Repair Topics > Fasteners — bolts and screws > Bolt size to wrench size chart for metric and U.S. bolts — Guide

Bolt size to wrench size chart for metric and U.S. bolts — Guide

Find the right wrench size using this bolt size to wrench size chart

As someone who has spent decades working with fasteners in automotive repair, machinery assembly, and home projects, I can tell you that knowing the proper wrench size for a given bolt isn’t just about convenience—it’s about doing the job correctly. If you don’t match the wrench to the bolt properly, you risk rounding off fasteners, damaging tools, and wasting valuable time. That’s why I always rely on a bolt size to wrench size chart to guide my tool selection.

In this article, I’ll walk you through the fundamentals of bolt sizing, explain how wrench sizes correspond, and share some tips that I’ve learned in the shop. With a Bolt size to wrench size chart, you’ll never struggle to pick the right tool again.

The Basics of Bolt Sizing

Bolts generally follow two standards: Metric and Imperial (SAE). Metric bolts are measured in millimeters. For example, an M10 bolt has a 10 mm shaft diameter. Imperial bolts, on the other hand, are measured in fractions of an inch. A 3/8-inch bolt, for instance, has a shaft that measures 0.375 inches across.

Bolts are measured by several factors, including diameter, thread pitch, and length. The diameter is the width of the bolt shaft, while the thread pitch refers to the distance between threads. Length is the distance from the underside of the bolt head to the tip. When we talk about bolt size in the context of wrench selection, we’re primarily concerned with the diameter of the bolt shaft.

Bolts come in two major standards: Metric and Imperial (SAE). Metric bolts are measured in millimeters, while Imperial bolts are measured in inches. For instance, a bolt labeled M10 has a 10mm diameter, whereas a 3/8″ bolt has a diameter of 0.375 inches.