Lumens calculator: how much light a room needs
This lumens calculator tells you how many lumens a room needs and how many LED bulbs, downlights, tubes or panels will deliver them. Choose a room type to load a recommended illuminance in lux (or foot-candles), enter the room size, pick a light fitting and you get the total lumens, the number of fittings, the installed wattage and what the lighting will cost to run each month.
Lighting is specified in lux — lumens per square metre — and the recommended level depends on the task: 100 lux is fine for a bedroom, 300 for a kitchen, 500 for a desk or a shop. Because walls, ceilings and the fittings themselves absorb light and LEDs dim with age, the lamp lumens you install must exceed the raw lux × area figure; the calculator divides by a light-loss and room factor (0.4–0.7 for most homes) to allow for that.
Values come from the usual IES and EN 12464-style recommendations for homes and workplaces and are a design guide rather than a photometric calculation.
How to use the Lumens & Lighting Calculator
- Choose metres and lux or feet and foot-candles, then enter the room length and width.
- Pick the room or task from the list; the target illuminance fills in automatically, or type your own value.
- Choose a light-loss and room factor: ×0.7 for light walls and a normal ceiling, ×0.55 for a typical home, ×0.4 for dark walls or high ceilings, or ×1 to compare raw lumens.
- Select the fitting you plan to use — the lumens and watts fill in — or enter custom values from the box, then your hours of use, tariff and price per fitting.
- Read the lumens, the number of fittings, total watts, the illuminance actually achieved and the running cost; copy or share the summary.
Lumens & Lighting Calculator features
Room-type lux table
Fifteen presets from bedroom (100 lux) to workbench (750 lux), with custom lux or foot-candles.
Light-loss factor
Accounts for wall colour, ceiling height, fitting efficiency and lamp ageing so the room really reaches the target.
Fitting presets
LED bulbs, downlights, battens, 2×2 and 1×4 panels, fluorescent tubes and incandescent lamps, or a custom lumen/watt pair.
Illuminance check
Shows the lux you will actually get with the rounded-up number of fittings.
Running and purchase cost
kWh per day and year, monthly and yearly electricity cost in your currency, and the cost of the fittings.
Metric or imperial
Square metres and lux, or square feet and foot-candles (1 fc = 10.76 lux).
How the lumens are calculated
Foot-candles convert at 1 fc = 10.764 lux, and 1 lm/ft² = 1 fc. The “lumens per m² installed” figure is simply installed lumens ÷ area — handy for comparing with the old rule of thumb of 150–300 lm/m² for living spaces.
- Raw lumens = lux × floor area in m². A 4 × 3 m living room at 150 lux needs 150 × 12 = 1,800 lm reaching the floor.
- Lamp lumens = raw lumens ÷ light-loss factor. With a typical factor of 0.55: 1,800 ÷ 0.55 = 3,273 lm.
- Fittings = lamp lumens ÷ lumens per fitting, rounded up: 3,273 ÷ 800 = 4.1 → 5 bulbs of 9 W (45 W total), giving about 183 lux.
- Cost = watts × hours ÷ 1,000 × tariff: 45 W × 5 h = 0.225 kWh a day, about ₹54 a month at ₹8 per unit.
Recommended illuminance by room
Light the whole room to the lower level and add task lighting (a desk lamp, under-cabinet strip) where the higher level is needed, rather than flooding the whole room to 500 lux.
| Room or task | Lux | Foot-candles | Lumens for 12 m² (÷0.55) |
|---|---|---|---|
| Bedroom, hallway, stairs | 100 | 10 | 2,200 |
| Living room, dining room | 150 | 15 | 3,300 |
| Bathroom | 200 | 20 | 4,400 |
| Kitchen (general), garage, classroom | 300 | 30 | 6,500 |
| Kitchen worktop, study desk, office, shop | 500 | 50 | 10,900 |
| Workbench, sewing, detailed work | 750 | 75 | 16,400 |
Choosing fittings
- Compare lamps by lumens, not watts: a 9 W LED gives the 800 lm of an old 60 W incandescent. Efficacy of 80–110 lm/W is typical for LED bulbs, 100–130 lm/W for panels and tubes.
- Colour temperature changes the feel, not the lux: 2700–3000 K warm white for living areas and bedrooms, 4000 K neutral for kitchens and offices, 5000–6500 K cool white for workshops.
- Spread the lumens over several fittings — four 1,100 lm downlights light a room more evenly than one 4,400 lm fitting in the middle.
- Lighting adds heat that the AC must remove; the AC tonnage calculator includes it as appliance watts. The electricity bill calculator adds lighting to the rest of your appliances.
Lumens & Lighting Calculator FAQ
How many lumens do I need for a 12 × 12 ft room?
144 ft² is 13.4 m². For a living room at 150 lux that is 2,000 lm reaching the floor, about 3,650 lm of lamps after a 0.55 light-loss factor — five 800 lm bulbs or four 1,100 lm bulbs. The lumens calculator gives the figure for your room type and fitting.
How many lumens per square foot should a room have?
Roughly 10–20 lm/ft² for living areas and bedrooms, 30–40 for kitchens and bathrooms and 50–75 for desks and workshops, before allowing for light loss. In metric that is about 100–200, 300–400 and 500–750 lm/m².
What is the difference between lumens and lux?
Lumens measure the light a lamp emits; lux measures how much lands on a surface — one lux is one lumen per square metre. The same bulb gives fewer lux in a bigger room.
Why is the light-loss factor needed?
Dark walls absorb light, a fitting’s diffuser blocks some, dust gathers and LEDs lose 10–30% of output over their life. Dividing by 0.55 for a typical home means the room still meets the target after all that.
How many LED bulbs do I need for a bedroom?
A 3.5 × 3.5 m bedroom at 100 lux needs about 1,225 lm of lamps with a 0.7 factor — two 800 lm bulbs or one 1,500 lm ceiling light, plus bedside lamps for reading.
Does the calculator work for offices and shops?
Yes. Office (500 lux), classroom (300), retail (500) and warehouse (150) presets are included, and panel fittings of 3,600 and 4,800 lm are in the list. For large spaces a lighting designer will also check uniformity and glare.