Recessed Lighting Calculator

Find how many recessed lights a room needs, the spacing on center and the setback from each wall, then check the count against a footcandle target for the room.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Space recessed cans at half the ceiling height, start each row half that spacing off the wall, and multiply the rows by the columns. A 16 by 12 ft kitchen with an 8 ft ceiling takes 12 cans on a 4 ft grid, 2 ft off every wall.

Your measurements

Plan view from above: area is length times width.

Room lengthftWall to wall, the long way. Measure the finished room, not the framing, because drywall takes about 1 in off each dimension.Room widthftThe short wall. For an L-shaped room, run the calculator twice and treat each leg as its own rectangle.Ceiling heightftFinished floor to finished ceiling. Spacing comes straight off this number, so a tray or a vault changes the layout under it.Can sizeNominal trim diameter. A 6 in can is the general-purpose size, 4 in suits halls, small baths and accent runs, and 5 in splits the difference. Housing-free wafer models need about 3 in of plenum for the driver box.Room useSets the light level the lumen check aims at. Kitchens and baths are designed brighter than living rooms, and halls are designed dimmer.Lumens per fixtureDelivered lumens from the trim, printed on the box next to the wattage. A 6 in LED wafer is usually 800 to 1,200 lumens at 10 to 15 W, and a 4 in trim 500 to 800.

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Results

Fixtures needed: 12 cans; A 4 by 3 grid across a 16 ft by 12 ft room, 192 sq ft of ceiling
Spacing on center: 4 ft x 4 ft; Ceiling height divided by 2 caps the spacing at 4 ft for a 8 ft ceiling, and the grid divides the room evenly inside that
Distance from wall: 2 ft and 2 ft; Half the spacing, which keeps the wash even and clears the front edge of 24 in base cabinets
Lumen method check: 10 cans; Target 40 fc over 192 sq ft is 7,680 lumens, divided by 800 per fixture. The grid asks for 2 more than the light level needs, so keep the grid for even coverage and order 640 lumen trims, or dim the run
Light delivered: 50 fc; 9,600 lumens from 12 trims. The published range for kitchen work is 30 fc to 40 fc. Undercabinet strips carry the light at the counter, so the cans only have to cover the room. Close to the 40 fc target for kitchen work
Planning estimate only: Licensed electrician; Cut holes are 6.25 in for this trim, and housings in an insulated ceiling must be IC-rated and airtight (NEC 410.116, IECC R402.4.5). Always consult a professional: a licensed electrician and the local inspector govern the circuit and the fixtures.

Recessed Lighting formula

Maximum spacing = Ceiling height / 2   (8 ft ceiling = 4 ft on center)
Fixtures = round up(Room length / spacing) x round up(Room width / spacing)
Actual spacing = Room length / columns;  Distance from wall = Actual spacing / 2
Lumen check: Fixtures = Target footcandles x Room area / Lumens per fixture

Worked example

An 18 by 12 ft kitchen with a 9 ft ceiling. Half the ceiling height is 4.5 ft, so 18 / 4.5 = 4 columns and 12 / 4.5 = 2.67, rounded up to 3 rows. That is 12 cans. Spacing works out to 18 / 4 = 4.5 ft one way and 12 / 3 = 4 ft the other, with the outer cans 2.25 ft and 2 ft off the walls.

The lumen check uses 216 sq ft at a 40 footcandle kitchen target, so 8,640 lumens. With 900 lumen trims that is 8,640 / 900 = 9.6, rounded up to 10 cans. The two answers differ, so keep the 12-can grid for even coverage and order 8,640 / 12 = 720 lumen trims, or run the 900 lumen trims on a dimmer set near 80 percent.

Quick reference

Cans on the 4 ft grid at an 8 ft ceiling, with 800 lumen trims
Room size	Grid	Cans	Spacing	Light delivered
10 x 10 ft (100 sq ft)	3 x 3	9	3.33 ft	72 fc
10 x 12 ft (120 sq ft)	3 x 3	9	3.33 x 4 ft	60 fc
12 x 12 ft (144 sq ft)	3 x 3	9	4 ft	50 fc
12 x 16 ft (192 sq ft)	3 x 4	12	4 ft	50 fc
12 x 20 ft (240 sq ft)	3 x 5	15	4 ft	50 fc
14 x 16 ft (224 sq ft)	4 x 4	16	3.5 x 4 ft	57 fc
16 x 20 ft (320 sq ft)	4 x 5	20	4 ft	50 fc
20 x 24 ft (480 sq ft)	5 x 6	30	4 ft	50 fc

Laying out the grid

Start with the ceiling height. Half of it is the widest the cans can sit apart and still overlap into an even wash, so 4 ft at 8 ft, 4.5 ft at 9 ft and 5 ft at 10 ft. Divide each room dimension by that number, round up, and you have the columns and rows. Then divide the room back by those counts so the grid is centered and the outer cans land half a spacing from the wall. Under 2 ft off the wall the beam scallops the paint and can sit over the upper cabinets, which is why a 10 ft dimension on the 4 ft rule reads 1.67 ft and wants one column pulled out.

Layout beats math in three places. Over a kitchen counter, set that row 24 to 30 in off the face of the upper cabinets so the light lands on the counter and not on your head. Over an island, forget the grid and center the cans on the island. In a hall, one can every 4 to 6 ft down the center is enough, with the first about 3 ft in from the end wall. Snap the whole layout on the joists before the drywall goes up and check it against joists, ducts, the bath fan and anything else already framed in.

Housings, IC rating and air sealing

Two ratings matter. IC means the housing may touch insulation; a non-IC housing needs 3 in of clearance all around and nothing over the top (NEC 410.116(B)). Any can that pokes into an insulated attic or a vaulted ceiling should be IC-rated and airtight, which the IECC requires at R402.4.5: 2.0 cfm or less of leakage at 75 Pa, sealed to the drywall with a gasket or caulk. A leaky can is a hole into the attic that carries house air and moisture into the insulation all winter.

Housing-free wafer downlights have taken over remodels because they drop through a 4-1/4 to 6-1/4 in hole and need only about 3 in of plenum for the remote driver box. They run typically $15 to $35 each in 2026 US retail, against $25 to $60 for a can plus a separate trim. A can over a tub or shower has to be listed damp or wet under NEC 410.10(D), and a can carrying a ceiling fan has to be a fan-listed box under 314.27(C). Wire the run to a dimmer listed for LED loads and check its minimum load, because many dimmers will not hold a steady low end under about 20 W of LED.

What this calculator leaves out

The result is a fixture count and a layout. Not included: the circuit, the 14/2 or 12/2 cable, the dimmer and its box, the switch leg, drywall patching on a retrofit, and the permit. It does not size the undercabinet strips, the vanity bar or the island pendants that carry the task light, and it does not account for wall color, a dark floor, a sloped ceiling or a beam that breaks the grid. Beam angle sits outside it too, since a 60 degree flood and a 25 degree spot at the same lumens light a room very differently.

Code limits that apply

NEC 410.116(A)(1)
Non-IC housings stay 1/2 in clear of combustible material except at the point of support
If you miss it: A header or blocking pushed tight to the can is written up; the wood is cut back or an IC-rated housing goes in instead
NEC 410.116(B)
Thermal insulation stays 3 in from a non-IC housing and never covers the top of it
If you miss it: Blown-in insulation piled over a non-IC can trips its thermal cutout, so the light cycles off and on until the can is dug out
IECC R402.4.5
Cans in the thermal envelope are IC-rated, sealed to the drywall, and leak 2.0 cfm or less at 75 Pa
If you miss it: The blower door test comes in over target, so every can gets a gasket, caulk at the ceiling, or an airtight retrofit trim
NEC 314.27(C)
A box used to support a ceiling fan is listed and marked for that use, up to 70 lb
If you miss it: A fan hung off a standard housing works loose and drops; the ceiling is opened to fit a fan-rated brace box
NEC 410.10(D)checked
Fixtures in the tub or shower zone are marked for damp locations, or wet where exposed to spray
If you miss it: A dry-listed trim over a shower corrodes and gets tagged at final; a wet-listed lensed trim replaces it
NEC 210.70(A)(1)
Every habitable room, hall, stair and bath gets at least one wall-switched lighting outlet
If you miss it: A room lit only by a switched receptacle or a pull chain fails final; a switch leg is fished up to the can circuit
NEC 110.3(B)
Housings and trims are installed per their listing, which means matching trim to housing family
If you miss it: One brand of trim clipped into another brand of housing voids both listings and is refused, even when the clips fit

Provision	Limit	If you miss it
NEC 410.116(A)(1)	Non-IC housings stay 1/2 in clear of combustible material except at the point of support	A header or blocking pushed tight to the can is written up; the wood is cut back or an IC-rated housing goes in instead
NEC 410.116(B)	Thermal insulation stays 3 in from a non-IC housing and never covers the top of it	Blown-in insulation piled over a non-IC can trips its thermal cutout, so the light cycles off and on until the can is dug out
IECC R402.4.5	Cans in the thermal envelope are IC-rated, sealed to the drywall, and leak 2.0 cfm or less at 75 Pa	The blower door test comes in over target, so every can gets a gasket, caulk at the ceiling, or an airtight retrofit trim
NEC 314.27(C)	A box used to support a ceiling fan is listed and marked for that use, up to 70 lb	A fan hung off a standard housing works loose and drops; the ceiling is opened to fit a fan-rated brace box
NEC 410.10(D)checked	Fixtures in the tub or shower zone are marked for damp locations, or wet where exposed to spray	A dry-listed trim over a shower corrodes and gets tagged at final; a wet-listed lensed trim replaces it
NEC 210.70(A)(1)	Every habitable room, hall, stair and bath gets at least one wall-switched lighting outlet	A room lit only by a switched receptacle or a pull chain fails final; a switch leg is fished up to the can circuit
NEC 110.3(B)	Housings and trims are installed per their listing, which means matching trim to housing family	One brand of trim clipped into another brand of housing voids both listings and is refused, even when the clips fit

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Cans centered on the room instead of the counter: the cook stands in their own shadow. Set that row 24 to 30 in off the cabinet face.
Non-IC housings buried under blown-in attic insulation: the thermal cutout blinks the light on and off until someone digs the can out.
Ordering trims in two lots: color temperature drifts between runs, so buy every can at once and check the CCT switch on each.
Wafer cans in a shallow bay: the remote driver box needs about 3 in of plenum, which an insulated 2x6 rafter bay does not have.
Hanging a ceiling fan on a standard housing: NEC 314.27(C) wants a fan-listed box, and the fix means opening the finished ceiling.
Cutting holes before the drywall is hung: mark the layout on the joists, then cut from below once the ceiling is up and the grid is snapped.

Key facts

The common layout rule is ceiling height divided by 2, so an 8 ft ceiling puts cans about 4 ft apart and a 10 ft ceiling about 5 ft apart.
One footcandle is one lumen per square foot; kitchens are usually designed to 30 to 40 footcandles and living rooms to 10 to 20.
A 6 in LED wafer downlight typically draws 10 to 15 W for 800 to 1,200 lumens, and a 4 in trim gives 500 to 800 lumens.
NEC 410.116 keeps a non-IC housing 1/2 in clear of combustible material and 3 in clear of thermal insulation.
IECC R402.4.5 requires recessed cans in an insulated ceiling to be IC-rated and to leak no more than 2.0 cfm at 75 Pa.

Frequently asked questions

How many recessed lights do I need for a 12 by 16 ft room?

With an 8 ft ceiling the 4 ft rule gives a 3 by 4 grid, so 12 cans 4 ft apart and 2 ft off each wall. At 800 lumens each that is 9,600 lumens over 192 sq ft, about 50 footcandles, which suits kitchen work and wants a dimmer in a living room.

How far apart should recessed lights be?

Half the ceiling height is the working maximum: 4 ft at an 8 ft ceiling, 4.5 ft at 9 ft and 5 ft at 10 ft. Start the outer row half that spacing from the wall, and never closer than about 2 ft, or the beam scallops the wall.

Do recessed lights have to be IC rated?

Any housing that touches or sits under insulation does. NEC 410.116(B) keeps a non-IC can 3 in from insulation with nothing over the top, and IECC R402.4.5 requires cans in an insulated ceiling to be IC-rated and to leak no more than 2.0 cfm at 75 Pa.

Should I use 4 inch or 6 inch cans?

6 in is the general-purpose size for kitchens and living rooms at 800 to 1,200 lumens. 4 in trims at 500 to 800 lumens suit halls, small baths, closets and accent runs, and a tighter 4 in grid reads cleaner on a low ceiling.

Sources and references

WF6 Downlight, Wafer LED 6-inch housing-free recessed downlight (Acuity Brands, Lithonia Lighting)
LED Basics, Solid-State Lighting program (U.S. Department of Energy)
NFPA 70, National Electrical Code (Article 410 Luminaires, 314.27) (NFPA)
Chapter 4 Residential Energy Efficiency, 2021 IECC (R402.4.5 Recessed lighting) (ICC)

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.