Daily Light Integral?
Multiply PPFD (µmol/m²/s) by the photoperiod in hours, then by 3,600 (seconds per hour), and divide by 1,000,000 (µmol → mol). For example: 500 µmol/m²/s × 14 h × 3,600 ÷ 1,000,000 = 25.2 mol/m²/day.
What's the difference between PPFD and DLI?
PPFD is light intensity at a single moment (µmol/m²/s) — what your quantum meter reads. DLI is PPFD integrated over the full day in mol/m²/day. PPFD says 'how bright', DLI says 'how much total light a plant got today'.
Can I use lux instead of PPFD?
Not reliably. Lux is weighted to the human eye, which peaks at 555 nm; chlorophyll absorbs strongest at the blue and red ends. A rough daylight conversion is PPFD ≈ lux × 0.0185, but for grow lights or single-colour LEDs the multiplier shifts a lot. Use a quantum/PAR meter for real numbers.
What DLI does a tomato plant need?
Tomatoes are heavy feeders: aim for 20–30 mol/m²/day for healthy fruiting. Below 15 mol/m²/day fruit set drops and plants stretch; above 30 you mainly add heat without yield gain.
What DLI is best for cannabis?
Cannabis vegetative growth: 25–35 mol/m²/day (often 18 h × ~500 µmol/m²/s). Cannabis flowering: 35–45 mol/m²/day (12 h × ~800–1,000 µmol/m²/s). Above ~45 you risk light burn and waste electricity.
How is DLI affected by season and location?
Outdoor DLI in mid-latitudes ranges from ~5–15 mol/m²/day in winter to 40–50 mol/m²/day in summer. Tropical regions stay near 50–65 mol/m²/day year-round. Cloud cover can cut a single day by 50–80%.
What if my DLI is too high?
Plants reach light saturation around 30–45 mol/m²/day for most C3 crops; above that, photosynthesis plateaus while heat and electricity costs keep climbing. Solutions: shorten the photoperiod, dim the fixture, raise it higher above the canopy, or add CO₂ enrichment to push the saturation point higher.
How much supplemental light do I need to hit my target DLI?
Find the gap between your target and your current natural DLI, then back-calculate PPFD: Required PPFD = Gap × 1,000,000 ÷ (fixture on-time × 3,600). Example: a 13 mol/m²/day gap with 12 h of supplemental lighting needs ≈ 301 µmol/m²/s of extra PPFD at the canopy.