A quick-reference sheet of the core lighting design formulas, with a link to a full explanation and worked example for each one.
How to use these
Most lighting design comes back to a handful of relationships: how much light a source makes, how it spreads over an area or distance, and how much power that takes. Each formula above links to a page that defines the terms and works a real example.
| Quantity | Formula | Reference |
|---|---|---|
| Lumens | Φ = E × A | Lumens Formula |
| Lux | E = Φ ÷ A | Lux Formula |
| Foot-candles | fc = Φ ÷ A | Foot Candle Formula |
| Inverse-square | E = I ÷ d² | Inverse Square Law |
| Intensity | I = Φ ÷ Ω | Luminous Intensity |
| Beam diameter | D = 2d × tan(θ/2) | Beam Angle Formula |
| Efficacy | lm/W = Φ ÷ P | Efficacy Formula |
| Voltage drop | VD = 2KIL ÷ CM | Voltage Drop Formula |
| Energy | kWh = P × h ÷ 1000 | Energy Consumption |
See the Lumens to Foot Candles Calculator and the Lux Calculator.
The formulas you’ll use most
Most everyday lighting design comes back to a handful of relationships. The lumen method: required lumens = target lux × floor area, then divide by lumens per fixture to get the fixture count. The inverse-square law: lux = candela ÷ distance², for a single source on-axis. Efficacy: lumens ÷ watts, which shows how efficiently a source turns power into light (modern LEDs reach 100–150+ lm/W). The coefficient of utilization and a light-loss factor (typically 0.7–0.8) refine the lumen method for losses to walls, fixture optics, dirt, and lamp ageing. Start with the lumen method for a quick target, then apply the loss factor so the design still meets the target years later.