How do you calculate lighting requirements?

You start by picking a target foot-candle level for the task at hand. A warehouse aisle might need only 10–20 foot-candles, while a machine shop or drafting table runs 50–100. Measure the room’s square footage (length × width in feet), then multiply by that target to get raw lumens required. That number assumes perfect conditions, which don’t exist. So apply two correction factors: a utilization factor (0.5 to 0.8 depending on wall/ceiling color and fixture height) and a maintenance factor (0.7 to 0.85 for dirt build-up and lamp fade over time). Divide the raw lumens by (utilization × maintenance) to find the total delivered lumens needed from your fixtures.

Once you have that total, decide on a fixture’s lumen output and beam spread. For high bays, take one unit’s lumens—say 25.000—and divide that into your total needed lumens to get the rough number of fixtures. Then check spacing: a rule of thumb for general lighting is to keep fixtures no farther apart than 1.5 times the mounting height (e.g., 20’ ceiling → max 30’ between lights). Adjust beam angles (60° for tall ceilings, 120° for low) and run a quick layout. Always add 10–15% extra for uneven walls or future dirt. For critical jobs, plug everything into a free tool like Visual or DIALux—the math gets you close, but a simulation nails it.