Lighting & Lumens Calculator
How many fixtures the room actually needs — the lumen method, with the two honesty factors (CU and LLF) on the page instead of buried.
Planning targets, not a lighting design. CU and LLF are the two honesty knobs — dark finishes and high ceilings push CU down; the fixture photometrics win over defaults.
How the math works
Required source lumens = target foot-candles × room area ÷ (CU × LLF). Foot-candles are lumens per sq ft delivered to the surface; CU (coefficient of utilization) is the share of a fixture’s lumens that survive the room — dark finishes and high ceilings push it down; LLF (light-loss factor) covers dirt and lamp aging. Common residential planning targets: living and bedrooms 10–20 fc, kitchens 30–40 general with 70–80 at counters, offices 30–50, baths 70–80 at the mirror. Targets are editable because the room, not the table, is the client.
A worked example
A 15 × 12 ft kitchen (180 sq ft) planned at 35 fc, CU 0.7, LLF 0.8: 35 × 180 ÷ 0.56 = 11,250 source lumens. At 800 lumens per fixture, 15 fixtures, delivering about 37.3 fc at 135 W connected. Drop the CU for dark cabinets and the count climbs — the factor is the design conversation.
Questions from the field
How many lumens does a room need? Area × target foot-candles, divided by CU × LLF. A 180 sq ft kitchen at general-lighting levels needs roughly 11,000 source lumens at typical factors.
What is a foot-candle? One lumen landing on one square foot. It measures the light at the counter or the desk — the number the work actually feels.
Why do designers layer light? Ambient, task, and accent do different jobs. This page sizes the total; the layout — under-cabinet at counters, cans off the walls — is design, and it is why a single bright center fixture feels wrong.
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