# 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.

- Calculator id: `room-lighting` · Category: Home Energy & Electrical (`energy`) · Tool name: `calculate_room_lighting`
- Canonical page: https://tttkmbb.com/energy/room-lighting · This document: https://tttkmbb.com/energy/room-lighting.md · JSON definition: https://tttkmbb.com/energy/room-lighting.json

## Purpose

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.

**Use when:** 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).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `floor_area_m2` | number | m² | required | Floor area of the room in square metres. (> 0, max 100000) |
| `room_type` | enum: living_room \| bedroom \| kitchen \| bathroom \| home_office \| workshop \| hallway \| dining \| custom |  | required | Room type selecting a typical maintained illuminance (EN 12464-1 / IES ranges); use custom with target_lux for other values. |
| `target_lux` | number | lux | optional | Optional illuminance in lux that overrides the room-type value; required when room_type is custom. (> 0, max 100000) |
| `lumens_per_bulb` | number | lm | optional, 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). (> 0, max 100000) |
| `light_loss_factor` | number |  | optional, 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). (min 0.3, max 1) |
| `luminous_efficacy_lm_per_w` | number | lm/W | optional, default 100 | Lumens per watt of the light source: LED 80–150, CFL 50–70, halogen 15–25, incandescent 10–17. (min 5, max 300) |

## Output

| Field | 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. |

## 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.

## Data Sources

- Wikipedia – Lux (typical illuminance values) — https://en.wikipedia.org/wiki/Lux (reference, retrieved 2026-09-24)
- Wikipedia – Luminous efficacy — https://en.wikipedia.org/wiki/Luminous_efficacy (reference, retrieved 2026-09-24)
- U.S. DOE Energy Saver – Lighting Choices to Save You Money — https://www.energy.gov/energysaver/lighting-choices-save-you-money (government, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/room-lighting?floor_area_m2=…&room_type=…`
- `POST https://tttkmbb.com/api/v1/calculate/room-lighting` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/room-lighting · OpenAPI operationId `calculate_room_lighting` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "room-lighting", "inputs": {…}}`

## Example

- 20 m² home office, 800 lm LED bulbs: inputs `{"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: inputs `{"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
```

## Limitations

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). 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Electricity Cost Calculator](https://tttkmbb.com/everyday/electricity-cost.md) — Running cost of the lights from total wattage and hours.
- [Circuit Load Calculator](https://tttkmbb.com/energy/circuit-load.md) — Check the lighting circuit's breaker load.
- [Electric Power Calculator](https://tttkmbb.com/physics/electric-power.md) — Watts, volts and amps of the fixtures.