Free Engineering Tool
Thread Size Converter
Cross-reference between metric (ISO), UNC, UNF, and BSW thread sizes. Search by any thread designation or select a metric size to find the closest equivalents.
Thread Cross-Reference
| Metric | Ø mm | Pitch mm | UNC | UNF | BSW | Stress Area mm² |
|---|
Thread Standards Overview
The four main thread standards used worldwide:
- Metric (ISO 261/262) — Designated M + diameter (e.g. M8). Pitch in mm. 60° thread angle. Most common worldwide.
- UNC (Unified National Coarse) — Diameter-TPI format (e.g. 5/16-18). 60° angle. Standard coarse pitch for USA/Canada.
- UNF (Unified National Fine) — Same system as UNC but with finer pitch (more TPI). Better vibration resistance.
- BSW (British Standard Whitworth) — 55° thread angle. Diameter in inches with TPI. Legacy standard, still found in older equipment.
Pitch vs TPI Conversion
For example: M8 × 1.25 mm pitch = 25.4 / 1.25 = 20.32 TPI (not a standard TPI, confirming it’s not directly interchangeable with any inch thread).
Stress Area
Where d is the nominal diameter and P is the pitch. Stress area is used to calculate bolt tensile and proof loads.
⚠️ Important: “Closest equivalent” does NOT mean interchangeable. Metric and inch threads have different pitches even when diameters are similar. Never force a metric bolt into an inch-tapped hole or vice versa — this will damage both components.
ℹ️ Tip: To identify an unknown thread: (1) Measure the major diameter with calipers. (2) Use a thread pitch gauge to determine pitch or TPI. (3) Cross-reference in the table above.
No. Metric and inch threads are NOT interchangeable, even when the diameters appear similar. The key differences:
- Pitch: Metric uses mm pitch; UNC/UNF uses TPI — they never match exactly
- Thread angle: Both metric and unified use 60°, but BSW uses 55°
- Root/crest: Different profiles even at the same angle
Forcing the wrong thread will strip the threads and damage both components. Always verify with a thread gauge.
Step-by-step identification:
- Measure major diameter with calipers (mm or inches)
- Use a thread pitch gauge to determine pitch (mm) or TPI (threads per inch)
- Check the thread angle: 60° = metric or unified; 55° = Whitworth/BSP
- Cross-reference in the table above to confirm the standard
If the pitch is a clean mm value (0.5, 0.7, 1.0, 1.25, 1.5, 2.0, 2.5, 3.0), it's almost certainly metric.
Pitch is the distance between adjacent thread crests, measured in millimeters. Used for metric threads.
TPI (Threads Per Inch) is the number of complete thread crests in one inch. Used for imperial threads.
Conversion: Pitch (mm) = 25.4 / TPI
Examples: 20 TPI = 1.27 mm pitch, 18 TPI = 1.411 mm pitch, 16 TPI = 1.5875 mm pitch.
BSW (British Standard Whitworth) was the world's first national standard thread, created by Sir Joseph Whitworth in 1841.
Key features:
- 55° thread angle (vs 60° for metric/unified)
- Rounded roots and crests
- Diameter in inches, pitch in TPI
BSW has been largely replaced by metric and unified standards but is still found in legacy machinery, plumbing (BSP is related), and some British equipment.
UNC (Unified National Coarse) — the standard/default thread:
- Fewer threads per inch → faster assembly
- More forgiving for tapped holes in soft materials
- Less likely to cross-thread
UNF (Unified National Fine) — the fine-pitch variant:
- More threads per inch → higher tensile strength
- Better vibration resistance (self-locking effect)
- Finer adjustment capability
- Requires more precise tapping
Rule of thumb: Use UNC for general assembly, UNF for precision and vibration-prone applications.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments and vibration analysis software. Precision tools for precision engineering. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
