Lighting Calculator
Lumens Add Up, but Fixtures Don't Divide Evenly
Lighting design has a well-established shortcut called the lumen method: figure out how much total light a space needs, then divide by how much light one fixture actually delivers once you account for real-world losses. It turns a subjective question — "does this room feel bright enough?" — into an arithmetic one.
The Formula
Fixtures Needed = Total Lumens ÷ (Lumens per Fixture × Light Loss Factor)
Required light level is entered in foot-candles (lumens per sq ft) or lux (lumens per sq m), depending on the unit system, and multiplying it by the room's area gives the total lumen output the space needs. That figure is divided by each fixture's rated lumen output, multiplied by a light loss factor — 0.75 by default — that accounts for lamp depreciation over time and dirt or dust accumulating on the fixture, both of which reduce real-world output below the rated figure.
Where This Calculation Matters
- Office and workspace lighting — general office work is commonly designed around 30 to 50 foot-candles, and undershooting that level is one of the most common complaints in a newly lit space.
- Retail and task lighting — display and task areas often call for meaningfully higher light levels than general ambient lighting in the same building.
- LED retrofits — replacing older fixtures with LED equivalents means re-running this calculation, since LED lumen output and light loss characteristics differ from the fixtures being replaced.
Worked Example
A 15 ft × 12 ft office (180 sq ft) requiring 50 foot-candles, using LED fixtures rated at 3,200 lumens each and the default 0.75 light loss factor:
| Step | Calculation | Result |
|---|---|---|
| Total lumens required | 180 × 50 | 9,000 lm |
| Usable lumens per fixture | 3,200 × 0.75 | 2,400 lm |
| Fixtures needed | ceil(9,000 ÷ 2,400) | 4 fixtures |
How to Use This Calculator
- Select Feet (foot-candles) or Meters (lux).
- Enter the room's Length and Width.
- Enter the Required Light Level for the space's intended use.
- Enter the Lumens per Fixture rating of the fixture you're planning to use.
- Optionally adjust the Light Loss Factor — defaults to 0.75.
- Select Calculate to see the number of fixtures needed.
Related Calculations
Wiring the circuits those fixtures sit on? The Construction Cost Calculator can help budget the electrical work, and the Insulation Calculator covers the ceiling cavity fixtures are often recessed into.