Metric to SAE Calculator - Fastener Size Converter

The metric to SAE calculator is a workshop-ready tool that converts metric fastener sizes in millimeters to the closest standard SAE fractional inch used by imperial sockets, wrenches, and taps. If you have ever stood in front of a parts drawer trying to decide whether your M8 bolt needs a 5/16 or 3/8 inch socket, this calculator removes the guesswork.

Metric fasteners are sized by the letter M followed by the nominal diameter in millimeters, e.g., M10 = 10 mm. SAE fasteners use fractional inches, e.g., 3/8 in, paired with standardized wrench openings. The two systems coexist on most modern vehicles and machinery, so being able to jump between them is essential for mechanics, fabricators, and DIY builders.

The tool also runs in reverse. Switch the mode and you can enter a SAE inch value to get the equivalent metric size, which is helpful when ordering metric replacements for older American equipment.

To convert metric lengths to plain inches without the SAE snap, explore our metric to inches calculator for the full fractional ruler output.

The conversion runs in two steps. First, the millimeter value is divided by 25.4 to obtain a decimal inch. Second, that decimal inch is compared to the standard SAE bolt series and snapped to the closest fractional size in the set 1.

For example, an M10 bolt is 10 mm. Dividing 10 by 25.4 yields 0.3937 inches. The nearest standard SAE fraction in the set is 3/8 inch (0.375 in), which is why a 3/8 in socket is the conventional match for M10 hardware. An M8 bolt at 8 mm gives 0.315 in, snapping to 5/16 in (0.3125 in).

According to the National Institute of Standards and Technology (NIST), the inch is defined as exactly 25.4 millimeters, providing the authoritative conversion constant used to map metric fastener sizes to SAE fractional inches.

To do the same conversion in the opposite direction, explore our inch converter for inch-to-mm and inch-to-cm support.

Understanding metric to SAE conversion is easier once you know the four ideas that connect the two worlds:

For a typical M6 to SAE mapping, 6 mm equals 0.236 in, which snaps to 1/4 in. The same logic scales all the way up: M20 (20 mm = 0.787 in) snaps to 3/4 in, and M24 (24 mm = 0.945 in) snaps to 1 in. This pairing is the reason most modern tool sets ship with both metric and SAE sockets.

To compare metric and customary lengths in general, explore our centimeter calculator for side-by-side mm, cm, and inch conversions.

This tool is built for the toolbox, so it updates in real time as you type. Follow these four steps to match any metric bolt to the right SAE socket:

If you regularly work on US-built vehicles and need to convert M10 bolts to SAE hardware, keep this calculator bookmarked. It also helps in reverse: enter a 3/8 in socket label and you will see the matching metric size is about 9.525 mm, useful when buying metric taps or drill bits for an SAE hole.

To convert larger travel distances between systems, explore our kilometer calculator for km, mile, and meter conversions.

Sorting through mixed metric and SAE hardware is faster when you have a reliable reference. Here are the key advantages of using this calculator:

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  Eliminates guesswork: Instantly tells you whether a metric bolt uses a 5/16, 3/8, or 1/2 inch socket.
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  Speeds up repairs: Avoid trial-and-error at the toolbox by knowing the right SAE tool before you walk over.
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  Standard-aligned results: Uses the NIST 25.4 mm/in constant and the SAE standard inch bolt series for trustworthy values.
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  Bidirectional: Convert from metric to SAE or from SAE back to metric without flipping between separate tools.

The same workflow also serves as a quick SAE bolt size chart. Whether you are ordering a replacement 7/16 in bolt for a vintage American truck or trying to identify the metric drill bit that matches a 3/8 in tap, the calculator outputs a label you can match against the parts drawer.

To switch between metric and customary length units for layouts, explore our feet to meters calculator for room, framing, and blueprint conversions.

The math behind metric to SAE conversion is exact, but the snap to a real wrench opening depends on a few physical factors you should keep in mind:

As published by SAE International, the standard inch bolt series (1/4, 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, 3/4, 7/8, 1) governs imperial wrench and socket sizing for automotive and machinery applications.

To flip back from imperial to full metric lengths, explore our inch to meter calculator for longer-distance unit swaps.