How the lighting calculator works
Bulbs are labelled in lumens, the total light they emit. What your eyes judge is illuminance: lumens landing on each square metre of floor or worktop, measured in lux (or foot-candles, lumens per square foot; 1 fc = 10.76 lux). Each room type has a recommended level, so multiplying that level by the floor area tells you how many lumens have to arrive. Not all emitted light arrives, though: shades absorb some, and dark walls reflect far less than white ones. The calculator divides by a light-loss factor to cover that.
Lumens = target lux × floor area in m² ÷ surface factorThe factor is 0.8 for a bright room with open fittings, 0.6 for average décor and 0.45 for dark rooms or heavy shades. The bulb count is the lumens divided by the output of one lamp, rounded up. Modern LEDs deliver around 100 lumens per watt, so the power drawn is the number of lamps × lumens per lamp ÷ 100.
Worked example
A 4 m × 3.5 m living room has 14 m² of floor. At 150 lux the room needs 150 × 14 = 2,100 lumens on the floor. With pale walls and open fittings (factor 0.8) the lamps must emit 2,100 ÷ 0.8 = 2,625 lumens. Four 800 lm LED bulbs give 3,200 lm, around 32 W of LED in total, against roughly 240 W with old 60 W incandescent bulbs. For a 13 × 11 ft room (143 ft², 13.3 m²) the same calculation gives about 2,490 lumens, still four bulbs.
Recommended light levels
| Space or task | Lux | Foot-candles | Lumens per m² of floor (factor 0.8) |
|---|---|---|---|
| Hallway, stairs, bedroom (general) | 100 | 9 | 125 |
| Living room | 150 | 14 | 190 |
| Dining room, bathroom | 200 | 19 | 250 |
| Kitchen, laundry | 300 | 28 | 375 |
| Desk, kitchen worktop, reading | 500 | 46 | 625 |
| Detailed hobby or workbench | 750 | 70 | 940 |
LED lamp equivalents
| Old incandescent | LED lumens | Typical LED watts |
|---|---|---|
| 25 W | about 250 lm | 3 W |
| 40 W | about 450 lm | 4–5 W |
| 60 W | about 800 lm | 7–9 W |
| 75 W | about 1,100 lm | 10–12 W |
| 100 W | about 1,600 lm | 14–16 W |
Planning tips
- Layer the light. The figure here is ambient light. A reading lamp, under-cabinet strips or a mirror light add task lighting exactly where 500 lux is useful, without flooding the whole room.
- Spread the fittings. Downlights are usually spaced at about 1 to 1.5 times the ceiling height apart, and no closer than half that spacing to the walls.
- Choose colour temperature by room. 2,700–3,000 K feels warm in living spaces and bedrooms; 3,500–4,000 K suits kitchens, bathrooms and workshops.
- Fit dimmers in living and dining rooms so one layout covers both evening and cleaning light; check the lamps are marked dimmable.
- Look at CRI. A colour rendering index of 90 or more makes skin, food and paint colours look right, which matters most in kitchens and bathrooms.
New circuits and fixed fittings in bathrooms fall under electrical regulations; use a qualified electrician where the law requires it.
A reproducible calculation with complete inputs
For a reproducible room lighting calculator example, use the metric input system and enter Room length: 4 m; Room width: 3.5 m; Room type: Living room – 150 lux (14 fc); Surfaces and fittings: Pale walls and ceiling, open fittings or downlights; Lumens per bulb or fitting: 800 lm. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Lumens needed | 2625 | 2625 | lm |
| Bulbs or fittings | 4 | 4 | |
| Approx. LED power | 32 | 32 | W |
| Target light level | 150 | 150 | lux |
| Floor area | 14 | 14.0 | m² |
How should I prepare these entries?
Room length: enter the value in m. Room width: enter the value in m. Room type: choose among Living room – 150 lux (14 fc), Bedroom or hallway – 100 lux (9 fc), Dining room or bathroom – 200 lux (19 fc), Kitchen, general – 300 lux (28 fc), Home office or kitchen worktop – 500 lux (46 fc), Workshop bench or craft room – 750 lux (70 fc). Surfaces and fittings: choose among Pale walls and ceiling, open fittings or downlights, Mid-tone walls or diffused shades, Dark walls, deep lampshades or high ceiling. Lumens per bulb or fitting: On the box: about 450 lm replaces a 40 W bulb, 800 lm a 60 W, 1,100 lm a 75 W. GU10 spots: 350–450 lm. Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
Frequently asked questions
How many lumens do I need for a living room?
About 150 lux of general light, so roughly 150–190 lumens per m² (15–18 per ft²) after losses. A 20 m² (215 ft²) living room needs around 3,750 lumens, for example five 800 lm bulbs.
How many lumens for a kitchen?
Around 300 lux for general light, so about 375 lumens per m² in a bright kitchen, plus 500 lux on the worktops from under-cabinet lights.
How many recessed lights do I need in a room?
Divide the lumens needed by the output of one downlight. A 12 m² bedroom at 100 lux needs 1,500 lm, which is four GU10 spots of 400 lm.
How many lumens is a 60 watt bulb?
About 800 lumens. An LED giving the same light uses 7–9 W.
What is the difference between lux and lumens?
Lumens measure the light a bulb emits; lux measures the light falling on one square metre. 1,000 lumens spread evenly over 10 m² gives 100 lux.