US Fastener Sizes and Conversion Charts (Screw, Bolt, Fraction & Metric Equivalents)

US fastener sizing can feel like it was designed to test your patience: numbered screws (#8, #10), fractional diameters (3/8, 1/2), and then a metric spec sheet that expects everything in millimeters. This Metric Specialties guide brings the most common conversions into one place so you can identify sizes faster, order with confidence, and avoid the classic “it almost fits” hardware headache.

Screw Size Chart (US Number, Decimal, Fraction, Metric)

Use these charts to translate common US machine screw number sizes into decimal inches, a nearest everyday fraction, and a practical metric equivalent in millimeters. The inch diameters listed follow standard US machine screw diameter conventions.

Bolt Diameter Chart (Fractional Inch to Metric)

For larger diameters, US hardware is typically specified in fractions of an inch. This chart converts common fractional diameters to decimal inches and millimeters for quick comparison.

Note: Fractions are exact. Metric values are computed using the internationally defined relationship where 1 inch equals exactly 25.4 mm.

Fraction-to-Metric Chart (Quick Reference)

If you’re working from fractional measurements but need millimeters (or vice versa), these quick references help you translate the most common sizes without stopping to do math mid-task.

(Conversions rely on the exact inch definition of 25.4 mm per inch.)

Where These Charts Matter in Real Work

Conversions aren’t just “nice to have.” They show up in everyday tasks where parts, drawings, and tools don’t all speak the same measurement language. Maintenance teams use them when a machine manual references metric replacements but the storeroom inventory is labeled in fractional inches. Fabricators and machinists rely on conversions when prints call out decimal inches while the measuring tools on the bench are set up for fractions. Assembly teams use them to keep builds consistent, especially when mixed-origin components are involved.

Typical use cases include industrial equipment servicing, manufacturing and automation, automotive repair, HVAC installs, electrical enclosures, and general construction. In those environments, you may be selecting Screws for panels and housings, stepping up to Socket Head Bolts for compact high-strength assemblies, or pairing parts with a Socket Head Cap Screw and a matching Hex Nut when you need a dependable, repeatable fit.

Frequently Asked Questions (FAQs)

Why do US fasteners use both screw numbers and fractions?

In the US, smaller diameters are often designated by screw numbers (#6, #8, #10), while larger diameters are commonly specified in fractional inches (1/4", 3/8", 1/2"). Metric sizing developed separately and is expressed in millimeters.

How to measure a screw’s size the right way

Use calipers to measure the outside thread diameter, then measure the length according to the head style (often from under the head to the tip for many screws). Once you have the diameter, match it using the charts above.

What does a “#10” or “#8” screw label actually indicate?

It refers to the screw’s nominal diameter. As the number increases, the diameter increases as well.

How to translate a US screw number into a metric equivalent

Find the screw number in the Screw Size Charts and use the metric column as a quick reference. For precision work, verify the measured diameter and confirm the required fit for the application.

Can these charts help with bolts as well as screws?

Yes. When you’re working with fractional diameters, the Bolt Diameter Charts are the quickest way to compare inch sizes to millimeters.

How does size affect how much load a fastener can handle?

Diameter typically influences load capacity because a larger cross-sectional area can carry more force. For critical applications, grade, material, thread engagement, and installation method are just as important as diameter.

Why does thread pitch still matter if the diameter matches?

Diameter tells you the general size, but pitch determines whether the threads will properly mate. A correct diameter with the wrong pitch can bind, cross-thread, or fail to seat securely.

Ready to Put the Charts to Work?

If you’ve identified your size and you’re ready to source parts, Metric Specialties makes it easy to move from measurement to ordering. Start with our Screws and Hex Head Bolts selections to cover the most common builds and replacements.