Metric vs imperial fasteners (short answer)
Metric sizes use millimeters (M6, M8, M10). Imperial (inch) sizes use fractions or decimal inches (1/4-20, 5/16-18). They are not interchangeable—thread pitch and diameter geometry differ even when the outside diameter looks “close.”
This chart helps US buyers map common inch callouts to the nearest metric stock sizes we publish in 304 / A2-70. It is a purchasing aid, not a drop-in substitute table for load-critical joints.
Shop metric stock: socket head cap screws · hex head screws · parametric catalog.
Why US RFQs mix both systems
US drawings and hardware stores often list UNC / UNF inch fasteners. Imported equipment, robotics, 3D-printer frames, and DIN/ISO prints use metric. Ordering “about 1/4 inch” as M6 without checking pitch is a common RFQ failure.
Related guides:
Approximate diameter cross-reference
Outside diameters are close but threads will not mate across systems.
| Imperial (inch) | Approx mm | Nearest metric | Notes |
|---|---|---|---|
| #4 (0.112") | 2.8 | M3 | Common in electronics |
| #6 (0.138") | 3.5 | M3 or M4 | Prefer drawing callout |
| #8 (0.164") | 4.2 | M4 | Very common panel screw |
| #10 (0.190") | 4.8 | M5 | Not M4 |
| 1/4" (0.250") | 6.4 | M6 | Closest catalog size |
| 5/16" (0.312") | 7.9 | M8 | Do not force M6 |
| 3/8" (0.375") | 9.5 | M10 | Structural / frames |
| 1/2" (0.500") | 12.7 | M12 | Check grip & wrench |
| 5/8" (0.625") | 15.9 | M16 | Quote if length odd |
| 3/4" (0.750") | 19.1 | M20 | Production / RFQ |
Rule: If the print says 1/4-20 UNC, order inch—or redesign to metric. If the print says M6 × 1.0, order metric coarse (see pitch guide).
Thread pitch is the trap
| System | Example | Meaning |
|---|---|---|
| Metric coarse | M8 × 1.25 | 1.25 mm between threads (default) |
| Metric fine | M8 × 1.0 | Explicit fine pitch |
| UNC | 1/4-20 | 20 threads per inch, coarse inch |
| UNF | 1/4-28 | 28 TPI, fine inch |
A 1/4-20 nut will not run on an M6 screw. Always copy pitch from the drawing onto the PO.
Length under head (quick map)
Lengths also convert roughly—confirm under-head length on the SKU page.
| Inch length | Approx mm | Common metric lengths to try |
|---|---|---|
| 1/2" | 12.7 | 12 / 14 / 16 |
| 5/8" | 15.9 | 16 / 18 |
| 3/4" | 19.1 | 20 |
| 1" | 25.4 | 25 / 30 |
| 1-1/4" | 31.8 | 30 / 35 |
| 1-1/2" | 38.1 | 40 |
| 2" | 50.8 | 50 / 55 |
How to RFQ metric when the old part is inch
- Measure major diameter (calipers) → pick nearest M size from the table
- Count threads / check pitch gauge → coarse vs fine
- Measure under-head length → nearest mm
- Match head style: socket (SHCS), hex, pan, countersunk
- Material: for stainless, write 304 / A2-70 (not only “18-8”)
Copy/paste:
Metric · [Head] · M[thread] × [length mm] · coarse (or fine + pitch) · 304 / A2-70 · [DIN/ISO] · qty · ship-to USA
Example:
Metric SHCS · M6 × 20 · coarse · 304 / A2-70 · DIN 912 / ISO 4762 · 500 pcs · USA
→ sales@pisenfasten.com or Contact
Shop metric 304 / A2-70
| Need | Go here |
|---|---|
| Socket head (SHCS) | Socket head cap screws |
| Hex head | Hex head screws |
| Nuts / washers | Hex nuts · Flat washers |
| Inch Grade 5 / 8 / A325 | Not in this catalog — ask sales only if you need a sourcing quote |
Related searches
- metric to imperial bolt conversion chart
- m6 equivalent to 1/4 inch
- unc to metric screw chart
- replace 1/4-20 with metric
- metric socket cap screw for US machine
FAQ
Q: Can I replace 1/4-20 with M6?
A: Only with engineering approval. Diameters are close (~6.35 vs 6.0 mm) but threads will not mate. Redesign the tapped hole or keep inch fasteners.
Q: Is M8 the same as 5/16?
A: Approximately in diameter only. Pitch and standards differ—order by the system on the drawing.
Q: Do you stock imperial UNC fasteners?
A: Published catalog is metric 304 / A2-70. Email sales with the inch print if you need a sourcing path; do not assume web SKUs are UNC.
Q: What should a US buyer put on the PO for metric SHCS?
A: Thread × length × A2-70/304 × DIN 912 or ISO 4762. See how to specify metric SHCS.
Q: Where is the standards library?
A: DIN/ISO standards & charts.