Grow Light Calculator
Calculate the number of grow light fixtures needed and Daily Light Integral (DLI) from PPFD, photoperiod, and canopy area for optimal indoor plant growth.
About this calculator
This calculator answers two separate questions for an indoor grow: how many fixtures do you need, and how much usable light are your plants actually getting? Fixtures Needed is simple coverage math — canopy area divided by each fixture's effective coverage area, rounded up to a whole fixture. Daily Light Integral (DLI), the more important number, converts your fixture's instantaneous light output (PPFD, in micromoles of photons per square meter per second) into a cumulative daily dose: PPFD × photoperiod hours × 3,600 seconds/hour, divided by 1,000,000 to convert micromoles to moles, giving mol/m²/day.
That total-daily-dose framing is what plants actually respond to — the same DLI can come from a dim light running long hours or a bright light running short hours — which is why the calculator also rates your DLI on a plain-language scale from "Low (leafy greens only)" below 12 mol/m²/day up to "Very High (cannabis, tomatoes)" above 45. Power estimates are a rougher approximation: the calculator assumes a fixed LED efficacy of 2.5 µmol of photons per joule of electrical power, a reasonable but not universal figure for modern horticultural LEDs (actual efficacy varies by fixture, spectrum, and driver quality), and it multiplies that estimate across every fixture at your photoperiod and a flat $0.12/kWh rate to project daily, monthly energy use and monthly cost. Because both PPFD and coverage-area inputs come from a fixture's own spec sheet — measured at a specific hanging height — mounting a light higher or lower than that spec assumes will make every downstream number less accurate; always re-check PPFD at your actual canopy height rather than relying on the manufacturer's default distance.
Inputs
Results
Fixtures Needed
2
Daily Light Integral (DLI)
34.56 mol/m²/day
How to Use This Calculator
- Enter your fixture's PPFD (from its spec sheet) and your light hours per day, along with your canopy area and the coverage area per fixture in sq ft.
- Review Fixtures Needed and Daily Light Integral (DLI) — most vegetables need 15–30 mol/m²/d for optimal growth.
- Check DLI Rating for a plain-language read on your light level, from Low (leafy greens only) up to Very High (cannabis, tomatoes).
- Use Est. Watts per Fixture and Total Power Draw (W) to verify your electrical circuit has sufficient capacity for your lighting setup.
- Review Daily and Monthly Energy Use (kWh) and Est. Monthly Electric Cost, calculated at a flat $0.12/kWh rate.
How the result changes with Canopy Area
| Canopy Area | Fixtures Needed | Daily Light Integral (DLI) |
|---|---|---|
| 16 | 1 | 34.56 mol/m²/day |
| 24 | 2 | 34.56 mol/m²/day |
| 48 | 3 | 34.56 mol/m²/day |
| 80 | 5 | 34.56 mol/m²/day |
What each input means
- Fixture PPFD
- Photosynthetic Photon Flux Density at canopy height in micromoles per square meter per second. Check your fixture's spec sheet for PPFD at your hanging height.
- Light Hours per Day
- Photoperiod in hours per day. Vegetative growth: 16-18 hrs. Flowering: 12 hrs. Leafy greens: 14-18 hrs.
- Canopy Area
- Total plant canopy area to illuminate in square feet. Measure the growing surface, not floor space.
- Fixture Coverage
- Area each fixture effectively covers in square feet at your hanging height. A 4x4 ft fixture covers 16 sq ft.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFixture PPFD = 600, Light Hours per Day = 16, Canopy Area = 32, Fixture Coverage = 16 = 4 input(s) provided
- Calculate Fixtures NeededFixtures Needed2 = 2
- Calculate Daily Light IntegralDaily Light Integral34.56 = 34.56
- Calculate DLI RatingDLI RatingExcellent (fruiting crops) = Excellent (fruiting crops)
- Calculate Est. Watts per FixtureEst. Watts per Fixture357 = 357
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between PPFD and DLI?
PPFD (Photosynthetic Photon Flux Density) is an instantaneous rate — how many photons per second are hitting each square meter of canopy at any given moment. DLI (Daily Light Integral) is the cumulative total over a full day, computed here as PPFD × photoperiod hours × 3,600 seconds/hour ÷ 1,000,000 to convert into mol/m²/day, which is the number that actually predicts plant growth response.
How does the calculator estimate my electricity cost?
It assumes a fixed LED efficacy of 2.5 µmol of photons per joule of electrical power, converts your fixture's PPFD and coverage area into total photon output, divides by that efficacy to estimate watts per fixture, then multiplies by fixture count, daily hours, and a flat $0.12/kWh rate. Because 2.5 µmol/J is only a typical figure and actual fixture efficacy varies by spectrum and driver quality, treat the cost estimate as a ballpark rather than your actual utility bill.
Why does mounting height matter if I already entered my fixture's PPFD?
PPFD and coverage-area numbers on a fixture's spec sheet are measured at one specific hanging height above the canopy. Mounting the light higher spreads the same photon output over a larger area (lowering real PPFD), while mounting it lower concentrates it (raising real PPFD), so every downstream number here is only as accurate as your actual height matching the spec sheet's assumed height.
How is Fixtures Needed calculated, and why might it round up more than expected?
It's your canopy area divided by each fixture's effective coverage area, then rounded up to the next whole fixture with Math.ceil — a canopy that's 33 sq ft with 16 sq ft fixtures needs 3 fixtures, not 2.06, because you can't buy a fraction of a light. This also means small canopies that just barely exceed a fixture's coverage will need a full extra unit.
What does the DLI Rating scale actually tell me about my crop choice?
It maps your computed DLI onto plain-language bands: below 12 mol/m²/day is rated 'Low (leafy greens only)', 12-20 is 'Moderate (herbs, lettuce)', 20-30 is 'Good (most vegetables)', 30-45 is 'Excellent (fruiting crops)', and above 45 is 'Very High (cannabis, tomatoes)'. It's a quick way to check whether your current PPFD and photoperiod combination is actually delivering enough cumulative light for the crop you're growing, not just a bright-looking fixture.
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