Small Metric Sizes in Inches
Precise conversions for 1–30 mm — screws, bits, sockets, straps and 3D printing.
Why Small Sizes Need Extra Care
At small sizes, the gap between a metric part and its “close” inch size is a big share of the part. A 6 mm drill bit and a 1/4″ bit differ by only 0.35 mm, but that is enough to make a screw loose or a dowel joint wobble. That is why this page shows the exact decimal inches next to the nearest tape-measure fraction.
Use the decimal column for machining, 3D printing, electronics and anything with a tolerance. Use the ≈ fraction column only for rough layout and everyday measuring.
Common Small Metric Sizes in Inches
| Metric size | Decimal inches | ≈ Nearest 1/16″ | Where you see it |
|---|---|---|---|
| 1.75 mm | 0.0689″ | ≈ 1/16″ | 3D printer filament diameter (standard) |
| 3 mm | 0.1181″ | ≈ 1/8″ | M3 screw — electronics, 3D printers, drones |
| 4 mm | 0.1575″ | ≈ 3/16″ | M4 screw — furniture fittings, brackets |
| 5 mm | 0.1969″ | ≈ 3/16″ | M5 screw; 5 mm LED |
| 6 mm | 0.2362″ | ≈ 1/4″ | M6 bolt; 6 mm drill bit (close to 1/4″ = 6.35 mm) |
| 8 mm | 0.3150″ | ≈ 5/16″ | M8 bolt; 8 mm dowel |
| 10 mm | 0.3937″ | ≈ 3/8″ | 10 mm socket — one of the most used socket sizes |
| 12 mm | 0.4724″ | ≈ 1/2″ | M12 bolt; 12 mm plywood (close to 1/2″ = 12.7 mm) |
| 13 mm | 0.5118″ | ≈ 1/2″ | 13 mm wrench (close to 1/2″ = 12.7 mm) |
| 18 mm | 0.7087″ | ≈ 11/16″ | 18 mm plywood / MDF sheet thickness |
| 19 mm | 0.7480″ | ≈ 3/4″ | 19 mm wrench (almost identical to 3/4″ = 19.05 mm) |
| 20 mm | 0.7874″ | ≈ 13/16″ | 20 mm watch strap width |
| 22 mm | 0.8661″ | ≈ 7/8″ | 22 mm watch strap width; 22 mm pipe (close to 7/8″) |
| 25 mm | 0.9843″ | ≈ 1″ | 25 mm (just under 1″ = 25.4 mm) |
Metric vs Inch “Near Equivalents”
Some metric and inch sizes are so close that people swap them — and some are not. A quick rule: if the difference is under about 0.1 mm, the parts are usually interchangeable for hand tools; over that, check before you force it.
| Metric | Inch | Difference | Swap? |
|---|---|---|---|
| 19 mm | 3/4″ (19.05 mm) | 0.05 mm | Usually yes (wrenches) |
| 13 mm | 1/2″ (12.70 mm) | 0.30 mm | Loose fit — avoid on tight bolts |
| 6 mm | 1/4″ (6.35 mm) | 0.35 mm | No for holes/dowels |
| 25 mm | 1″ (25.40 mm) | 0.40 mm | No for pipes and fittings |
| 10 mm | 3/8″ (9.525 mm) | 0.475 mm | No |
All Sizes from 1 to 30 mm
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