18×20 home office:
25 lights
25fixtures A 18×20 ft home office (360 sq ft) needs approximately 25 recessed lights at 800 lumens each — about 19,438 lumens in total — laid out 5 across by 5 deep.
Space them 4 ft apart along the 20 ft wall and 3.6 ft apart along the 18 ft wall, starting 2 ft from the end walls.
Assumed·8 ft ceiling·working plane 2.5 ft·light surfaces·UF 0.65·MF 0.8
SOURCE·IES lumen-method recommendations·published recommendations, quoted as ranges·REVIEWED JUL 2026
Sheet 01 · layoutWhere the 25 go, and how far the light gets
Drawn to scale from the computed centres. The dashed rings are iso-lux contours: the line each fitting's own light crosses on the way down from 89 lux beneath it.
Read the contours off any one fitting. Directly beneath it, a 800 lm downlight at 5.5 ft above the working plane puts about 89 lux on that plane; 5.5 ft away it is down to 22 lux. The target for a home office is 300 lux. No single ring gets anywhere near it, and that is the point of the drawing: the level in the room is built out of overlapping contours plus the light that bounces off the walls and ceiling, which is what the utilisation factor of 0.65 pays for. 25 fittings is the count that closes that gap — it is not the room's area divided by a rule of thumb.
SOURCE·Computed from the lumen method·no dataset, every figure reproducible with a calculator·REVIEWED JUL 2026
How this is worked out
- Set the ambient target at 300 lx. The published band for a home office is 400–500 lx. The ceiling layer carries this room on its own, so it is sized inside the band rather than at the floor of it.
- Restate the room in metres, and measure to the plane. 360 sq ft is 33.4 m². Mounting height is taken to the working plane, not the floor: 5.5 ft under a 8 ft ceiling.
- Take the losses off before sizing anything. A room index of 1.71 gives a utilisation factor of 0.65 against light surfaces; the maintenance factor is fixed at 0.8 for dirt and lamp depreciation. Between them 48% of everything the fittings emit never lands on the plane, and it has to be bought up front.
- Divide, then fit a grid to the room's own proportions. 19,438 lm required; at 800 lm each that is 24.3 fittings, resolved to 25 in a 5 × 5 grid, because an even grid beats a lopsided one at the same total.
- Check it against the spacing criterion. 1.3 × 5.5 ft allows up to 7.2 ft between fittings. This layout sits at 4 ft, inside the limit.
N = (E × A) / (F × UF × MF) = (300 × 33.4) / (800 × 0.65 × 0.8) = 24.3 fittings → 5 × 5 = 25 installed E0 = F / (π × Hm²) contour datum, one fitting, direct only = 800 / (π × 1.69²) = 89 lx on the plane directly beneath a fitting
Sheet 03 · scheduleRe-lamp it: what a brighter fitting buys you
If the fitting is already chosen, read across from its lumen output. Fewer, brighter fittings raise the contour datum and widen the rings; more, dimmer ones flatten the floor.
| Fixture output | Rough equivalent | Fixtures needed | Suggested grid |
|---|---|---|---|
| 450 lm | 40 W incandescent | 42 | 7 × 6 |
| 650 lm | 50 W incandescent | 30 | 6 × 5 |
| 800 lm | 60 W incandescent | 25 | 5 × 5 |
| 1000 lm | 75 W incandescent | 20 | 5 × 4 |
| 1200 lm | 100 W incandescent | 16 | 4 × 4 |
Fewer, brighter fixtures give more contrast; more, dimmer fixtures give flatter, more even light. Both rows deliver the same total.
SOURCE·Computed from the lumen method·no dataset, every figure reproducible with a calculator·REVIEWED JUL 2026
Sheet 04 · ledgerEvery input behind the 25, line by line
| Target illuminance for a home office | 300 lux | published range 400–500 lux |
|---|---|---|
| Floor area | 33.4 m² | 360 sq ft |
| Mounting height above the working plane | 5.5 ft | 8 ft ceiling, 2.5 ft working plane |
| Room index | 1.71 | (L × W) ÷ (Hm × (L + W)) |
| Utilisation factor | 0.65 | from the room index, light surfaces |
| Maintenance factor | 0.8 | dirt and lamp depreciation |
| Lumens required | 19,438 lm | 300 × 33.4 ÷ (0.65 × 0.8) |
| Fixtures at 800 lm | 25 | delivering about 309 lux |
The brightest room on this list. The ceiling layer is sized to 300 lux and a desk lamp is assumed to take the working plane the rest of the way — lighting a whole office to 500 lux from the ceiling alone is glary and wasteful.
SOURCE·IES lumen-method recommendations·published recommendations, quoted as ranges·REVIEWED JUL 2026
Sheet 05 · variationRaise the ceiling and the whole sheet moves
A taller ceiling spreads each fitting's contours wider, which permits wider centres — and drops the level on the plane, which asks for more total output. Both at once.
| Ceiling | Fixtures | Grid | Spacing | Max spacing |
|---|---|---|---|---|
| 8 ft | 25 | 5 × 5 | 4′ × 3.6′ | 7.2′ |
| 9 ft | 25 | 5 × 5 | 4′ × 3.6′ | 8.5′ |
| 10 ft | 30 | 6 × 5 | 3.3′ × 3.6′ | 9.8′ |
| 12 ft | 30 | 6 × 5 | 3.3′ × 3.6′ | 12.4′ |
SOURCE·Computed from the lumen method·no dataset, every figure reproducible with a calculator·REVIEWED JUL 2026
Sheet 06 · queriesWhat an electrician asks next
How many recessed lights do I need for a 18x20 home office?
About 25 fixtures at 800 lumens each, arranged 5 across by 5 deep. That gives roughly 309 lux across 360 square feet, against a target of 300 lux for a home office.
How many lumens does a 18x20 home office need?
Roughly 19,438 lumens in total. That comes from 300 lux over 33.4 m², divided by a utilisation factor of 0.65 and a maintenance factor of 0.8 — the light that never reaches the working plane has to be paid for up front.
How far apart should the lights be in a 18x20 room?
4 ft apart along the 20 ft wall and 3.6 ft apart along the 18 ft wall, with the outer row about 2 ft in from the end walls — half a spacing. With a 8 ft ceiling the maximum sensible spacing is around 7.2 ft before the floor starts to show scallops.
Does the ceiling height change the answer?
Yes, in two ways. A higher ceiling spreads each fixture's light over a wider area, so fixtures can sit further apart — but it also drops the light level reaching the floor, so a taller room generally needs more total lumens. At 8 ft the room index works out at 1.71, giving a utilisation factor of 0.65.
Sheet 07 · adjacentThe same footprint, commissioned for another room
Identical geometry, different target illuminance. The count is the only thing that changes.
Sheet issued for planning only. Method, assumptions and the four places this sheet is weak.
Planning guidance only, not an electrical design. Fixture counts are a starting point — follow your local wiring rules and use a licensed electrician for the installation.
