Illuminance, measured in lux (lumens per square metre) or foot-candles, is how bright a surface is lit. Recommended levels depend on the task: relaxing needs far less light than chopping vegetables or sewing. Dark walls and high ceilings absorb light, so add 10–20% in those rooms.
How to use the Lighting Calculator
- Room / activity (lux)
- Custom illuminance
- Room length
- Room width
- Bulb brightness
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the room and activity (or enter a custom lux level), enter the room length and width and the lumens of the bulbs you plan to use.
Tips
- Use the figures from your own bills and product labels – prices and appliance ratings vary widely.
- Look at yearly totals: small daily costs add up and make it easier to compare options.
- Round quantities up to whole packs, rolls or bulbs when you shop.
- Re-check the numbers when prices change; most results scale directly with the unit price.
Common mistakes to avoid
- Confusing watts (power) with watt-hours or kilowatt-hours (energy).
- Comparing prices without bringing them to the same unit first.
- Ignoring one-off costs such as installation, delivery or replacement parts.
Inputs explained
| Field | What to enter |
|---|---|
| Room / activity (lux) | Choose one: Living room – general (150 lux), Living room – reading / task (400 lux), Kitchen – general (300 lux), Kitchen – worktop task (750 lux) … |
| Custom illuminance | In lux |
| Room length | Units: m, ft, cm, in |
| Room width | Units: m, ft, cm, in |
| Bulb brightness | Choose one: 450 lm (40 W equivalent), 800 lm (60 W equivalent), 1,100 lm (75 W equivalent), 1,600 lm (100 W equivalent) … |
What each result means
- Room area = [Room length]×[Room width]
- Lumens needed = [Illuminance used]×[Room area]
- Bulbs needed = ceil([Lumens needed]÷[Bulb brightness] – 0.000001)
- Approximate LED watts in total = [Lumens needed]÷100
Example values
How the main result changes when Custom illuminance changes and everything else stays at the example values above:
| Custom illuminance (lux) | Lumens needed (lm) |
|---|---|
| 150 | 4,200 |
| 225 | 4,200 |
| 300 | 4,200 |
| 375 | 4,200 |
| 450 | 4,200 |
| 600 | 4,200 |
Formula
Lumens needed = illuminance (lux) × area (m²)1 foot-candle = 10.764 luxBulbs = ceil(lumens ÷ lumens per bulb)Worked example
Inputs — Room / activity (lux): Kitchen – general (300 lux); Custom illuminance: 300 lux; Room length: 4 m; Room width: 3.5 m; Bulb brightness: 800 lm (60 W equivalent).
Results — Room area = 150.7 ft²; Illuminance used = 300 lux; Lumens needed = 4,200 lm; Bulbs needed = 6; Approximate LED watts in total = 42 W.
Frequently asked questions
How many lumens does a living room need?
About 150 lux for general lighting – a 20 m² (215 ft²) room needs about 3,000 lumens.
How many lumens for a kitchen?
About 300 lux overall and 500–750 lux on worktops.
What is the difference between lux and lumens?
Lumens measure the light a bulb emits; lux measure how much light lands on each square metre.
What formula does the Lighting Calculator use?
Lumens needed = illuminance (lux) × area (m²).