How this calculator works
It works out two counts and uses whichever needs more lights.
1. Spacing
If you don't know the beam angle, it keeps recessed lights no more than half the ceiling height apart, a common rule of thumb. If you pick a beam angle, it uses geometry instead: the gap equals the width of one light's beam at counter height (30 in), so neighbouring beams just meet there. Either way, the first light sits half a gap from each wall so the room is lit to the edges.
2. Brightness
This uses the lumen method: average light on the work surface = lumens × coefficient of utilization × light loss factor ÷ area. Not all of a bulb's light reaches the table or floor; walls, the ceiling and the fixture itself absorb some. Engineering ToolBox's worked example uses a coefficient of utilization of 0.6 and a light loss factor of 0.8, which means 48% arrives. The calculator uses that by default.
So the lumens you need from the bulbs = area × target ÷ 0.48. A 14 × 12 ft living room at 20 foot-candles needs 7,000 lumens, or 11 lights at 650 lumens.
If brightness needs more lights than spacing does, the grid grows until both are met. The result line tells you which of the two set your count.
Recessed light spacing by ceiling height
| Ceiling height | Max gap (rule of thumb) | Distance from wall |
|---|---|---|
| 8 ft | 4 ft | 2 ft |
| 9 ft | 4.5 ft | 2.25 ft |
| 10 ft | 5 ft | 2.5 ft |
| 12 ft | 6 ft | 3 ft |
With a known beam angle the gap changes. On an 8 ft ceiling, 60° lights can be about 6.4 ft apart, so the same 14 × 12 ft room needs a 6-light grid for coverage, and brightness then decides whether you need more (12 here).
Brightness targets by room
| Room | Target used (fc) | IES range (fc) | Nearest IES space type |
|---|---|---|---|
| Living room | 20 | 20–30 | Dormitory, living quarters |
| Bedroom | 20 | 20–30 | Dormitory, living quarters |
| Kitchen (general) | 30 | 30–75 | Kitchen / food prep (low end) |
| Kitchen (task / counters) | 50 | 30–75 | Kitchen / food prep (middle) |
| Dining room | 20 | 20–30 | Cafeteria, eating |
| Bathroom (general) | 20 | 10–30 | Restroom / toilet (middle) |
| Bathroom (vanity) | 30 | 10–30 | Restroom / toilet (top of range; no vanity value listed) |
| Home office | 40 | 30–50 | Office, private / closed |
| Hallway | 10 | 5–10 | Corridor, general |
| Basement / rec room | 20 | 10–30 | Lounge / breakroom |
| Garage | 10 | 5–10 | Parking, interior |
| Workshop | 50 | 30–75 | Workshop |
The ranges are the IESNA Lighting Handbook levels listed by Archtoolbox. That table covers commercial and institutional spaces rather than homes, so each room uses the closest space type. A foot-candle is one lumen per square foot arriving on the surface; 10 lux is roughly 1 foot-candle.
Worked example
Living room, 14 × 12 ft, 8 ft ceiling, 650-lumen 9 W lights, beam angle not known.
- Maximum gap: 8 ÷ 2 = 4 ft
- Spacing grid: 14 ÷ 4 → 4 across, 12 ÷ 4 → 3 deep = 12 lights
- Brightness: 168 sq ft × 20 fc ÷ 0.48 = 7,000 lm ÷ 650 → 11 lights
- Spacing needs more, so the answer is 12 lights, 3.5 ft × 4 ft apart, 1.75 ft and 2 ft from the walls, giving about 22 foot-candles.
- Power: 12 × 9 W = 108 W. At 3 hours a day and 18.31¢ per kWh that's about $21.65 a year.
Common questions
How many recessed lights do I need?
Work out two counts and use the larger. Spacing: divide each side of the room by half the ceiling height, round up and multiply. Brightness: room area × target foot-candles ÷ 0.48 (the share of light that reaches the work surface) ÷ lumens per light. A 14 × 12 ft living room at 20 fc with 650-lumen lights gives 12 by spacing and 11 by brightness, so you need 12.
How far apart should recessed lights be?
About half the ceiling height if you don't know the beam angle: 4 ft apart on an 8 ft ceiling, 4.5 ft on a 9 ft ceiling, 5 ft on a 10 ft ceiling. Wide-beam lights can go further apart; narrow ones need to be closer.
How far should recessed lights be from the wall?
About half the spacing between lights, usually 1.5 to 3 ft. That keeps light even to the edges without harsh scallops on the wall.
What beam angle should recessed lights have?
Narrow beams (around 40°) make pools of light for accents and art. Wide beams (60° to 90°) spread light for general lighting and can be spaced further apart. The number is on the box or spec sheet.
How much does it cost to run recessed lights?
Total watts × hours per day × 365 ÷ 1,000 gives kWh per year; multiply by your electricity rate. 12 lights at 9 W for 3 hours a day use 118 kWh a year, about $21.65 at the US average of 18.31¢ per kWh.
4-inch or 6-inch recessed lights?
6-inch lights usually give more light, so you need fewer. 4-inch lights look neater but usually need tighter spacing. Use the lumens printed on the box.
Do I need permits?
Many US jurisdictions require a permit for new circuits or fixtures. Check with your local building department or electrician.
Sources
- Archtoolbox, Recommended Lighting Levels: IESNA Lighting Handbook light levels by space type
- Engineering ToolBox, Illuminance: lumen method formula, coefficient of utilization 0.6 and light loss factor 0.8 in the worked example
- U.S. Energy Information Administration, Electricity Monthly Update: average residential price, 18.31¢ per kWh in July 2026