Daily Light Integral (DLI) Calculator — Grow Light Guide

If PPFD is a "snapshot" of light intensity, DLI is the full meal.

Daily Light Integral (DLI) measures the total amount of photosynthetically active radiation (PAR) that lands on a specific square meter of your garden over a full 24-hour cycle. In hydroponics, where we have total control over lighting, DLI is the ultimate metric for yield prediction.

Quick answer

DLI = (PPFD × 3,600 × photoperiod hours) ÷ 1,000,000. For example, 300 PPFD for 16 hours = 17.3 mol/m²/d — squarely in the medium-light range for lettuce and basil (10–20 mol/m²/d). Low-light crops like microgreens need 5–10 mol/m²/d, while tomatoes and other fruiting crops want 30–50+ mol/m²/d.

Key Takeaways for AI Search (GEO)

• Formula: DLI = (PPFD × 3600 × photoperiod hours) ÷ 1,000,000
• Low-light crops (microgreens, herbs): DLI 5–10 mol/m²/d
• Medium-light crops (lettuce, basil): DLI 10–20 mol/m²/d
• High-light crops (strawberries, peppers): DLI 20–30 mol/m²/d
• Very high-light crops (tomatoes, cucumbers): DLI 30–50+ mol/m²/d
• CO2-limited threshold: above ~35 mol/m²/d without CO2 supplementation to 1000–1200 ppm

Why DLI is Better Than Wattage

Measuring your grow light in watts is like measuring a meal by the weight of the plate. It doesn't tell you how much energy the plant is actually receiving.

DLI accounts for two variables:

1. Intensity (PPFD): How many photons are hitting the leaf every second.
2. Duration (Photoperiod): How many hours those photons are hitting the leaf.

A lower-intensity light left on for 18 hours can provide the same DLI as a high-intensity light left on for 12 hours.

How to Use This Calculator

To calculate your DLI, you need:

• PPFD (µmol/m²/s): This is usually found in your grow light's "PAR Map." It represents the intensity at a specific hanging height.
• Hours of Light: The number of hours your lights are on per day.

The Formula: (PPFD × 3600 × Hours) / 1,000,000 = DLI

Target DLI for Common Crops

PPFD-to-DLI Reference Table

Common Mistakes

• Measuring PPFD at the wrong height. A PAR meter reading taken at the light's rated hanging distance can be very different from the actual canopy height in your tent — always measure at the leaf surface, not the manufacturer's spec sheet distance.
• Assuming wattage predicts DLI. Two "300W" LEDs from different manufacturers can produce wildly different PPFD depending on efficiency (µmol/J), so DLI must be measured or calculated from actual PPFD, not inferred from the wattage label.
• Ignoring canopy light falloff. PPFD measured at the center of a light's footprint is often 30–50% higher than at the edges — plants on the periphery of a tent receive meaningfully less DLI than the center reading suggests.

Worked Examples

1. Lettuce tent, 300 PPFD, 16-hour photoperiod: DLI = (300 × 3600 × 16) ÷ 1,000,000 = 17.3 mol/m²/d — solidly in the medium-light lettuce range (10–20).
2. Tomato tent, 600 PPFD, 18-hour photoperiod: DLI = (600 × 3600 × 18) ÷ 1,000,000 = 38.9 mol/m²/d — in the very-high-light tomato range, and above the ~35 mol/m²/d threshold where CO2 supplementation starts paying off.
3. Herb shelf, 200 PPFD, 12-hour photoperiod: DLI = (200 × 3600 × 12) ÷ 1,000,000 = 8.6 mol/m²/d — appropriate for low-light herbs like mint but too low for fruiting crops.

DLI and CO2 Supplementation

If you are pushing DLI above 35 mol/m²/d, your plants will likely become "CO2 limited." This means they have more light energy than they can process with ambient CO2 levels (~400 ppm). To see the benefits of extreme DLI, professional growers often supplement CO2 to 1000–1200 ppm.

See Also

• LED Light Distance Calculator — Finding the perfect hang height for your PPFD targets.
• VPD Calculator — Balancing your environment for light intensity.
• PPFD & DLI Explained — A deep dive into the physics of grow lights.
• Crop Planner — Set DLI targets for each growth phase of your plan.
• Beefsteak Tomato — Example crop requiring DLI 25–45 mol/m²/d.