How many light fittings does the room actually need? Floor area × target illuminance gives total lumens; divide by the output per fitting and you have a count. Lux bands: 150 general, 300 workshop, 500 detailed, 750 precision — typical lighting-industry guidance.
What Your Results Mean
First, the two units, because they're the whole calculation. Lumens measure the total light a fitting emits — the figure printed on the box. Lux measure illuminance: how much light lands on a surface, at one lumen per square metre. A 1,600 lm fitting spread evenly over 4 m² gives about 400 lux in theory; over 40 m², the same fitting delivers just 40 lux. The calculator works backwards from the lux you want to the lumens — and therefore the fittings — that deliver it.
The lux bands are typical lighting-industry guidance for the working plane (roughly bench height for task assessment): about 150 lux for general living spaces, 300 for workshop and utility work, 500 for detailed work like bench repairs or kitchen prep, and 750 for precision or small-detail work such as fine assembly. Recommended levels and any regulations vary by country, so treat these as planning figures rather than requirements — and note that ageing eyes generally want more light for the same task, not less.
One big caveat: task lighting is localised. The ceiling layer doesn't have to carry precision-level illuminance on its own — a bench lamp or under-cabinet fitting close to the work delivers high lux exactly where it's needed. The practical approach is to light the room to the general or workshop band for safe circulation and even coverage, then add task light at the bench, desk or machine. Trying to light an entire garage to 750 lux from the ceiling wastes energy and creates glare.
Layered light beats one bright centre. Several fittings spaced across the ceiling spread light evenly, soften shadows and keep you from casting your own shadow across the work; a single central source leaves dark corners and harsh contrast. The coverage-per-fitting row helps here — with six fittings over 30 m², each covers about 5 m², which you can turn into a spacing pattern. Fit them in a grid, roughly evenly, at normal residential ceiling heights.
Two practical adjustments worth knowing. First, light levels decline as fittings age and collect dust, so a 10–20% margin over the calculated count is common practice — or choose a brighter fitting. Second, mounting height and surface colour matter: high ceilings and dark walls eat light, while pale, matt surfaces reflect it. The calculator assumes typical residential conditions; if your workshop has a 4 m ceiling painted battleship grey, add a fitting or two.
What the Results Mean
- Room Area (m²): the floor area the fittings must cover.
- Total Lumens: area × target lux — the combined output all fittings need to deliver.
- Fittings: total lumens ÷ your fitting's output, rounded up to whole fittings.
- Coverage per Fitting: the area each fitting serves on average — your spacing guide.
- Total: fittings and lumens in one line for the shopping list.
The Math Behind the Calculator
The lumen method: illuminance in lux is lumens per square metre, so the required lumens are simply the area multiplied by the target lux. Dividing by the output per fitting gives the count.
area_m² = room_length × room_width
total_lumens = area_m² × target_lux
fittings = ceil(total_lumens ÷ lumens_per_fitting)
coverage_m²_per_fitting = area_m² ÷ fittings
The theory assumes every lumen lands evenly on the working plane with typical reflectance from walls and ceiling — in real rooms some light is absorbed, and distribution is never perfectly even. That's why the bands are guidance rather than guarantees, and why adding a fitting's worth of margin is common. Lux bands (150 / 300 / 500 / 750) reflect typical lighting-industry guidance for general, workshop, detailed and precision tasks respectively; actual recommendations vary by country and application.
Worked Example: 6 × 5 m Workshop
A home workshop measures 6 m by 5 m. The plan: ceiling fittings for general workshop light (300 lux), using 1,600 lm fittings — the common general-purpose output.
- Area: 6 × 5 = 30 m²
- Total lumens: 30 × 300 = 9,000 lm
- Fittings: 9,000 ÷ 1,600 = 5.625 → 6 fittings
- Coverage: 30 ÷ 6 = 5 m² per fitting — on a 6 × 5 m room, a 3 × 2 grid keeps spacing roughly even
- Margin: a seventh fitting (7 × 1,600 = 11,200 lm) gives about 373 lux average — covering ageing and dust with room to spare
- Task layer: a bench where close work happens wants 500 lux locally. A 1,000 lm task lamp over a 2 m² bench delivers roughly that without touching the ceiling grid.
For comparison, the same room at general living level (150 lux) needs only 30 × 150 = 4,500 lm — three of the same fittings — while precision level (750 lux) from the ceiling alone would demand 22,500 lm, or fifteen fittings. That's the arithmetic behind "don't light the whole room to task level": put the intensity where the work is.
Reminder
These are planning figures for fitting counts and light output. Circuit design — loads, switching and cable sizing — is separate electrical work; fixed wiring should be carried out by a qualified electrician to local regulations.
Assumptions & Limitations
Every number above rests on these constants. If your room differs, change the matching input — or read the linked guide for the full reasoning.
- Lux bands (150 / 300 / 500 / 750) are typical lighting-industry guidance, not regulations — recommendations vary by country
- Theory assumes even distribution, typical wall and ceiling reflectance, and normal residential ceiling heights
- No maintenance factor applied — add 10–20% margin for ageing fittings and dust
- Task lighting is localised and additional to the ceiling layer
- Quantities only — circuit and switching design is electrical work for a qualified installer