How much recessed lighting do I need?
Type the room’s length, width and ceiling height. The recessed lighting calculator lays the lights out on an even grid at half the ceiling height apart, and shows how many you need, the gap between them, and how far the outer lights sit from the walls.
- Lights
- 12
You need 12 recessed lights: 4 along the length and 3 along the width.
- Lights along the length
- 4
- Lights along the width
- 3
- Gap along the length (ft)
- 3.75
- Gap along the width (ft)
- 4
- From the end walls (ft)
- 1.88
- From the side walls (ft)
- 2
- Gap along the length (m)
- 1.14
- Gap along the width (m)
- 1.22
Lights: 12. You need 12 recessed lights: 4 along the length and 3 along the width.
How to calculate
Computes how many recessed (can) lights a rectangular room needs on an even grid, the spacing between them, the distance from the walls, and the lumens per square foot.
Example with the default inputs (Room length 15 ft, Room width 12 ft, Ceiling height 8 ft): You need 12 recessed lights: 4 along the length and 3 along the width.
Method: Spacing = ceiling height ÷ 2 (or the spacing you type); lights along each wall = wall length ÷ spacing, rounded up; lights = along × across; gap = wall length ÷ lights on it; distance from the wall = gap ÷ 2.
- Lights at most half the ceiling height apart (Amico and Lumary: an 8 ft ceiling gives 4 ft).
- An even grid, with the outer lights half a gap from the walls (Amico).
- Brightness is the fixtures’ total lumens over the floor area; Lumary uses about 10 to 20 lumens per ft² for a living room or bedroom.
Worked examples
Each example is checked against the calculator on every build.
- Room length 15 ft, Room width 12 ft, Ceiling height 8 ft gives Lights along the length 4, Lights along the width 3, Lights 12, Gap along the length (ft) 3.75, Gap along the width (ft) 4, From the end walls (ft) 1.875, From the side walls (ft) 2.Source: Amico, Where to Place Recessed Lights (ceiling height ÷ 2; perimeter lights half the spacing from the walls), https://amicolight.com/blogs/news/where-to-place-recessed-lights-the-rule-of-thumb-for-every-room
- Room length 20 ft, Room width 14 ft, Ceiling height 10 ft, Lumens per light 800 gives Lights along the length 4, Lights along the width 3, Lights 12, Gap along the width (ft) 4.666667, Total lumens 9,600, Lumens per ft² 34.285714.Source: Lumary, What is the recommended spacing between each recessed light? (roughly half the ceiling height; a 10 ft ceiling about 5 ft apart), https://www.lumarysmart.com/blogs/news-and-blogs/what-is-the-recommended-spacing-between-each-recessed-light
- Room length 13.12 ft, Room width 9.843 ft, Ceiling height 7.874 ft gives Lights along the length 4, Lights along the width 3, Lights 12, Gap along the length (m) 1, Gap along the width (m) 1.Source: Amico, Where to Place Recessed Lights (ceiling height ÷ 2), https://amicolight.com/blogs/news/where-to-place-recessed-lights-the-rule-of-thumb-for-every-room
- Room length 15 ft, Room width 12 ft, Ceiling height 8 ft, Spacing between lights 6 ft gives Lights along the length 3, Lights along the width 2, Lights 6, Gap along the length (ft) 5, Gap along the width (ft) 6.Source: Lumary, recommended spacing between recessed lights (4 to 6 ft for typical downlights), https://www.lumarysmart.com/blogs/news-and-blogs/what-is-the-recommended-spacing-between-each-recessed-light
How it works
- Spacing s = ceiling height ÷ 2, or the spacing you type (which then replaces it).
- Lights along the length = room length ÷ s, rounded up to a whole number (at least 1). Lights along the width = room width ÷ s, rounded up (at least 1).
- Lights = lights along the length × lights along the width.
- Gap along the length = room length ÷ lights along the length; gap along the width = room width ÷ lights along the width. Each gap is at most s.
- From the walls = half the gap on that side, so each row of lights is centred: half a gap, then whole gaps, then half a gap.
- Total lumens (when you type lumens per light) = lights × lumens per light; lumens per ft² = total lumens ÷ (length × width in square feet).
- Gaps show in feet and in metres (1 ft = 0.3048 m exactly).
Exact arithmetic. Each size is read as the exact decimal you typed, in the unit you typed it in (12 ft 6 in is exactly 150 in). When you type in a unit, the page stores the size in metres and reads it back as the decimal with the fewest significant digits, in one of the box's units, that gives the same stored number (a feet-and-inches pair counts by its value in inches; on a tie the first unit in the menu wins). When no decimal of up to 17 significant digits in those units gives the stored number (a value that was not typed on the page), the stored number's own shortest decimal, in metres, is used. Every step is an exact fraction, so 12 ÷ 4 is exactly 3 lights, not 3.0000001 rounded up to 4.
Output format. Light counts show as whole numbers; gaps and wall distances with at most 2 decimals; lumens per ft² with at most 1 decimal and total lumens with none, each rounded half up from its exact value, with trailing zeros left off.
Assumptions
- Spacing: half the ceiling height by default (Amico and Lumary). Lengths, the ceiling height and the spacing are from 0.1 m (about 4 in); the room up to 100 m, the ceiling and spacing up to 30 m.
- The room is a rectangle with an even grid of lights, the outer lights half a gap from the walls (Amico).
- Lumens per light: from 1 to 100,000, from the fixture’s box. Lumary’s guide of 10 to 20 lumens per ft² for living rooms and bedrooms is for comparison only.
Worked examples by hand
A 15 × 12 ft room with an 8 ft ceiling. s = 8 ÷ 2 = 4 ft. 15 ÷ 4 = 3.75 → 4 along; 12 ÷ 4 = 3 → 3 across; 12 lights. Gaps 15 ÷ 4 = 3.75 ft and 12 ÷ 3 = 4 ft; from the walls 1.875 ft and 2 ft.
A 20 × 14 ft room with a 10 ft ceiling, 800 lm lights. s = 5 ft. 20 ÷ 5 = 4 along; 14 ÷ 5 = 2.8 → 3 across; 12 lights; gap across 14 ÷ 3 = 4.67 ft. 12 × 800 = 9,600 lm over 280 ft² = 34.3 lm per ft².
A 4 × 3 m room with a 2.4 m ceiling. s = 1.2 m. 4 ÷ 1.2 = 3.33 → 4 along; 3 ÷ 1.2 = 2.5 → 3 across; 12 lights, each gap 1 m.
The 15 × 12 ft room with 6 ft spacing typed. 15 ÷ 6 = 2.5 → 3 along; 12 ÷ 6 = 2 across; 6 lights, gaps 5 ft and 6 ft.
Other questions people ask
How far apart should recessed lights be?
A common rule from lighting makers is half the ceiling height: 4 ft apart under an 8 ft ceiling, 4.5 ft under 9 ft, and 5 ft under 10 ft. Lumary says most rooms end up with lights 4 to 6 ft apart.
How far from the wall should recessed lights be?
About half the gap between the lights, so the light from the outer row meets the light from the next row. Under an 8 ft ceiling that is about 2 ft from the wall.
How many recessed lights do I need for a 12 × 15 room?
With an 8 ft ceiling the lights go at most 4 ft apart. 15 ÷ 4 = 3.75, so 4 lights along the 15 ft wall, and 12 ÷ 4 = 3 along the 12 ft wall: 12 lights in all, 3.75 ft and 4 ft apart.
How many lumens do I need in a room?
Multiply the floor area by the lumens per square foot the room needs. Lumary’s example is 1,000 to 2,000 lumens for a 100 ft² living room or bedroom, so 10 to 20 lumens per square foot. Kitchens and work areas need more.
Why round the number of lights up?
Rounding down would put the lights further apart than the spacing you chose and leave dark patches between them. The calculator rounds up, then spreads the lights evenly, so the real gap is never wider than your spacing.
Can I use a different spacing?
Yes. Type your own spacing, for example from the fixture maker’s spacing guide, and the calculator uses it in place of half the ceiling height.