SAE to Metric Calculator

What is an SAE to metric calculator?

An SAE to metric calculator converts a tool or fastener size given in inches (the SAE convention) into millimeters (the metric convention), and back again. SAE stands for the Society of Automotive Engineers, and in everyday workshop language an “SAE” wrench or socket is one sized in fractions of an inch — 1/4”, 3/8”, 1/2”, and so on. A metric tool, by contrast, is sized directly in millimeters — 6 mm, 10 mm, 13 mm.

Because hardware is sold in both standards, anyone working on cars, bicycles, machinery, furniture, or plumbing eventually runs into a bolt head that does not quite match the wrenches on hand. This converter takes the guesswork out of that situation: enter a size in either inches or millimeters and read off the equivalent in the other unit instantly.

SAE (fractional-inch) versus metric (millimeter) sizing

The two systems describe the same physical dimension — usually the distance across the flats of a hex head or the nominal diameter of a fastener — but label it differently:

• SAE sizes are written as fractions of an inch, such as 1/4”, 7/16”, or 9/16”. They step up in sixteenths (and sometimes thirty-seconds) of an inch.
• Metric sizes are written as whole or decimal millimeters, such as 10 mm or 14 mm. They step up in one- or two-millimeter increments.

The sizes interleave but rarely line up exactly, which is why a 1/2” socket and a 13 mm socket feel “almost” interchangeable but should not be forced onto the wrong fastener.

How does the calculator work?

One inch is defined as exactly 25.4 millimeters, so the conversion is a single multiplication:

$$mm = in \times 25.4$$

To go the other way, divide instead:

$$in = \frac{mm}{25.4}$$

Where $in$ is the size in inches and $mm$ is the size in millimeters. The calculator is bidirectional: type a value into the inches field and the millimeters field updates, or type into the millimeters field and the inches field updates.

Worked examples

Example 1 — inches to millimeters. Convert a 1/2” (0.5 inch) wrench to metric:

$$mm = 0.5 \times 25.4 = 12.7\,mm$$

Example 2 — a full inch. Convert 1 inch to millimeters:

$$mm = 1 \times 25.4 = 25.4\,mm$$

Example 3 — millimeters to inches. Convert a 13 mm socket to inches:

$$in = \frac{13}{25.4} \approx 0.5118\,in$$

This is just over 1/2” (0.5”), which is why a 13 mm socket is a slightly loose fit on a 1/2” fastener and vice versa.

Example 4 — back to a whole inch. Convert 25.4 mm to inches:

$$in = \frac{25.4}{25.4} = 1\,in$$

Common SAE to metric equivalents

The following reference values show the exact millimeter equivalent of frequently used SAE sizes, computed with the same $mm = in \times 25.4$ formula. A 1/4” size equals 6.35 mm, a 5/16” size equals 7.9375 mm, a 3/8” size equals 9.525 mm, a 7/16” size equals 11.1125 mm, a 1/2” size equals 12.7 mm, a 9/16” size equals 14.288 mm, a 5/8” size equals 15.875 mm, and a 3/4” size equals 19.05 mm. Notice that none of these land on a whole millimeter, which is exactly why nearby metric sizes (such as 6 mm near 1/4”, or 13 mm near 1/2”) are close but not perfect substitutes.

Practical notes

• The 25.4 factor is exact by definition, so the conversion itself introduces no rounding error — any rounding you see comes only from limiting the displayed decimal places.
• When a converted value falls between two available tool sizes, choose the size that fits snugly rather than the closest number; a wrench that is even a fraction too large can round off a fastener’s corners.
• The same arithmetic applies whether you are converting drill bits, wrench openings, bolt diameters, or sheet thicknesses — anything measured in inches converts to millimeters by multiplying by 25.4.