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Daily Light Integral (DLI) Calculator

Calculate your grow space's Daily Light Integral from PPFD and photoperiod, and check it against real published target ranges for your crop category.

Your DLI17.3 mol/m²/day
Target range10-20 mol/m²/day ✓ in range

How to measure your inputs

  • PPFD: from your grow light's spec sheet, or measured directly with a PAR meter at canopy height for an exact reading.
  • Photoperiod: hours per day your lights are on.

Methodology

DLI (mol/m²/day) = PPFD (µmol/m²/s) × 3,600 × photoperiod hours ÷ 1,000,000. Formula and crop target ranges from Virginia Tech Cooperative Extension publication SPES-720.

DLI: the single most load-bearing number in indoor growing

Why DLI matters more than instantaneous light intensity

PPFD tells you how bright your light is at one instant; DLI tells you the total light a plant actually receives over a full day — which is what actually drives photosynthesis and growth over time. Two setups with very different PPFD and photoperiod combinations can deliver the exact same DLI, and plants generally respond to that total, not the instantaneous reading alone.

Why target ranges vary so much by crop

Leafy greens evolved to photosynthesize efficiently at moderate light and can suffer tip burn or bolt if pushed too high; fruiting crops like tomatoes need substantially more light to support both leaf growth and fruit development. Matching your light setup to your crop’s real target range — not just "more is always better" — avoids wasting electricity and avoids stressing the crop.

Adjusting DLI without buying new lights

Since DLI is PPFD × photoperiod, you can hit a target DLI two ways: brighter light for fewer hours, or dimmer light for longer — useful if your fixture is fixed-output but you can control the timer, or vice versa.

Frequently asked questions

What is a good DLI for lettuce?

Leafy greens including lettuce generally target 10-20 mol/m²/day, per Virginia Tech Cooperative Extension’s published guidance — higher within that range for faster growth, lower for a more compact, tip-burn-resistant result.

What's the difference between PPFD and DLI?

PPFD (Photosynthetic Photon Flux Density) is an instantaneous measurement — light intensity at one moment. DLI (Daily Light Integral) is the total accumulated over a full day, combining PPFD and photoperiod. PPFD is like speed; DLI is like distance traveled.

Can too much DLI hurt my plants?

Yes — light-sensitive crops (especially some lettuce varieties) can develop tip burn or bolt prematurely when DLI runs significantly above their target range, even if temperature and nutrients are otherwise fine.

How do I measure my actual PPFD?

A PAR (photosynthetically active radiation) meter measured at canopy height gives a real reading; your grow light’s spec sheet PPFD figure is usually measured at a specific fixed distance, which may not match your actual setup’s canopy distance.