How many fixtures, in what grid, delivering how much light
This calculator uses the lighting industry’s lumen method: the maintained light level a room needs, times its area, divided by what each fixture actually delivers after the room’s surfaces and ageing take their shares. It then does the part most calculators skip: it arranges the fixtures into a real grid for your room’s shape, reports the spacing both ways, and checks that spacing against the fixture height, because a count without a layout is how pools of light and dark corners get built. Results are in footcandles and lux, the two dialects of the subject.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. The lumen method gives average illuminance; task lighting, code minimums and final design belong to the fixture photometrics and your lighting professional.
What this calculator does
Lighting fails in showrooms because brightness is easy to feel and hard to count. The lumen method is the counting discipline the industry has used for a century: maintained illuminance is a budget, the room’s area sets the total lumens that must land on the work plane, and each fixture contributes only the fraction of its output that survives the room (the coefficient of utilization, set by the fixture’s distribution and the room’s reflectances) and survives time (the light loss factor, set by dirt and depreciation). The calculator keeps those two factors as visible, changeable inputs rather than hiding them in a fixture count, because they are where honest estimates differ from optimistic ones, and then it lays the resulting fixtures out geometrically, which is the only way spacing, and with it uniformity, gets checked at all.
How to use it
- Choose the target by task, not by room name. The guidance table in the content below gives ranges by activity; a kitchen’s counters want task levels even when the room’s ambient target is modest, which is an argument for layers of light rather than one heroic ceiling.
- Lumens come from the fixture’s specification, never its wattage. Two 15-watt LED fixtures can differ by half in output; the lumens figure is printed on every compliant spec sheet and is the only honest input.
- CU and LLF are the estimate’s conscience. Defaults of 0.6 and 0.8 suit a light-finished room maintained normally. Dark walls, high ceilings with narrow beams, or a dusty workshop each pull the real CU or LLF down, and the fixture count follows them up.
- The layout is a starting grid. It centres fixtures in equal cells. Real rooms then adjust for joists, beams, pendants over islands and the furniture plan, keeping spacings near the grid rather than abandoning it.
The method
- Fixtures needed N = (target illuminance × area) / (lumens per fixture × CU × LLF), rounded up. Illuminance converts at 1 footcandle = 10.764 lux.
- Grid: columns = the ceiling of the square root of (N × length / width), rows = the ceiling of N / columns; the installed count is rows × columns, which can exceed N by the grid’s rounding.
- Spacing = length / columns and width / rows, centre to centre, with half a spacing to each wall.
- Uniformity check: the larger spacing is compared with SHR × mounting height, the fixture’s published maximum spacing criterion; exceeding it warns of scalloping and dark gaps between fixtures.
- Maintained level from the installed grid = installed count × lumens × CU × LLF / area, printed so the grid’s rounding shows its (small) bonus.
Target light levels (guidance)
| Space and task | Footcandles | Lux |
|---|---|---|
| Living room, ambient | 10 to 20 | 100 to 200 |
| Kitchen, general | 30 | 300 |
| Kitchen counters, bathroom vanity | 50 to 75 | 500 to 750 |
| Home office, reading | 30 to 50 | 300 to 500 |
| Workshop and garage benches | 50 to 75 | 500 to 750 |
| Hallways and stairs | 5 to 10 | 50 to 100 |
These are the commonly published recommended ranges for homes; workplaces follow their own standards and codes, and older eyes need measurably more light for the same task, which is worth building in rather than discovering.
Worked examples
A 20 by 12 kitchen at 30 footcandles. The room needs 30 × 240 = 7,200 effective lumens on the work plane. A 1,600-lumen downlight at CU 0.6 and LLF 0.8 delivers 768 of them, so the count is 9.375, rounded to 10 fixtures. The grid for this room’s shape is 5 along the length by 2 across, spacing 4.0 ft by 6.0 ft, comfortably inside the 9.75 ft maximum that a 1.5 spacing ratio allows at 6.5 ft above the counters. The installed grid maintains 32.00 footcandles (344 lux): a small, honest surplus from the rounding.
Why the factors matter. Refinish the same room in dark colours and drop CU to 0.4, and the identical fixtures deliver 512 effective lumens: the count climbs to 15. Nothing about the room’s size changed; its appetite for light did, which is the entire content of the coefficient of utilization and the reason it is an input here rather than a buried constant.
Reading your results
Read the grid before the count: if the spacings come out wildly unequal (a long thin room forces this), the layout wants rethinking into rows along the room, or two fixture types, rather than blind acceptance. Read the spacing check as the uniformity verdict it is; a failed check with the right average is still a badly lit room, striped at the work plane. Read the maintained level’s small surplus over target as normal grid rounding, not as licence to dim the specification elsewhere.
Limitations
- The lumen method computes an average, not a map. It cannot see the dark corner behind a tall cabinet or the glare spot under a pendant; point-by-point photometric software (using the fixture’s IES file) is the professional tool where uniformity or glare is critical.
- CU from a single number is a summary. The true coefficient comes from the fixture’s photometric table against your room’s proportions and reflectances; the input here is the planning value, and critical projects read it from the table.
- Daylight is a separate design. Windows change everything by day and nothing at night; this method sizes the electric lighting for the night case, and daylight harvesting is an additional, worthwhile, separate exercise.
- Colour quality is not in these numbers. Colour temperature and rendering index decide whether a correctly counted room looks right; they are specification choices the lumens cannot make for you.
Frequently asked questions
How many lumens does a room need in total?
Multiply the target footcandles by the square footage for the lumens that must arrive at the work surface, then divide by CU and LLF (together often near 0.5) for the lumens the fixtures must emit. The kitchen example: 7,200 arriving lumens from fixtures emitting 15,000 or so before the room’s shares are taken. Any rule that quotes fixture lumens per square foot without the two factors is assuming your room resembles its author’s, which is precisely what the factors exist to correct.
What is the spacing criterion and why does it matter?
Every fixture throws a cone of light; spaced too far apart for its height, the cones stop overlapping at the work surface and the room stripes. Manufacturers publish a maximum spacing-to-mounting-height ratio (often around 1.2 to 1.5 for downlights) at which uniformity stays acceptable. The layout check compares your grid’s larger spacing with that ratio times the height above the work plane, because a room can pass on average brightness and fail on evenness, and only the spacing check sees it.
Can I just add more, brighter fixtures to be safe?
Overlighting is a real defect: glare, flattened contrast, wasted energy and, with dimmers absent, no way back at night. The maintained-illuminance method aims at the target because targets come from visual-task research, not from caution. Where flexibility is wanted, the honest route is more fixtures on dimmers or switched layers (ambient plus task), which the grid arithmetic supports by letting you design each layer to its own target.
Does ceiling height change the fixture count?
Indirectly, twice: a higher ceiling widens each fixture’s spread (helping spacing) but the same lumens land spread over the same floor either way, so the count follows the CU, which does fall as walls grow taller relative to the room and absorb more light. Height enters this calculator through the spacing check directly and through your CU choice; a tall, narrow room honestly takes a lower CU, and the count responds.
Related calculators
- Home & Construction Calculators
- Electricity Cost Calculator
- Ceiling Grid Calculator
- Area and Volume Calculator
Method: the lumen method with user-set coefficient of utilization and light loss factor, a near-square fixture grid fitted to the room’s proportions, spacings checked against the fixture’s spacing-to-height criterion, and maintained illuminance reported for the installed grid. Last reviewed: 15 October 2026.