Convert between inches and millimetres — fractions included — built for lighting hardware, optics, extrusions and CNC work, not just a number box.
Visual ruler — 1 inch is 25.4 mm
Common fixture & optic sizes
| Imperial | Metric | Typical use |
|---|---|---|
| 1″ | 25.4 mm | Small optics & spot lenses |
| 2″ | 50.8 mm | MR16 / small downlights |
| 3″ | 76.2 mm | Mini cylinders, track heads |
| 4″ | 101.6 mm | Standard recessed downlights |
| 5″ | 127.0 mm | Mid-size recessed cans |
| 6″ | 152.4 mm | Large recessed cans / high-output |
| 8″ | 203.2 mm | Commercial downlights, high-bay optics |
Fraction → decimal → mm (shop cheat sheet)
| Fraction | Decimal in | mm |
|---|---|---|
| 1/32″ | 0.03125 | 0.79 mm |
| 1/16″ | 0.0625 | 1.59 mm |
| 3/32″ | 0.09375 | 2.38 mm |
| 1/8″ | 0.125 | 3.18 mm |
| 3/16″ | 0.1875 | 4.76 mm |
| 1/4″ | 0.250 | 6.35 mm |
| 5/16″ | 0.3125 | 7.94 mm |
| 3/8″ | 0.375 | 9.53 mm |
| 7/16″ | 0.4375 | 11.11 mm |
| 1/2″ | 0.500 | 12.70 mm |
| 5/8″ | 0.625 | 15.88 mm |
| 3/4″ | 0.750 | 19.05 mm |
| 7/8″ | 0.875 | 22.23 mm |
| 1″ | 1.000 | 25.40 mm |
Real-world build dimensions
LED strip widths
8, 10 and 12 mm tapes ≈ 5/16″, 3/8″ and 1/2″ — match to channel width.
Aluminium LED channel
Common profiles 17×15 mm and 20×20 mm; 20 mm T-slot ≈ 0.79″.
Optic / reflector diameters
20–50 mm reflectors ≈ 3/4″ to 2″.
Fasteners
M3 ≈ 0.118″, M4 ≈ 0.157″; a #8 screw shank ≈ 4.2 mm.
CNC tooling
1/8″ bit = 3.175 mm, 1/4″ bit = 6.35 mm; a 6 mm collet ≈ 0.236″.
Sheet & plate
1/16″ = 1.59 mm, 1/8″ = 3.18 mm — handy for brackets and heat-sink plate.
Why 25 mm isn’t 1 inch
The 0.4 mm that ruins a press-fit
One inch is exactly 25.4 mm — never 25. Rounding to 25 drops about 1.6% per inch: a “25 mm” hole is ~0.4 mm undersized, and over a 6″ fixture that compounds to ~2.4 mm — enough to bind an optic seat or misalign mounting holes.
For fits that matter, always convert with 25.4 and respect the manufacturer’s tolerance (often ±0.1–0.5 mm). Imperial fasteners and metric holes are not interchangeable, even when they look close.