Recessed Lighting Calculator

Use this recessed lighting calculator to plan can lights for a room. Enter the room size, the ceiling height and the light you’re buying. It tells you how many lights you need, how far apart to put them and how far from the walls, and draws the layout to scale. If you know where your ceiling joists are, it also flags any light that would land on one.

About the recessed lighting calculator

Planning recessed lights is two questions that most tools answer separately. How many lights does the room need to be bright enough? And where do they go so the light is even? This calculator answers both, then checks the answer against two things that catch people out on the day: the beam of the light and the joists in the ceiling.

• A count from brightness. Room area times the light level you want, divided by the lumens of one light.
• A layout that fits your room. Rows and columns spaced evenly in both directions, with the first light half a space from each wall.
• Tape-measure marks. The distance of every row and column from the wall, in feet and inches to the nearest quarter inch.
• A beam check. It works out the pool of light each fixture throws on the floor and adds lights if a narrow beam would leave dark bands.
• A joist check. Enter your joist spacing and direction, and it tells you which lights clash and how far to move them.
• A plan drawn to scale. So you can see the layout before you cut anything.

It runs in your browser. Nothing you enter is sent anywhere or saved.

How to use the recessed lighting calculator

1. Enter the room. Length, width and ceiling height in feet. Use decimals for inches: 10 ft 6 in. is 10.5.
2. Choose the room use. This fills in a light level from the middle of the published range. Change it if you like a brighter or dimmer room.
3. Enter the light. Lumens and beam angle are on the box or the spec sheet.
4. Add joists, if you know them. Which way they run, how far apart, and where the first one is.
5. Read the layout. The count, the spacing, the plan and the table of marks.

The recessed lighting calculator formula

There are three steps. You can check each one by hand.

1. How many, for brightness

lumens needed = length x width x foot-candles
lights = lumens needed / lumens per light, rounded up

A foot-candle is one lumen per square foot, so the first line is just area times brightness. Take a 12 ft by 12 ft living room at 15 foot-candles: 144 x 15 = 2,160 lumens. With 800 lumen lights, that’s 2.7, so 3 lights.

2. The grid

Three lights in a square room can’t be spaced evenly. In a line, they’d be 4 ft apart one way and 12 ft the other. So the calculator takes the smallest grid where the spacing in one direction is no more than twice the other. Here that’s 2 rows of 2: four lights.

spacing = room length / columns   (and room width / rows)
first light from the wall = spacing / 2

For the 12 ft room: 12 / 2 = 6 ft apart, and 3 ft from each wall. Four 800 lumen lights give 22.2 foot-candles, more than the 15 you asked for. The calculator tells you that lights of about 540 lumens each, or a dimmer, bring it back.

3. The beam

pool of light on the floor = 2 x ceiling height x tan(beam angle / 2)

The beam angle is the cone where the light is at least half as strong as it is in the centre. If the pools from neighbouring lights meet, there’s no dark band between them. With a 90 degree beam on an 8 ft ceiling, the pool is 16 ft across, so any sensible grid passes. With a narrow beam it’s a different story, as the example below shows.

How bright should the room be?

These are the ranges behind each room use. They’re published figures, not ours, and the sources don’t agree exactly, so the calculator starts in the middle and lets you move.

Search results attribute similar ranges to the Illuminating Engineering Society. We haven’t read its handbook, so we don’t cite it as a source.

Light level is partly taste. Older eyes need more. Dark walls and floors soak light up. If you’re unsure, plan for the upper half of the range and fit a dimmer. You can always turn lights down. You can’t turn them up past full.

The level changes the count quickly. Here’s the 12 ft by 12 ft room with 800 lumen lights:

Recessed lighting spacing: the rules of thumb

Lighting guides give spacing as a multiple of ceiling height. They don’t agree on the multiple.

The recessed lighting calculator doesn’t force your room into any one of them. It fits the grid to the room, shows your spacing beside the whole range, and warns you if you’re wider than the widest rule. When that happens, it offers a tighter grid and the lumens per light that would give the same brightness with more, smaller lights.

For the wall, FSG’s formula is half the spacing, which is what an evenly divided room gives you anyway. Cans and Fans suggests 2 to 3 ft from the wall for general lighting.

Beam angle: the check most layouts skip

Two lights with the same lumens can light a room very differently. A wide flood spreads its light over a big circle. A narrow spot puts the same light into a small one.

That’s the width of the pool on the floor. At counter height, 3 ft up, the pools are smaller: 3.6 ft, 5.8 ft and 10.0 ft for 40, 60 and 90 degree beams on an 8 ft ceiling.

Example: a narrow beam. Take the 12 ft square room again and swap in a 40 degree light. For brightness you still need 3 lights. But each one lights only 5.82 ft of floor, so three, or even four, would leave dark bands between bright circles. The recessed lighting calculator raises the grid to 3 by 3: nine lights, 4 ft apart. Nine 800 lumen lights would be far too bright at 50 foot-candles, so it suggests lights of about 240 lumens each, or a wider beam.

Recessed lighting calculator examples

A kitchen

A kitchen is 15 ft by 12 ft with a 9 ft ceiling. You want 40 foot-candles of general light from 800 lumen fixtures.

1. Lumens needed: 180 x 40 = 7,200.
2. Lights: 7,200 / 800 = 9.
3. Grid: 3 rows of 3.
4. Spacing: 15 / 3 = 5 ft along the length, 12 / 3 = 4 ft across the width.
5. Columns at 2 ft 6 in., 7 ft 6 in. and 12 ft 6 in. from the end wall. Rows at 2 ft, 6 ft and 10 ft from the side wall.

This is general light. Counters want 70 to 100 foot-candles, which usually means under-cabinet lights or a row of fixtures lined up over the counter edge, not more lights in the grid.

A living room

A living room is 20 ft by 14 ft with an 8 ft ceiling, at 15 foot-candles.

1. Lumens needed: 280 x 15 = 4,200.
2. Lights: 4,200 / 800 = 5.25, so 6.
3. Grid: 2 rows of 3, spaced 6 ft 8 in. by 7 ft.
4. Estimated level: 17.1 foot-candles.

Seven feet is wider than every rule of thumb for an 8 ft ceiling, so the calculator adds a note: 3 rows of 4, twelve lights of about 350 lumens each, gives the same light more evenly. More, dimmer lights cost more to install. Fewer, brighter ones can look patchy. That trade is yours to make.

A hallway

A hallway is 20 ft by 4 ft, at 8 foot-candles. That’s only 640 lumens, one light’s worth. But one light in the middle of a 20 ft hall leaves the ends dim, and a 16 ft pool doesn’t reach along 20 ft. The calculator gives two lights, at 5 ft and 15 ft, and suggests about 320 lumens each.

A bathroom

A bathroom is 8 ft by 5 ft at 40 foot-candles: 1,600 lumens, two lights in one row, at 2 ft and 6 ft along the length and 2 ft 6 in. off the side wall. Lights over a shower or tub need a rating for wet or damp locations. Check the listing on the fixture.

Checking against joists

This is the part of the recessed lighting calculator that saves a patched ceiling. A layout on paper can put a light exactly where a ceiling joist is. You find out with the hole saw in your hand. If you know where the joists are, from the attic, a stud finder or the nails in the drywall, enter them.

The sum is simple. A hole needs its centre at least half the hole size plus 3/4 inch from the centre of a joist, because a solid lumber joist is 1 1/2 inches thick. Engineered joists can be wider, so measure yours. For a 6 inch hole that’s 3 3/4 inches.

Example. In the kitchen above, joists run across the width, 16 inches apart, with the first one 16 inches from the end wall. They sit at 16, 32, 48 inches and so on. The first column of lights is planned at 2 ft 6 in., which is 30 inches. That’s 2 inches from the joist at 32. It clashes, and has to move 1 3/4 inches toward the wall. The other two columns, at 90 and 150 inches, are each 6 inches from a joist and clear.

When one light in a line has to move, move the whole line. A row that’s all 1 3/4 inches off its mark looks straight. One light out of line looks like a mistake.

Housings and clips need room too, and some need more than the hole. Check the installation sheet for the fixture’s clearance, and always confirm the joist on site before you cut.

Choosing the light

The recessed lighting calculator needs two numbers from the fixture: lumens and beam angle. Both are on the spec sheet.

• Lumens, not watts. Watts tell you what a light uses. Lumens tell you what it gives.
• Size is a rough guide to output. FSG’s calculator uses about 400 lumens for a 4 inch LED downlight, 650 for a 5 inch, 1,000 for a 6 inch and 1,400 for an 8 inch. Real products vary, so use the number on the box.
• Dimmable. If the layout gives more light than your target, a dimmer is the easy fix. Check that the dimmer suits the fixture.
• Ratings. A fixture that touches insulation needs a rating for it, and wet areas need a wet or damp rating. These are safety matters. Follow the fixture’s listing and your local code.

Reading your result

• Lights. The number in the layout. A note says so if it’s more than brightness alone needs, and why.
• Spacing. Centre to centre. Two figures if the room isn’t square.
• First light from the wall. Half the spacing.
• Estimated light level. Total lumens divided by floor area. Compare it with your target.
• Pool of light. How wide one fixture’s beam is at the floor and at counter height.
• The plan. Drawn to scale, with the distances along the top and down the side.
• Where to mark the ceiling. One line per row and column. Measure each from the same wall, snap a chalk line, and the crossings are your lights.

Common mistakes

• Counting by room size alone. A kitchen needs two to three times the light of a living room the same size.
• Ignoring the beam. Narrow spots in a grid make pools, not even light.
• Putting lights too close to the wall. You get bright arcs on the wall and a dim middle.
• Measuring from different walls. Pick one end wall and one side wall and measure everything from those.
• Forgetting what’s above the ceiling. Joists, ducts and pipes all live there.
• Using a grid where the room has a job. Over a kitchen island, a desk or a vanity, line lights up with the work surface.

What the recessed lighting calculator can’t tell you

• Light lost in the room. The sum assumes every lumen reaches the room. Dark walls, deep trims and dirty lenses absorb some. Professional designs apply correction factors for this. If your room is dark, aim higher.
• Sloped and vaulted ceilings. The height is treated as flat.
• Task and accent lighting. It plans general light. Counters, artwork and reading chairs want their own.
• Obstructions. Ceiling fans, beams, vents and cabinets all shift a grid.
• Wiring and code. Circuit loads, box fill, fixture ratings and permits are an electrician’s job.

More project calculators

Pricing the whole job? The job estimate calculator turns materials and labor into a quote. For other parts of a remodel, the concrete block calculator counts blocks and mortar, and the cubic yard calculator handles concrete and fill. Outdoors, there are the sand and rock calculators. All of them are on the home and garden tools page.

Sources

• FSG, recessed lighting calculator: the lumen formula, spacing at 0.6 times ceiling height, wall offset at half the spacing, and approximate lumens by downlight size.
• Home Project Calculator, recessed lighting calculator: foot-candle ranges by room and spacing at 0.8 times ceiling height.
• Cans and Fans, room size guide: spacing at half the ceiling height, 2 to 3 ft from walls, and lumens per square foot by room.

All read on October 8, 2026. The beam and joist sums are geometry.