Lighting Design Calculator

Fixtures Needed is calculated with the raw lumen-method result rounded up to the next whole fixture (Math.ceil), since you can't install a fraction of a light. Actual Illuminance is then recalculated from that rounded-up fixture count, so it will typically land a bit above your target lux rather than matching it exactly — the only way to hit the target precisely would be a fixture count that divides evenly, which is rare.

Utilization Factor sits in the denominator alongside lumens per fixture and maintenance factor, so it has a direct inverse relationship with fixture count — halving UF roughly doubles the number of fixtures needed. Because UF depends heavily on room geometry and surface reflectance rather than a fixed formula here, a wrong guess in this field is the single biggest source of error in the whole calculation.

No — it's estimated by dividing total lumens by 100, assuming LED fixtures average roughly 100 lumens per watt, then converting to watts per square foot using the room area. If your actual fixtures are less efficient than 100 lm/W, or you need code-compliance verification, check installed wattage against your jurisdiction's energy code (such as ASHRAE 90.1) rather than relying on this lumens-based estimate.

The lumen method formula used here (N = E×A / (F×UF×MF)) only accounts for total room area and the utilization/maintenance factors, not fixture geometry — mounting height influences UF indirectly in real installations (through beam spread and spacing-to-height ratio), but this calculator treats UF as a direct input rather than deriving it from mounting height, so changing that field alone won't move the fixture count.