Buying lights by wattage stopped meaning anything the day LED arrived, and the number on the box now tells you what the fitting produces rather than what the room receives. Most rooms are dim not because the bulbs are weak but because the light they produce never gets to where it is wanted. Give this the room area, what you do in it, and how reflective its surfaces are, and it works backwards from the light you actually need on the worktop to the lamp lumens you have to install, then splits that figure into what lands and what the room absorbs.
The lighting budget
Lumens are what a lamp produces. Lux is what arrives on the surface you are working on. The gap between the two is the whole reason rooms end up dim.
The part of the floor the light has to cover. If you do not know it yet, the room area calculator will get it from the wall measurements.
Sets the target in lux. If two activities share the room, design for the harder one.
Printed on the packaging as lumens, not watts.
Result
What this calculator works out
Three figures, and the relationship between them is the useful part. The first is the target: how much light should be falling on the surface where the work happens, measured in lux. The second is the lamp output you have to install to achieve it, measured in lumens. The third is the number of fittings that output divides into, which is the number you actually take to the till.
The second figure is always larger than the first, sometimes by more than half, and that gap is not inefficiency you can design away. Light leaves a fitting in all directions. Some of it goes straight to the surface you care about; some of it bounces once off a wall and arrives second-hand; some of it is absorbed by a dark cabinet and never arrives at all. The stacked bar under the dial shows that split, because it is the single most useful thing to understand about why a room with plenty of bulbs in it still feels gloomy.
Lux measures arrival, lumens measure output
These two units get used interchangeably in shop copy and they are not the same thing. A lumen is a quantity of light leaving a source. A lux is one lumen landing on one square metre of surface. So the same 1,100 lumen bulb delivers about 3 lux spread over a large room and about 300 lux concentrated onto a small worktop.
A fitting described as 1,100 lm is telling you what it produces at the moment it is switched on for the first time. By the time that light has crossed the room, it has been reduced by every surface it met on the way, and the fitting itself will have dimmed a little with age.
This is why the same bulb can feel generous in a small white bathroom and useless in a large room with dark furniture. Nothing about the bulb changed. The room changed what happened to the light.
The two factors that quietly halve your light
Lighting design uses two multipliers to bridge the gap between lamp lumens and lux on a surface. Both are less than one, and both are doing real work rather than padding the calculation.
The utilisation factor accounts for the room. It asks what proportion of the light leaving the fittings ends up on the working surface, and it is driven almost entirely by how reflective the room is. Light bouncing off a white matte ceiling is still useful light; light hitting a dark wood bookcase is essentially gone. In a pale room the utilisation factor sits near 0.65. In a room with dark walls, timber panelling or a lot of gloss, it drops towards 0.5 or below, which means you need meaningfully more lamp output for the same result.
The light loss factor accounts for time. Lamps dim as they age, and dust and grime on the fitting and on the ceiling cut output further. A value around 0.9 is a fair working assumption for a domestic LED installation: the room will still meet its target towards the end of the lamps' life rather than only on the day they were fitted.
| Room surfaces | Utilisation factor | Lamp lumens required | Equivalent fittings at 1100 lm |
|---|---|---|---|
| White matte walls and ceiling | 0.65 | 8,205 lm | 8 |
| Mid tones, some timber | 0.57 | 9,356 lm | 9 |
| Dark, panelled or glossy | 0.51 | 10,458 lm | 10 |
| Dark room with a 3 m ceiling | 0.51 | 13,073 lm | 12 |
If you want the surface areas behind that table, the wall area calculator and the ceiling area calculator give them without a tape measure on a ladder. Read the table above as an argument for paint rather than for more bulbs. Moving a room from dark panelling to a pale matte finish saves two to four fittings for the same measured light on the worktop, and it saves the running cost of those fittings every year afterwards. The difference is largest exactly where people are most tempted to solve the problem by adding lamps.
Choosing the target level honestly
The single biggest lever in this calculation is the lux target, and it is the one people set by guesswork. Two hundred lux is comfortable for sitting and talking. Three hundred is the general figure quoted for kitchens. Five hundred is what you want where a knife or a pen is being used deliberately. Below is a working table; if a room has two uses, design for the more demanding one and put the other on a separate switch.
| Activity | Target lux | What under-lighting looks like |
|---|---|---|
| Passing through, stairs, landings | 100-150 | Missed steps, shadows at the turn |
| Sitting, talking, watching | 200 | Flat, slightly oppressive in the evening |
| General kitchen use and dining | 300 | Worktops in the shadow of your own body |
| Food preparation | 500 | Hard to see whether something is properly done |
| Desk work, reading for long periods | 500-750 | Eye fatigue after half an hour |
| Sewing, model work, fine craft | 750-1000 | Colour and edge detail disappear |
One level is usually not enough: the three layers
A single number for a whole room is a starting point, not a design. Rooms that work well have light arriving from more than one direction and at more than one intensity, and the reason is that a single bright overhead source does two unhelpful things: it puts your own shadow on whatever you are looking at, and it makes the room feel like a corridor.
The general wash
- The background level that lets you cross the room safely.
- Usually from the ceiling, spread wide and fairly even.
- This is the layer the calculator above is sizing.
- On its own it is comfortable but flat.
Light where the work is
- Concentrated light on a worktop, desk or hob.
- Almost always under-cabinet or a directed fitting.
- Lets the ambient layer sit lower and saves energy.
- Should light the surface, not the person using it.
Light that gives the room shape
- Wall washers, picture lights, a lamp in the corner.
- Contributes almost nothing to the measured level.
- Stops a bright room from feeling like an office.
- The cheapest layer to add and the easiest to get wrong.
The practical consequence: you can set the ambient target lower than the activity implies if you are also installing task lighting. A kitchen designed at 300 lux throughout needs considerable installed output. The same kitchen at 200 lux ambient with 500 lux on the worktop from under-cabinet fittings feels brighter where it matters and costs less to run.
A worked example, end to end
A kitchen-diner, 4.0 m by 4.0 m, with a 2.4 m ceiling, mid-tone walls, used for both cooking and eating, lit with 1,100 lm fittings.
- Size the floor4.0 × 4.0 = 16 m². Measure what the light has to cover, not the footprint of the building.
- Pick the harder useCooking wants 500 lux, eating wants 200. Design for 500 and put the dining end on a dimmer or a separate switch.
- What has to arrive16 m² × 500 lux = 8,000 lm landing on the work plane.
- Convert to lamp outputMid-tone surfaces put utilisation at 0.57, and light loss at 0.9 gives a combined 0.513. So 8,000 ÷ 0.513 = 15,591 lm to install.
- Divide into fittings15,591 ÷ 1,100 = 14.2, so 15 fittings.
- Sanity-check itFifteen fittings in 16 m² sounds like a lot, and it is: 500 lux across a whole kitchen genuinely is a lot of light. This is the point at which most people decide to light the worktop separately and drop the ambient layer to 200 lux. It halves the fitting count.
Almost every under-lit room is under-lit because somebody chose the fittings first and worked out the light level afterwards. A pack of six downlights is a tidy number to buy, so six get bought, and the room gets whatever six downlights produce.
Working the other way round feels worse at the point of purchase and better every evening afterwards. The calculator is deliberately structured so the fittings are the last number it gives you, not the first.
Things that make a room dimmer than the maths says
Do
- Measure the area the light has to cover, not the whole floor plan.
- Design for the most demanding thing that happens in the room.
- Treat pale matte walls as part of the lighting scheme.
- Put task light under cabinets where your body casts a shadow.
- Install fittings on separate switches so a bright room can be dimmed.
- Check the lumen figure on the box, not the wattage or the equivalent wattage.
Avoid
- Adding more of the same fitting when the problem is absorption.
- A single central pendant over a worktop you stand at.
- Glossy surfaces where you meant to bounce light, which throws it elsewhere instead.
- Fittings you cannot reach to clean, which lose output steadily and invisibly.
- Designing for the room empty, then filling it with dark furniture.
- Reading equivalent wattage on the box as though it were a light output.
Where this estimate stops being reliable
This is a room-level estimate built on a utilisation factor, and a utilisation factor is a statement about an average room. It will be wrong in predictable directions.
It under-reads in rooms where the geometry fights you: very deep rooms with light only at one end, rooms with a gallery or a void, and rooms where tall furniture blocks the bounce path between ceiling and worktop. It over-reads where you have deliberately installed task lighting, because the calculator assumes one uniform level across the whole floor.
It says nothing about glare, about colour rendering, or about where fittings should physically go. A room with a high ceiling needs a different approach from one with a low ceiling, and the room volume calculator puts a number on that gap. A room can hit its lux target on paper and still be unpleasant to be in if the light arrives from the wrong direction or at the wrong colour temperature. Those are design questions, not arithmetic ones.
How this calculator is put together
The target level comes from the activity you select. Delivered lumens are the target multiplied by the floor area. Lamp lumens are delivered lumens divided by the combined utilisation and light loss factor, with an uplift for ceiling height because light has further to travel and more surfaces to meet on the way down. Fittings are lamp lumens divided by the output per fitting, rounded up, because you cannot buy a fraction of a lamp.
The utilisation factor of 0.65 for a light matte room, and the light loss factor of 0.9, are domestic working figures rather than anything measured in your room. The surface finish control scales the utilisation factor down to 0.57 and 0.51 for mid and dark rooms. If you want a figure specific to your own space, measure it: a lux meter app on a phone is not laboratory-grade but is easily good enough to tell 150 lux from 400 lux, which is the range that actually matters.
Related tools that feed into this one: the room area calculator for getting the floor area from wall measurements, the room volume calculator if you also need the cubic figure, the ceiling area calculator if you are measuring surfaces rather than space, and the TV viewing distance calculator if the same room also has to work as a place to watch something.
Questions people actually ask
Why does the calculator want so many fittings?
Because a uniform target across a whole floor area is demanding. Five hundred lux over every square metre of a kitchen is genuinely a great deal of light, and the honest answer is often that you should not do it: drop the ambient layer to around 200 lux and light the worktop separately. Try the lower target and see how the fitting count falls.
Do LED watts still tell me anything?
Only about running cost. Two bulbs of the same wattage can differ in output by a factor of two depending on quality and on how warm the light is. Lumens are the output figure; the wattage the calculator reports is derived from lumens at roughly 85 lm per watt purely so you can sanity-check the circuit.
Does a higher ceiling really need more light?
Yes, and more than people expect. The light has further to travel, spreads over a wider cone on the way down, and meets more wall area that will absorb some of it. The uplift here is 12 per cent at 2.7 m and 25 per cent at 3 m and above.
Should I use one big fitting or several small ones?
Several, almost always. A single bright source creates hard shadows and puts your own body between the light and the work. Several sources at the same total output light the room far more evenly and let you switch parts of it off.
What about warm versus cool white?
It does not change the calculation at all, because colour temperature and output are independent. Very warm lamps are marginally less efficient in lumens per watt, but the difference is small next to choosing the right target level and the right surface finishes.
Can I just buy the biggest panel that fits?
You can, and it will hit the target, but a single very bright ceiling panel is the classic cause of a room that measures well and feels like a supermarket. Spread the same total output across more fittings if you can.