Socket and wrench sizes: mm to inches, and which swaps round a bolt

What a wrench size measures

A wrench or socket size is the distance across the flats of the bolt head or nut, not the thread diameter. An M8 bolt, for example, usually has a 13 mm head; the 8 is the thread. A tool that is 0.5 mm too large can look fine on the bench and still slip once torque is applied, because it only grips the corners of the hex.

The conversion is exact: millimeters ÷ 25.4 gives inches. For any value not in the tables, use the mm to inches converter. If you already know the fractions, the fractions of an inch guide lists them in mm.

Common metric sizes in inches

The nearest 1/32 inch is shown, along with the gap in millimeters. A minus sign means the fractional tool is smaller than the bolt head.

Common fractional sizes in millimeters

For SAE or imperial hardware, here is each size in mm with the nearest whole-millimeter wrench and how it differs.

Pairs that nearly match

The closest metric-to-fraction match in the first table is 19 mm and 3/4 inch, only 0.05 mm apart. 19 mm and 3/4 inch (19.05 mm) are within 0.05 mm. 13 mm and 1/2 inch are different: 1/2 inch is 12.7 mm, so a 1/2 inch wrench on a 13 mm head has 0.3 mm of room across the flats, which in practice means a chance of slipping. In the other direction, a 13 mm wrench will not fit on a 1/2 inch head at all.

The rule behind the pattern is that a tool slightly larger than the head slips, and one slightly smaller does not go on. So it is the larger sizes that give trouble in swaps, not the smaller ones.

Rules for swapping

• Do not swap when the bolt is tight, rusted or torqued: use the exact size or a six-point socket that grips the flats.
• A 0.1 to 0.2 mm gap is usually workable on light fasteners such as covers or brackets.
• Prefer a six-point socket over a twelve-point one on any head you suspect is not the right size.
• If the head is already rounded, no unit conversion will fix it: use a bolt extractor socket.
• Keep your two sets in separate trays. Mixing mm and inch sockets in one drawer is the most common cause of the wrong tool being picked.

Sockets in a mixed toolbox

A practical way to avoid swaps is to know which metric sizes have no close imperial partner. Using the 1/32 table, the gap is largest where a metric size falls between two fractions. In the list above the biggest gap is 0.33 mm for 17 mm, so keep a real 17 mm tool instead of improvising. Cars from Europe and Asia are almost all metric; older American vehicles and plumbing are fractional, so a mixed set pays for itself.

Convert a size yourself

• Millimeters to inches: 17 ÷ 25.4 = 0.6693 inch. The nearest 1/32 is 21/32 inch (16.67 mm), which is 0.33 mm under, so that wrench will not go on a 17 mm head.
• Inches to millimeters: 9/16 inch is 9 ÷ 16 × 25.4 = 14.29 mm, which sits between the 14 mm and 15 mm wrenches.
• Check the inches to cm chart for sizes given in centimeters, and the inches to cm converter for anything with a decimal.

Where this fits in the series

This is the second post in the tools series; the first covers drill bit sizes in mm and fractional inches, where the same tolerance logic applies. For the same method on screens, see laptop screen sizes in cm. Reading the fractions off a tape comes next in how to read a tape measure.