Lighting Fixture Count Calculator (Zonal-Cavity Lumen Method)
Required luminaire count via the IES zonal-cavity lumen method, from a target illuminance and room area, plus a Room Cavity Ratio check and an achieved-illuminance verification step — imperial (fc/ft²) or metric (lux/m²).
What it calculates
- Room Cavity Ratio from cavity height and room length/width, to confirm the fixture's photometric CU value applies
- Number of luminaires required: N = (target illuminance × area) / (lumens per fixture × CU × LLF), rounded up
- Achieved illuminance for a given fixture count — the check/verify step once N is rounded to a whole number of fixtures
- Automatic fc↔lux and ft²↔m² conversion when switching between imperial and metric
Typical inputs
- Target illuminance E (foot-candles or lux)
- Room area (ft² or m²)
- Lumens per fixture, from the luminaire's photometric data
- Coefficient of Utilization (CU) and Light Loss Factor (LLF), or their metric equivalents (Utilization Factor / Maintenance Factor)
Known limits
- This is a design-planning estimate, not an NEC section — actual fixture placement still has to satisfy the photometric layout and any project-specific uniformity ratio.
Frequently asked
Why does the calculator ask for a Room Cavity Ratio if I already have the fixture's CU?
A luminaire's Coefficient of Utilization is only valid for the room proportions its manufacturer tested against. The Room Cavity Ratio (5 × cavity height × (length + width) / (length × width)) is what you look up in the fixture's photometric table to confirm — or correct — the CU value before it goes into the fixture-count formula.
What's the difference between the required count and the achieved-illuminance check?
The lumen-method formula almost never returns a whole number, so the result is always rounded up to the next full fixture. The achieved-illuminance step runs the formula in reverse with that rounded count, so you can confirm the real installed illuminance still meets or exceeds the target — not just check the math in one direction.
Autodesk and Revit are registered trademarks of Autodesk, Inc. SYMATEQ is not affiliated with, endorsed by, or sponsored by Autodesk, Inc. SYMATEQ Electrical Calculators is an independent add-in for Autodesk® Revit®. Calculations are engineering aids and do not replace review by a licensed professional engineer or confirmation with the authority having jurisdiction.