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Room Lighting Calculator

Computes the lumens a room needs from its floor area and a target illuminance (by room type or a custom lux value) divided by a light-loss factor, then the number of bulbs and the wattage at a given luminous efficacy.

Floor area (m²)Floor area of the room in square metres. Room typeRoom type selecting a typical maintained illuminance (EN 12464-1 / IES ranges); use custom with target_lux for other values. Target illuminance (lux) (optional)Optional illuminance in lux that overrides the room-type value; required when room_type is custom. Lumens per bulb (lm) (optional)Light output of one bulb or fixture (800 lm ≈ a 60 W incandescent equivalent, 1,100 lm ≈ 75 W, 1,600 lm ≈ 100 W). Light loss factor (optional)Maintenance / utilisation factor for dirt, lamp ageing and light absorbed by walls (0.8 typical for light rooms, 0.6–0.7 for dark ones). Luminous efficacy (lm/W) (optional)Lumens per watt of the light source: LED 80–150, CFL 50–70, halogen 15–25, incandescent 10–17.

When to use

You are choosing how many LED bulbs or fixtures a room needs, or converting a lux requirement into lumens and watts.

Do not use when: You need a photometric design with fixture spacing, beam angles or uniformity (use lighting-design software), or only the running cost of the lights (use electricity-cost).

Formula

required_lumens = target_lux × floor_area_m2 / light_loss_factor; bulbs_needed = ceil(required_lumens / lumens_per_bulb); installed_lumens = bulbs_needed × lumens_per_bulb; total_wattage_w = required_lumens / luminous_efficacy_lm_per_w; watts_per_m2 = total_wattage_w / floor_area_m2

Lumen method for uniform general lighting: illuminance (lux = lm/m²) × area gives the lumens reaching the work plane, and the light-loss factor accounts for depreciation and room absorption. Typical maintained illuminances: 100 lux circulation, 150–200 lux living, 300 lux kitchens, 500 lux desk work, 750 lux fine workshop tasks.

Inputs

Parameter	Type	Unit	Required	Description
`floor_area_m2`	number	m²	yes	Floor area of the room in square metres. Range: > 0, ≤ 100000
`room_type`	enum: living_room \| bedroom \| kitchen \| bathroom \| home_office \| workshop \| hallway \| dining \| custom		yes	Room type selecting a typical maintained illuminance (EN 12464-1 / IES ranges); use custom with target_lux for other values.
`target_lux`	number	lux	no	Optional illuminance in lux that overrides the room-type value; required when room_type is custom. Range: > 0, ≤ 100000
`lumens_per_bulb`	number	lm	default 800	Light output of one bulb or fixture (800 lm ≈ a 60 W incandescent equivalent, 1,100 lm ≈ 75 W, 1,600 lm ≈ 100 W). Range: > 0, ≤ 100000
`light_loss_factor`	number		default 0.8	Maintenance / utilisation factor for dirt, lamp ageing and light absorbed by walls (0.8 typical for light rooms, 0.6–0.7 for dark ones). Range: ≥ 0.3, ≤ 1
`luminous_efficacy_lm_per_w`	number	lm/W	default 100	Lumens per watt of the light source: LED 80–150, CFL 50–70, halogen 15–25, incandescent 10–17. Range: ≥ 5, ≤ 300

Outputs

Output	Type	Unit	Description
`target_lux_used`	number	lux	Illuminance used for the calculation.
`required_lumens`	number	lm	target_lux × floor_area_m2 / light_loss_factor.
`bulbs_needed`	integer		required_lumens / lumens_per_bulb, rounded up.
`installed_lumens`	number	lm	bulbs_needed × lumens_per_bulb.
`total_wattage_w`	number	W	required_lumens / luminous_efficacy_lm_per_w.
`watts_per_m2`	number	W/m²	total_wattage_w / floor_area_m2.

Example

20 m² home office, 800 lm LED bulbs: {"floor_area_m2":20,"room_type":"home_office"} → {"target_lux_used":500,"required_lumens":12500,"bulbs_needed":16,"installed_lumens":12800,"total_wattage_w":125,"watts_per_m2":6.25}

12 m² bedroom, 470 lm bulbs at 90 lm/W: {"floor_area_m2":12,"room_type":"bedroom","lumens_per_bulb":470,"luminous_efficacy_lm_per_w":90} → {"target_lux_used":100,"required_lumens":1500,"bulbs_needed":4,"total_wattage_w":16.7,"watts_per_m2":1.39}

GET https://tttkmbb.com/api/v1/calculate/room-lighting?floor_area_m2=20&room_type=home_office

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/room-lighting (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/room-lighting · Markdown: https://tttkmbb.com/energy/room-lighting.md · JSON definition: https://tttkmbb.com/energy/room-lighting.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="room-lighting"
OpenAPI operationId: calculate_room_lighting
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Lux (typical illuminance values) (reference)
Wikipedia – Luminous efficacy (reference)
U.S. DOE Energy Saver – Lighting Choices to Save You Money (government)

FAQ

Lux, lumens and watts – what is the difference?

Lumens measure the light a bulb emits, lux the light landing on a surface (1 lux = 1 lm/m²), and watts the electrical power; LED efficacy (lm/W) links watts to lumens.

Why divide by a light-loss factor?

Not all emitted light reaches the work plane: dark walls absorb it, fixtures and lamps get dirty and LEDs dim with age. A factor of 0.8 adds a 25 % allowance.

Are these lux values mandatory?

No. They are typical maintained illuminances from EN 12464-1 (workplaces) and IES recommendations; homes are not regulated, and older people generally need more light.

Related calculators

Electricity Cost Calculator — Running cost of the lights from total wattage and hours.
Circuit Load Calculator — Check the lighting circuit's breaker load.
Electric Power Calculator — Watts, volts and amps of the fixtures.