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Metric / Imperial Thread Conversion
Cross-reference and convert between Metric (ISO 261), Unified Inch (UNC/UNF per ASME B1.1), and British Standard Whitworth (BSW/BSF per BS 84) thread sizes. Find the nearest equivalent thread in another standard system.
Quick Lookup
M6 — Thread Equivalents
Metric (ISO 261)
M6
d = 6 mm
Coarse P = 1 mm
Fine P = 0.75 mm
Coarse P = 1 mm
Fine P = 0.75 mm
Unified (ASME B1.1)
1/4"-20
d = 6.35 mm (0.250")
UNC: 20 TPI (P = 1.270 mm)
UNF: 28 TPI (P = 0.907 mm)
UNC: 20 TPI (P = 1.270 mm)
UNF: 28 TPI (P = 0.907 mm)
BSW (BS 84)
1/4" BSW
d = 6.35 mm
20 TPI (P = 1.270 mm)
20 TPI (P = 1.270 mm)
Complete Cross-Reference Table
| Metric | d (mm) | P coarse | P fine | UNC | UNF | BSW |
|---|---|---|---|---|---|---|
| M1.6 | 1.6 | 0.35 | — | — | — | — |
| M2 | 2 | 0.4 | — | #2-56 | #2-64 | — |
| M2.5 | 2.5 | 0.45 | — | #3-48 | #3-56 | — |
| M3 | 3 | 0.5 | 0.35 | #4-40 | #4-48 | — |
| M3.5 | 3.5 | 0.6 | — | #6-32 | #6-40 | — |
| M4 | 4 | 0.7 | 0.5 | #8-32 | #8-36 | 5/32" BSW |
| M5 | 5 | 0.8 | 0.5 | #10-24 | #10-32 | 3/16" BSW |
| M6 | 6 | 1 | 0.75 | 1/4"-20 | 1/4"-28 | 1/4" BSW |
| M8 | 8 | 1.25 | 1 | 5/16"-18 | 5/16"-24 | 5/16" BSW |
| M10 | 10 | 1.5 | 1.25 | 3/8"-16 | 3/8"-24 | 3/8" BSW |
| M12 | 12 | 1.75 | 1.25 | 1/2"-13 | 1/2"-20 | 1/2" BSW |
| M14 | 14 | 2 | 1.5 | 9/16"-12 | 9/16"-18 | 9/16" BSW |
| M16 | 16 | 2 | 1.5 | 5/8"-11 | 5/8"-18 | 5/8" BSW |
| M18 | 18 | 2.5 | 1.5 | 3/4"-10 | 3/4"-16 | 3/4" BSW |
| M20 | 20 | 2.5 | 1.5 | 3/4"-10 | 3/4"-16 | 3/4" BSW |
| M22 | 22 | 2.5 | 1.5 | 7/8"-9 | 7/8"-14 | 7/8" BSW |
| M24 | 24 | 3 | 2 | 1"-8 | 1"-12 | 1" BSW |
| M27 | 27 | 3 | 2 | 1-1/8"-7 | 1-1/8"-12 | — |
| M30 | 30 | 3.5 | 2 | 1-1/4"-7 | 1-1/4"-12 | 1-1/4" BSW |
| M33 | 33 | 3.5 | 2 | 1-3/8"-6 | 1-3/8"-12 | — |
| M36 | 36 | 4 | 3 | 1-1/2"-6 | 1-1/2"-12 | 1-1/2" BSW |
Important Notes
- Not interchangeable: Metric and Unified threads are NOT directly interchangeable even when close in diameter. Thread angles differ (60° for both ISO and Unified, but 55° for BSW). Pitch values also differ.
- Closest equivalent: This table shows the nearest size in other standards for reference and cross-ordering. Always verify compatibility with go/no-go gauges.
- UNC vs UNF: UNC (Unified National Coarse) has fewer threads per inch and is the default for general use. UNF (Unified National Fine) provides higher tensile strength and better vibration resistance.
- BSW/BSF: British Standard Whitworth (BSW coarse, BSF fine) is largely obsolete but still found in legacy equipment. Thread angle is 55° vs 60° for ISO/Unified.
Reference: ISO 261:1998, ASME B1.1-2019, BS 84:2007.