Recessed Lighting Calculator

A recessed lighting calculator answers three questions: how many lights the room needs, how far apart they go, and how far they sit from the walls. This one does all three from the room size, the ceiling height, a target light level by room type, and the lumens of the light you plan to buy.

It makes two checks and uses the larger result. The first check is light level. The target is in foot-candles, the planning unit used in lighting guidance, and one foot-candle is one lumen per square foot. Area times foot-candles gives the lumens the room needs. The second check is the grid. A common rule of thumb is to space recessed lights no further apart than half the ceiling height, and the calculator fits the smallest even grid that respects that limit.

For the default 14 by 12 ft living room the light level needs 4 lights of 800 lumens and the grid needs 12. The grid is the larger, so the answer is 12. The two numbers are shown separately so you can see which one drives the count.

Penn State's energy course notes give planning ranges in foot-candles: 10 to 20 for living rooms and bedrooms, 30 to 40 for the general light in kitchens and dining rooms, 50 to 100 for kitchen task areas, 20 to 50 for bathrooms and home offices, and 5 to 20 for hallways. The calculator uses the middle of each range and lets you enter a custom value.

Multiply the area by the foot-candles. A 12 by 12 ft kitchen at 35 fc needs 5,040 lumens, and a living room of the same size at 15 fc needs 2,160. Divide by the lumens in one light. Light output is printed on the box, and 800 lumens is only a placeholder. Brighter lights mean fewer of them, as the count table shows.

The optional light loss allowance raises the lumens needed when you expect to lose some output to dark ceilings, deep trims or ageing lamps. It starts at zero, so the count is the bare minimum for the target. One foot-candle is about 10.764 lux, so a living room at 15 fc is about 161 lux.

The spacing rule of thumb that lighting suppliers give is ceiling height divided by 2. Lights are about 4 feet apart under an 8 foot ceiling, 4.5 feet under 9 feet and 5 feet under 10 feet. The idea is that the beam from each light spreads wider as the ceiling gets higher, so the lights can sit further apart without dark patches between them.

The rule assumes a typical beam. A narrow spot needs closer spacing, and a wide flood can go further. Fixture makers publish a spacing criterion, a number that multiplies the mounting height to give the largest spacing for even light. The calculator's spacing factor is that number, and 0.5 is the default. Try 0.75 or 1.0 if your fixture data allows it. The spacing table shows the effect for ceilings from 8 to 14 feet.

The calculator rounds the number of columns and rows up, so the actual spacing is always equal to or smaller than the limit. For the default room, the actual spacing is 3.50 ft along the length and 4.00 ft across the width, both within the 4 ft limit.

A layout is a grid. The calculator divides the room length by the spacing limit and rounds up to get the columns, then does the same for the width to get the rows. The lights sit at the centre of each grid cell, which puts the first light half a spacing from the wall. The 14 by 12 ft room gives 4 columns and 3 rows, with the first light 1.75 ft from the end walls and 2.00 ft from the side walls.

Distance from the wall matters. Too close, and the wall shows scallops of light. Too far, and the corners go dark. Half the spacing is the standard compromise, and one supplier suggests about 3 feet from the surrounding walls. You can mark the grid on the ceiling with a chalk line and then shift individual lights to avoid joists, ducts, beams and other fixtures, keeping the spacing within the limit.

The grid table shows layouts for common room sizes. A 12 by 12 ft room at 8 ft needs a 3 by 3 grid of nine lights, and 20 by 16 ft needs 5 by 4, a total of twenty. The grid grows quickly with the room, which is why designers often use fewer, brighter, wider-beam lights and a dimmer.

Lumens, not watts, measure light. LED fixtures differ widely in how many lumens each watt gives, so enter the lumens and watts from the box. Total power is lights times watts, and the energy per year is watts times hours per day times 365 divided by 1,000. The default 12 lights of 9 W at 3 hours a day use 118.3 kWh, which costs $20.10 a year at a placeholder $0.17 per kWh.

The fixture price is also a placeholder: 12 lights at $20 are $240. The cost of cable, boxes, switches, dimmers, labour and permits is not included. Check that the dimmer is rated for LED loads and for the lights you buy.

To see what a circuit carries, convert the watts to amps with the watts to amps calculator. If you run the lights for part of an hour, the hours to decimal calculator turns hours and minutes into the decimal hours the calculator needs. Both are planning tools only. Circuit design belongs to a licensed electrician.

A living room is the easy case: low light level, relaxed spacing, and a dimmer. The grid usually decides the count, and the lights can often be a wider beam. A kitchen needs more light, and Penn State's notes put task areas at 50 to 100 fc. Plan general light for the room, then add the extra lights over the counters, sink and range. The 15 by 12 ft kitchen at 35 fc needs 8 lights of 800 lumens, and at 75 fc it needs 17.

Bathrooms and home offices sit at 20 to 50 fc and are small, so a few lights do the job. Put one over the sink or desk rather than relying on the grid. Hallways are 5 to 20 fc and narrow, so a single row down the centre at the spacing limit is usually enough. For the narrow room, enter the width as the hall width, and the calculator rounds the rows up to one.

Ceiling height changes everything. A 14 by 12 ft room needs 12 lights in the grid at 8 ft and 6 at 12 ft with the 0.5 rule, because the higher ceiling lets the lights spread out. Sloped and vaulted ceilings need a plan from a designer, because the mounting height varies.

This calculator plans a layout. It is an estimate and not a design. Recessed lights are electrical equipment, so follow the National Electrical Code and your local code, and have a licensed electrician do or check the wiring. Many areas require a permit and an inspection.

Where a fixture touches ceiling insulation it must be IC-rated. The Building America Solution Center explains that codes require fixtures in direct contact with insulation to be IC-rated, and that non-IC fixtures need insulation kept at least 3 inches away from the enclosure and wiring. It also recommends air-tight rated fixtures where the ceiling is below an unconditioned space, such as an attic, to limit air leakage. Check the label on the housing and the manufacturer's instructions.

The spacing rule and the foot-candle values are planning guidance, not a lighting design to a standard. Real rooms have dark surfaces, windows, furniture and uneven ceilings that change the result. High ceilings and commercial spaces need a designer. For the circuit load use the watts to amps calculator, and to compare the cost of two lamp options use the percentage calculator.