Screw Size Converter
Convert US screw sizes, fractional inch screws, and metric M sizes to major diameter in inches and millimeters. Use it to compare fasteners before you buy or sort parts.
Convert a screw size
Enter a size like #8, 1/4, M6, 6 mm, or 0.164 inches. Results use nominal major diameter.
A #8 screw is about 0.164 inches or 4.17 mm in nominal major diameter.
How to use screw size conversions
- Enter the screw size from a package, parts list, plan, or caliper measurement.
- Compare the inch and millimeter diameter to identify nearby US and metric sizes.
- Before replacing a fastener, also confirm thread pitch, length, head type, drive type, and material.
- For tapped holes or safety-critical work, use the exact fastener specification rather than only the closest diameter.
Common screw size chart
| Size | Diameter in | Diameter mm | Type |
|---|
Screw size conversion FAQ
What does a #8 screw size mean?
#8 is a US numbered screw size. Its nominal major diameter is 0.164 inches, or about 4.17 mm. The number is not the same thing as thread pitch or screw length.
What does M6 mean on a screw?
M6 means a metric screw with a nominal major diameter of 6 millimeters. Metric fasteners also have a pitch, such as M6 × 1.0, which describes the spacing between threads.
Are US and metric screw sizes interchangeable?
Not automatically. A nearby diameter can help you compare sizes, but thread pitch and thread form differ. Match the complete fastener spec when the screw threads into an existing hole or nut.