This grow light coverage calculator determines how many grow lights are needed for any indoor garden area based on total square footage, individual light wattage, and plant light requirements (30W/sq ft for herbs to 75W/sq ft for flowering).
Grow Light Coverage Calculator
Calculate how many grow lights you need for your indoor garden area.
Turning Grow Light Coverage Calculator Numbers into a Working Setup
Watts per square foot is a quick sizing rule, but plants respond to light, not electricity. Use the wattage figure as a starting point, then refine the layout with the points below.
Check what the wattage actually means
Modern LED fixtures turn far more of each watt into usable light than older HID or fluorescent lamps, so two fixtures with the same wattage can deliver very different amounts of light. Look for the actual power draw at the wall, not an ‘equivalent’ figure on the box, which is often inflated. If the manufacturer publishes PPF (total light output useful to plants, in micromoles per second) or a PPFD map (light intensity at the canopy), those numbers tell you more than watts alone.
Spread the light evenly
Total wattage can be right while coverage is still patchy. A single powerful light in the middle of a large area leaves the edges dim, so several smaller fixtures often give more even results than one big one. A few habits help:
- Hang lights at the height the manufacturer recommends, and raise them if leaves bleach, curl upward or crisp at the tips.
- Lower the lights, or add one, if seedlings stretch and turn pale, a sign they are reaching for more light.
- Use reflective walls or a grow tent in small spaces so less light escapes the growing area.
- Rotate pots every few days on shelves where the light falls off toward the ends.
Match the hours to the plant
Intensity and duration work together. Leafy greens and herbs are usually happy with long days under moderate light, while many flowering plants need a set dark period to trigger blooms. A timer is the cheapest upgrade you can make: it keeps the schedule consistent and stops lights running all night by accident.
Plan for heat and power
Every watt ends up as heat in the room. In a closed tent or cupboard, a few hundred watts can push temperatures up quickly, so budget for a small fan or extractor. Check that your circuit can handle the total load, use grounded plugs, and keep cables and connections away from water and drip trays.
Once plants are growing, let them be the judge. Compact growth with good colour means the light is about right; stretching means too little, and scorched or bleached leaves mean too much or too close. For background on light spectra and fixture types, see the grow light article on Wikipedia, then rerun the grow light coverage calculator whenever you change the space or the fixtures.