20×20 workshop:
36 lights
36 fixtures A 20×20 ft workshop (400 sq ft) needs approximately 36 recessed lights at 800 lumens each — about 27,415 lumens in total — laid out 6 across by 6 deep.
Space them 3.3 ft apart along the 20 ft wall and 3.3 ft apart along the 20 ft wall, starting 1.7 ft from the end walls.
The ceiling plan
Drawn to scale, with each fixture at its computed position and the pale circles showing roughly where each one lands on the working plane.
Same room, different fixtures
If you have already chosen a fitting, read across from its lumen output.
| Fixture output | Rough equivalent | Fixtures needed | Suggested grid |
|---|---|---|---|
| 450 lm | 40 W incandescent | 64 | 8 × 8 |
| 650 lm | 50 W incandescent | 42 | 6 × 7 |
| 800 lm | 60 W incandescent | 36 | 6 × 6 |
| 1000 lm | 75 W incandescent | 30 | 5 × 6 |
| 1200 lm | 100 W incandescent | 25 | 5 × 5 |
Fewer, brighter fixtures give more contrast; more, dimmer fixtures give flatter, more even light. Both rows deliver the same total.
How the 36 was calculated
| Target illuminance for a workshop | 400 lux | published range 500–750 lux |
|---|---|---|
| Floor area | 37.2 m² | 400 sq ft |
| Mounting height above the working plane | 5 ft | 8 ft ceiling, 3.0 ft working plane |
| Room index | 1.98 | (L × W) ÷ (Hm × (L + W)) |
| Utilisation factor | 0.68 | from the room index, light surfaces |
| Maintenance factor | 0.8 | dirt and lamp depreciation |
| Lumens required | 27,415 lm | 400 × 37.2 ÷ (0.68 × 0.8) |
| Fixtures at 800 lm | 36 | delivering about 420 lux |
The highest requirement here. The ceiling is sized to 400 lux with task lighting assumed over the bench — machine and detail work needs both quantity and good colour rendering.
Other ceiling heights
| Ceiling | Fixtures | Grid | Spacing | Max spacing |
|---|---|---|---|---|
| 8 ft | 36 | 6 × 6 | 3.3′ × 3.3′ | 6.6′ |
| 9 ft | 36 | 6 × 6 | 3.3′ × 3.3′ | 7.9′ |
| 10 ft | 40 | 5 × 8 | 4′ × 2.5′ | 9.2′ |
| 12 ft | 42 | 6 × 7 | 3.3′ × 2.9′ | 11.8′ |
Questions
How many recessed lights do I need for a 20x20 workshop?
About 36 fixtures at 800 lumens each, arranged 6 across by 6 deep. That gives roughly 420 lux across 400 square feet, against a target of 400 lux for a workshop.
How many lumens does a 20x20 workshop need?
Roughly 27,415 lumens in total. That comes from 400 lux over 37.2 m², divided by a utilisation factor of 0.68 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 20x20 room?
3.3 ft apart along the 20 ft wall and 3.3 ft apart along the 20 ft wall, with the outer row about 1.7 ft in from the end walls — half a spacing. With a 8 ft ceiling the maximum sensible spacing is around 6.6 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 1.98, giving a utilisation factor of 0.68.
Computed with the lumen method — see how it works and where it 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.