Lumens Calculator
Find how many lumens a room needs and how many bulbs that is, from the room size and how bright you want it. Advanced adds ceiling height, wall colour, light losses, watts, running cost and a lux, foot-candle and lumens converter.
Room values are typical guide values, not an official standard. Change the number if you like it brighter or softer.
Typical LED bulb sizes. The lumens are printed on the box; the old-bulb watts are rough.
Up to 9 ft (2.74 m) counts as normal. Higher ceilings get more light in proportion, a rough adjustment.
Dark walls soak up light, so you need more. A rough adjustment, not a measured one.
Share of the bulbs' light that reaches the room after shades, dust and ageing. 0.8 is a rough guess; 1 means no losses.
Lumens ÷ watts on the box. LED bulbs are often 80–150 lm/W.
Find it on your bill as the unit rate.
Lumens are spread over the room's floor area above.
How your lumens are worked out
Lumens = lux × floor area in m². 1 foot-candle = 10.764 lux.
- 1
Floor area
12 ft × 15 ft= 180 sq ft= 16.72 m²
- 2
How bright
Living room150 lux= 13.94 fc
- 3
Lumens needed
150 × 16.72 m²= 2,508 lm
- 4
Bulbs to fit
2,508 ÷ 800 lm= 3.14→ 4 bulbs
Read more: the formula, a worked example, room guide values, bulb sizes and running cost
Lux is how much light lands on each square metre (1 lux = 1 lumen per m²). So the lumens a room needs are the brightness you want times the floor area. The room values and adjustments here are typical guide values, not an official standard, so treat the result as a starting point. A lighting designer works out a full plan for big or special rooms.
The formula
area (m²) = length × width (1 sq ft = 0.09290304 m²)
lumens needed = target lux × area in m²
(or foot-candles × area in sq ft; 1 foot-candle = 10.7639 lux)
Advanced: lumens to fit = lumens needed × ceiling (height ÷ 9 ft, if over 9 ft) × walls (light 1, medium 1.1, dark 1.25) ÷ utilisation factor
bulbs = lumens to fit ÷ lumens per bulb, rounded up
LED watts = lumens to fit ÷ LED lumens per watt
old bulb watts = lumens ÷ 15 to ÷ 12 (rough)
cost a year = watts ÷ 1000 × hours a day × 365 × price per kWh
Worked example
A living room 12 ft × 15 ft = 180 sq ft. In square metres that is 180 × 0.09290304 = 16.72 m². The typical living room value is 150 lux (150 ÷ 10.764 = 13.94 foot-candles). Lumens needed = 150 × 16.7225 = 2,508 lumens. With 800 lumen bulbs, 2,508 ÷ 800 = 3.14, so 4 bulbs. Four bulbs give 3,200 lumens, which is 3,200 ÷ 16.7225 = 191 lux, a little brighter than the target.
In Advanced, with an 8 ft ceiling (no change), light walls (× 1) and a utilisation factor of 0.8: 2,508.38 ÷ 0.8 = 3,135 lumens to fit, so 3,135 ÷ 800 = 3.92, still 4 bulbs (giving 3,200 × 0.8 ÷ 16.7225 = 153 lux). At 100 lumens per watt the LEDs need 3,135.48 ÷ 100 = 31.35 W. On for 5 hours a day, that is 31.35 × 5 × 365 ÷ 1000 = 57.22 kWh a year, which costs $8.58 at $0.15 per kWh. Old incandescent bulbs would need about 3,135 ÷ 15 = 209 W to 3,135 ÷ 12 = 261 W. At the middle figure of 13.5 lumens per watt (232.26 W) they would use 423.87 kWh and cost $63.58 a year, so the LEDs save $55.00.
Typical brightness by room (guide values, not a standard)
| Room | Lux | Foot-candles | Lumens for 10 m² (108 sq ft) |
|---|---|---|---|
| Living room | 150 | 13.9 | 1,500 |
| Bedroom | 100 | 9.3 | 1,000 |
| Kitchen | 300 | 27.9 | 3,000 |
| Bathroom | 200 | 18.6 | 2,000 |
| Home office or study | 400 | 37.2 | 4,000 |
| Dining room | 150 | 13.9 | 1,500 |
| Kids' room or playroom | 200 | 18.6 | 2,000 |
| Laundry or utility room | 300 | 27.9 | 3,000 |
| Hallway or stairs | 100 | 9.3 | 1,000 |
| Garage or workshop | 300 | 27.9 | 3,000 |
These are typical values from home lighting guides for general light across the room. Workplace and school rules in many countries set their own levels for desks and tasks; check your local rules for those. Older people and detailed work such as sewing usually need more light, often from a task lamp.
Bulb sizes (typical)
| LED lumens | Old incandescent bulb (rough) | LED watts at 100 lm/W |
|---|---|---|
| 450 lm | 40 W | 4.5 W |
| 800 lm | 60 W | 8 W |
| 1,100 lm | 75 W | 11 W |
| 1,600 lm | 100 W | 16 W |
Buy bulbs by lumens, not watts. Watts only tell you how much power a bulb uses. Old incandescent bulbs gave roughly 12–15 lumens per watt; LED bulbs give about 80–150.
Tips
- Layer your light: general light for the room, task light for desks and worktops, and accent light for mood.
- Use a dimmer if the bulb count rounds up a lot, so you can bring it back to the brightness you like.
- Warm white (2,700–3,000 K) feels cosy for living rooms and bedrooms; cool white (4,000 K and up) suits kitchens, garages and work areas.
- Count only lights that light the room: a lamp with a thick shade gives much less useful light than its lumens suggest.
Common questions
How many lumens do I need for a room?
Multiply the brightness you want in lux by the floor area in square metres. A 12 × 15 ft living room is 16.72 m²; at a typical 150 lux it needs 150 × 16.72 = about 2,508 lumens. In feet, multiply foot-candles by square feet: 13.94 × 180 gives the same 2,508 lumens.
What is the difference between lumens and lux?
Lumens measure how much light a bulb gives out in total. Lux measures how much of that light lands on each square metre of a surface. So 1 lux is 1 lumen per square metre, and 1 foot-candle is 1 lumen per square foot, which is about 10.76 lux.
How many 800 lumen bulbs do I need?
Divide the lumens the room needs by 800 and round up. A room that needs 2,508 lumens needs 2,508 ÷ 800 = 3.14, so 4 bulbs. Four bulbs give 3,200 lumens, a little brighter than the target, or use a dimmer.
How many lumens is a 60 W bulb?
An old 60 W incandescent bulb gives about 800 lumens. Rough matches for LED bulbs: 450 lumens for 40 W, 800 for 60 W, 1,100 for 75 W and 1,600 for 100 W. An 800 lumen LED usually uses only 8 to 10 W.
How bright should a home office be?
Typical guide values are about 300 to 500 lux on the desk for reading and screen work, and this page uses 400 lux. A desk lamp can add light where you need it without lighting the whole room that brightly.
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