Two numbers, in this order: how much light, then where it goes
Lighting a room is two calculations that people usually collapse into one. The first is how much light the space needs in total, which comes from what happens in the room and how big it is. The second is how to distribute it so it arrives evenly, which comes from the ceiling height. They are independent, and they frequently disagree, which is the most useful thing this calculator shows you.
The output side is straightforward. Lumens are the measure of light a fixture produces, and a target expressed in lumens per square foot times the floor area gives a total. Multiply, done.
The distribution side is the ceiling height rule: space recessed fixtures roughly half the ceiling height apart, and set the outer row about half a spacing from the wall. On an eight foot ceiling that is four feet between centres and two feet off the wall. The reason is geometry — a downlight throws a cone, the cone widens with distance, and at about half the mounting height the cones from adjacent fixtures overlap enough that the pool of light on the floor reads as continuous rather than as a row of spots. Space them further and the ceiling starts to scallop. This rule is about evenness, not brightness, and it produces a minimum number of positions independent of how bright each one is.
Lumens per square foot, and how much to trust it
These are design guidance and every source states them slightly differently, because they are conversions of footcandle recommendations that were themselves ranges. Use them as a starting bracket rather than a specification.
| Room | Lumens per sq ft | What sets the number |
|---|---|---|
| Hallway, stairs | 5 - 10 | Circulation only, uniformity matters more than level |
| Living room, bedroom | 10 - 20 | Ambient. Reading and hobbies get their own lamps. |
| Dining room | 20 - 30 | The table gets a pendant, this is the surround |
| Bathroom | 20 - 40 | General light. Mirror lighting is separate and non-negotiable. |
| Kitchen, general | 30 - 40 | Plus dedicated task light on every work surface |
| Home office | 30 - 50 | Screen work needs less, paper and detail work more |
| Garage, workshop | 50 - 75 | Small tolerances, sharp tools, poor surface reflectance |
Three things push you toward the top of a range or beyond it. Dark walls and floors, because a large part of what you perceive is light bounced off surfaces and a dark room absorbs it. High ceilings, because illuminance falls off with the square of the distance from the fixture. And age, because the light a sixty-year-old eye needs to read comfortably is substantially more than a twenty-year-old needs.
One honest limitation. The lumens-per-square-foot method uses the light leaving the fixture, and a proper lighting calculation uses the light arriving at the working plane. The professional version, the lumen method, multiplies the fixture output by a coefficient of utilisation that accounts for room proportions and how reflective the surfaces are, and by a light loss factor for dirt accumulation and lamp output falling over time. Together those commonly remove 30 to 50 percent. The simple method is calibrated by habit to roughly absorb that, which is part of why the published ranges are as wide as they are.
When the grid and the lumens disagree
Run a 12 by 14 living room on an 8 foot ceiling and this happens immediately. The grid at half the ceiling height wants something like 12 positions. The brightness target is around 2,500 lumens, which is three or four ordinary downlights. Twelve fixtures at 210 lumens each is not a product anyone sells for the purpose.
Both numbers are right and they are answering different questions. The grid is telling you the maximum spacing for an even wash; the lumen figure is telling you the total output. The resolution is almost always the same: keep the even layout, fit the fixtures you actually want, and put them on a dimmer. A dimmer costs less than the extra cans and it makes the room work at two in the afternoon and at eleven at night, which no fixed installation does. The alternative, spacing wider and accepting some scalloping, is legitimate in a room that has table lamps and floor lamps doing the real work, where the ceiling lights are fill.
Where you should not compromise is regularity. An uneven grid is visible from anywhere in the room, permanently, and no amount of correct total output fixes it. If the arithmetic asks for eleven fixtures, fit ten or twelve in a rectangle rather than eleven in a pattern that has to explain itself.
Things the grid does not know about
The layout this produces is a rectangle on an empty ceiling, and no ceiling is empty. Joists run one way and a can has to fit between them. Ducts, plumbing stacks and the odd steel beam will take out a position and move the whole row. In a kitchen the row over the base cabinets belongs at the front edge of the wall cabinets rather than on the grid, because a downlight on the grid line puts the upper cabinet between the lamp and the countertop and you end up working in your own shadow. Beds, sofas and dining tables have a say too: a downlight directly over the head of a bed is unpleasant to lie under, and a downlight directly over a dining table competes with the pendant that belongs there.
So take the grid as the starting position, mark it on the ceiling in pencil, and then go up and look. Move what has to move, keep the rows parallel and the spacing within a row equal, and accept a slightly larger gap where the beam runs rather than one fixture that is six inches out of line. Then, before you cut fifteen holes, cut one, fit one fixture and switch it on at night. What a beam angle and a colour temperature actually look like in your room, with your walls, is not something any number on this page will tell you.
Questions people ask
How many recessed lights for a 12 by 14 room?
The layout and the brightness give different answers and you need both. With an 8 foot ceiling, the half-the-ceiling-height rule wants fixtures about 4 feet apart, which is a 4 by 3 grid, so 12 positions with the outer ones about 2 feet off the wall. The brightness side: 168 square feet of living room at 10 to 20 lumens per square foot is 1,700 to 3,400 lumens, which is three to five typical 800 lumen downlights. Most people fit six to eight in a room that size, spaced evenly, on a dimmer. That is a compromise between the two, and it is the right one.
How far apart should recessed lights be?
Roughly half the ceiling height between centres, and about half that from the wall. Eight foot ceiling, four feet apart, two feet off the wall. Nine foot ceiling, four and a half feet apart. The rule exists because the light cones from adjacent fixtures need to overlap before they reach the floor, and how far down that happens scales with the mounting height. Space wider and you get scalloping on the walls and pools on the floor; space tighter and you get a very even wash, which is what you want over a work surface and can feel clinical in a living room. The rule sets a maximum, not a target, so erring tighter is safer than erring wider.
How many lumens do I need per square foot?
It depends on the room and the source you ask, which is why this page gives ranges rather than a figure. Broadly: 5 to 10 for a hallway, 10 to 20 for a living room or bedroom, 20 to 40 for a bathroom, 30 to 40 for general kitchen lighting, 30 to 50 for an office and 50 to 75 for a workshop. These are design guidance and not a code requirement. They also describe the light leaving the fixtures rather than the light landing on the work surface, so dark walls, a high ceiling or narrow-beam trims all mean aiming higher in the range. Task lighting on counters, desks and at mirrors sits on top of these numbers, not inside them.
Is it better to have more dim lights or fewer bright ones?
More fixtures at lower output, if you can afford the holes, because evenness is what makes a ceiling look intentional and glare is what makes it look cheap. A small number of high-output downlights produces bright spots you can see the source of, hard shadows, and noticeable dark ground between them. The same total lumens spread across more fixtures gives softer shadows and a wall that is lit from top to bottom. The practical limit is cost and ceiling clutter. Whichever way you go, dim the circuit — a room lit at full output all evening is the most common complaint about recessed lighting and the easiest thing to have fixed at the wiring stage.