Indoor Growing Light Hours Calculator

About the Indoor Growing Light Hours Calculator

Plants don't measure light intensity alone — they respond to the total amount of photosynthetically active light received over an entire day, measured as the Daily Light Integral (DLI) in mol/m²/day. DLI combines light intensity (PPFD, measured in µmol/m²/s) with duration (photoperiod, in hours).

This page converts your grow light's PPFD output and the hours it runs per day into a DLI value. Compare that to your crop's DLI requirement to determine if you're providing enough light — or if you need to adjust intensity or photoperiod.

Indoor lettuce needs 12-17 mol/m²/d, tomatoes need 20-30+, and cannabis thrives at 30-45. Low-light herbs like parsley can manage with 8-12. The point is to connect light schedule and fixture intensity to a daily light target the crop actually responds to.

When This Page Helps

DLI is useful because it ties electricity use and plant response together. This page helps tune that balance instead of guessing photoperiod.

How to Use the Inputs

1. Enter the PPFD output of your grow light at canopy level (µmol/m²/s).
2. Enter the number of hours the light runs per day.
3. Review the calculated DLI in mol/m²/day.
4. Compare to your crop's target DLI range.
5. Adjust photoperiod or light height to achieve the target DLI.

Example Calculation

Tips & Best Practices

• Measure PPFD at canopy height — intensity drops rapidly with distance from the light.
• Most leafy greens need 12-17 DLI; fruiting crops need 20-30+ DLI.
• Adding 2 hours of light is often cheaper than buying a brighter fixture.
• Use a light meter or quantum sensor for accurate PPFD readings.
• Don't exceed 18 hours for most crops — plants need a dark period for respiration.
• Calculate electricity cost: Watts × Hours / 1000 × $/kWh = daily cost per fixture.

Understanding DLI for Different Crops

Low-light crops (microgreens, lettuce, herbs) thrive at 8-17 mol/m²/d. Medium-light crops (peppers, strawberries) need 18-25. High-light crops (tomatoes, cannabis) need 25-45+. Providing DLI above the crop's saturation point wastes electricity without increasing yield.

Energy Efficiency in Indoor Farming

Lighting is the largest operating cost in indoor farming, often 50-70% of total energy. LED fixtures deliver 2.5-3.0+ µmol/J (photon efficacy), far better than older HPS lamps at 1.7 µmol/J. Choosing efficient fixtures and optimizing photoperiod saves significant operating cost.

Uniformity Matters

A high average PPFD means little if distribution is uneven. Map PPFD across the canopy at 9-16 points. Aim for a coefficient of uniformity above 0.8 (minimum PPFD / average PPFD). Overlapping fixtures and adjusting height improve uniformity.

Frequently Asked Questions

• What is PPFD?

  Photosynthetic Photon Flux Density — the number of photosynthetically active photons (400-700 nm) hitting a square meter per second, measured in µmol/m²/s. It's the standard for measuring grow light intensity at the plant canopy.

• What is a good DLI for indoor growing?

  Lettuce: 12-17. Herbs: 10-18. Peppers: 18-25. Tomatoes: 22-30. Cannabis: 30-45. Microgreens: 8-12. Higher DLI generally increases growth rate and yield up to the species' saturation point.

• How do I measure PPFD?

  Use a quantum sensor (PAR meter) like the Apogee MQ-500. Budget options include smartphone-based meters, though they are less accurate. Measure at multiple points across the canopy and average for uniformity.

• Can I run lights 24 hours?

  Most plants need a dark period (6-8 hours) for metabolic processes including respiration and starch mobilization. Some crops like lettuce tolerate 20+ hours but rarely benefit from no darkness. 16-18 hours is a practical maximum.

• Does light distance affect DLI?

  Yes, dramatically. PPFD follows the inverse square law approximately — doubling the distance from the light reduces intensity by ~75%. Keep lights at the manufacturer's recommended height and measure PPFD at canopy level.

• How do I supplement natural light?

  Measure natural DLI from sunlight entering your growing area. Subtract from your crop target. Calculate the supplemental PPFD needed: Supplemental DLI / Hours / 0.0036 = required PPFD from grow lights.