Metric to Imperial Thread Size Reference for Industrial Fasteners
Introduction
Selecting the correct thread size is essential for ensuring compatibility, strength and proper assembly. Engineers and procurement professionals often encounter both metric (ISO) and imperial (Unified) thread systems when sourcing or replacing fasteners.
This thread size conversion chart provides a convenient reference between common ISO metric threads and their nearest imperial equivalents, including UNC and UNF thread series. It is intended as a quick identification guide only and should not be used to substitute one thread system for another, as thread pitch, profile and tolerances differ between standards. (RivCut CNC)
What This Chart Includes
- ISO Metric Thread Sizes
- Unified Coarse (UNC) Thread Sizes
- Unified Fine (UNF) Thread Sizes
- Major Diameter Comparison
- Metric Pitch
- Threads per Inch (TPI)
- Nearest Equivalent Sizes
Important Note
Metric and imperial threads are not directly interchangeable.
Although some thread sizes have very similar outside diameters, differences in thread pitch, thread angle and tolerance class mean they should not be considered direct replacements. Always verify the applicable engineering standard before selecting a substitute fastener.
Thread Size Conversion Chart
| Metric Coarse | Metric Fine | BSF/BSW | UNC | UNF |
|---|---|---|---|---|
| M2 × 0.4 | – | – | 1-64 | 1-72 |
| M2.5 × 0.45 | – | – | 3-48 | 3-56 |
| M3 × 0.5 | – | – | 4-40 | 4-48 |
| M3.5 × 0.6 | – | – | 6-32 | 6-40 |
| M4 × 0.7 | – | – | 8-32 | 8-36 |
| M5 × 0.8 | – | – | 10-24 | 10-32 |
| M6 × 1 | M6 × 0.75 | 1/4″ | 1/4-20 | 1/4-28 |
| M8 × 1.25 | M8 × 1 | 5/16″ | 5/16-18 | 5/16-24 |
| M10 × 1.5 | M10 × 1.25 | 3/8″ | 3/8-16 | 3/8-24 |
| M12 × 1.75 | M12 × 1.25 | 1/2″ | 1/2-13 | 1/2-20 |
| M14 × 2 | M14 × 1.5 | 9/16″ | 9/16-12 | 9/16-18 |
| M16 × 2 | M16 × 1.5 | 5/8″ | 5/8-11 | 5/8-18 |
| M20 × 2.5 | M20 × 1.5 | 3/4″ | 3/4-10 | 3/4-16 |
| M24 × 3 | M24 × 2 | 1″ | 1-8 | 1-12 |
| M30 × 3.5 | M30 × 2 | 1-1/8″ | 1 1/4-7 | 1 1/8-12 |
| M36 × 4 | M36 × 3 | 1-3/8″ | 1 1/2-6 | 1 1/4-12 |
How to Read the Chart
Metric Threads
Example:
M8 × 1.25
- M = Metric thread
- 8 = Major diameter (mm)
- 1.25 = Pitch (mm)
UNC Threads
Example:
5/16-18 UNC
- 5/16 = Nominal diameter
- 18 = Threads per inch
- UNC = Unified National Coarse
UNF Threads
Example:
5/16-24 UNF
- Same nominal diameter
- 24 Threads per inch
- Fine thread series
Typical Equivalent Sizes
These are nearest reference sizes, not direct replacements.
| Metric | Approximate Imperial |
|---|---|
| M3 | #4-40 UNC |
| M4 | #8-32 UNC |
| M5 | #10-24 UNC |
| M6 | 1/4-20 UNC |
| M8 | 5/16-18 UNC |
| M10 | 3/8-16 UNC |
| M12 | 1/2-13 UNC |
| M16 | 5/8-11 UNC |
| M20 | 3/4-10 UNC |
Related Technical References
- ISO Metric Thread System
- Fastener Nomenclature & Designation System
- Standard Dimensional Tolerances for Common Metric Fasteners
- DIN ISO Standards for Fasteners
Frequently Asked Questions
Can metric and imperial threads be interchanged?
No. Similar major diameters do not guarantee compatibility. Differences in thread pitch, profile and tolerances mean they should not be used interchangeably.
What is the closest imperial equivalent to M6?
The nearest commonly referenced size is 1/4-20 UNC, although the two thread systems are not interchangeable.
What does UNC mean?
UNC stands for Unified National Coarse, the standard coarse thread series used in the Unified Thread Standard.
What does UNF mean?
UNF stands for Unified National Fine, the fine thread series of the Unified Thread Standard.
Is this chart suitable for selecting replacement fasteners?
The chart is intended as a quick reference for identifying similar thread sizes. Always verify the applicable thread standard, pitch and tolerance class before selecting a replacement fastener.
