Enter a light source's luminous flux and the area it illuminates to calculate illuminance in lux.
How to Use the Lumens to Lux Calculator
Enter the fixture's total Luminous Flux and the Illuminated Area it's covering in square meters — Illuminance recalculates as soon as either field changes.
800 lumens spread across a small desk and 800 lumens spread across an entire room produce two very different lighting experiences, even though the bulb itself hasn't changed.
lux = lumens ÷ areaShrink the illuminated area and the same flux concentrates onto less surface, so lux climbs. Spread it wider and lux drops, even with the bulb's total output held constant. That's the whole calculation — flux divided by area, no beam geometry involved, because this version assumes the light is already spread evenly across the area entered.
Unlike the candela-based calculators in this category, there's no beam angle here — this version assumes the flux already lands evenly across the stated area, which holds for a diffused fixture but not for a tightly focused spotlight. Take that same 800 lm fixture: spread over a 2 m² desk it delivers 400 lux, comfortably bright for close work; spread over a 40 m² room it delivers just 20 lux, barely enough to move around by.
This is also the reason lighting codes and recommended-light-level tables specify a lux figure for a workspace, warehouse aisle, or reading nook rather than telling you which bulb to buy. A desk task and a warehouse floor might both call for a similar lux target, but the fixture that gets a small desk there and the fixture that gets a large floor there are not remotely the same wattage, because area is doing half the work in that lux number. Lumens on a box describes the bulb; lux describes the actual lighting outcome in the room.