Are metric and inch threads interchangeable?

No. Metric and inch threads are NOT interchangeable, even when the diameters are similar. The thread pitch and flank angle differ. Metric threads use a 60° angle with pitch in mm. UNC/UNF also use 60° but with TPI (threads per inch). BSW uses a 55° angle. Forcing a wrong thread will damage both bolt and nut.

How to identify thread type?

Use a thread pitch gauge to measure the pitch. Measure the major diameter with calipers. If pitch is in whole mm values (0.5, 0.7, 1.0, 1.25, 1.5, 2.0, etc.), it's likely metric. Count threads per inch for imperial. Check if the thread angle is 60° (metric/unified) or 55° (BSW/BSP).

What is the difference between pitch and TPI?

Pitch is the distance between adjacent thread crests in millimeters (used for metric threads). TPI (Threads Per Inch) is the number of threads in one inch (used for imperial threads). They are inversely related: Pitch (mm) = 25.4 / TPI.

BSW (British Standard Whitworth) is a thread standard developed by Sir Joseph Whitworth in 1841. It uses a 55° thread angle (vs 60° for metric and unified). BSW has largely been replaced by metric and unified standards but is still found in legacy equipment, plumbing (BSP), and some British machinery.

What is the difference between UNC and UNF?

UNC (Unified National Coarse) has fewer threads per inch — it's the standard/default thread. UNF (Unified National Fine) has more threads per inch for the same diameter. UNF provides: better vibration resistance, higher tensile strength per diameter, finer adjustment capability. UNC is preferred for: general assembly, tapped holes in softer materials, and ease of assembly.

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.

Metric ISO UNC / UNF BSW Whitworth

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.