Allen Wrench / Hex Key Size Chart — Metric & Inch

Allen wrench and hex key usually mean the same six-sided tool. The correct key is the one that fills the fastener socket without rocking, forcing, or rounding the corners. Use the exact key system and size shown for the fastener. A metric key that looks close to an inch key can still damage the socket.

Quick Allen key size chart

Use the search box to find a key size, a metric fastener, or an inch fastener designation. Use the system filter to narrow the results. The fastener mappings in this chart cover the selected socket-head cap screw rows. They are not a universal mapping for every screw with an internal hex socket.

How to read the chart

Exact key dimension is the nominal across-flats size of the hex key. Metric rows show the key in millimeters, and inch rows show the key as an inch fraction or decimal. The conversion uses 25.4 millimeters per inch:

• millimeters = inches × 25.4
• inches = millimeters ÷ 25.4

Common fastener/thread size identifies the socket-head cap screw row used for the lookup. Thread designation does not change the required key size when the listed head style and standard apply, but a different head style can use a different socket.

Nearest other-system key is a dimensional comparison. The difference is the absolute difference between the two key dimensions in millimeters. “Nearest” does not mean “fits.” The chart intentionally labels every comparison as non-interchangeable.

Allen wrench vs. hex key

In everyday workshop language, Allen wrench, Allen key, hex key, and hex wrench often describe the same L-shaped, T-handle, bit, or driver tool with a hexagonal cross-section. The name does not identify the size or the fastener standard. Check the socket itself, the fastener drawing, or the product designation before choosing a tool.

Metric vs. SAE hex keys

Metric keys are sized in millimeters. Inch-series, often called SAE in tool sets, are sized in fractions of an inch. Some neighboring sizes are separated by only a few hundredths of a millimeter. For example, a 5/16-inch key is 7.9375 mm, which is close to an 8 mm key. The dimensions are not equal, so one should not be used as a replacement for the other.

A key that is slightly undersize can turn at first and then slip under load. A key that is slightly oversize can enter only after force or damage the socket opening. Either condition can round the recess, reduce usable engagement, and make later removal harder. Stop if the key does not seat fully by hand. Clean the socket and check for coating, debris, burrs, corrosion, or a damaged fastener before applying torque.

Which key fits a socket-head cap screw?

Start with the complete fastener designation and head style. The chart includes common metric socket-head cap screws in the ISO 4762 family and common inch socket-head cap screw rows in the ASME B18.3 scope. It also uses an official manufacturer catalog and technical manual to cross-check the displayed product rows and recess sizes.

Do not infer the key from the screw diameter alone when the head style is unknown. Button-head, low-head, countersunk, shoulder, and other socket screw families can use different recess dimensions. A product catalog or drawing controls when it differs from a general lookup.

Which key fits a set screw?

Do not use this socket-head cap screw mapping as a universal set-screw chart. Set-screw socket sizes can depend on the point style, series, diameter, and controlling standard. For a production or high-torque application, use the exact part drawing or manufacturer specification.

Ball-end vs. standard hex keys

A ball-end key can reach a socket at an angle, which is useful when access is restricted. The ball contact area is smaller than the full flat-end engagement, so it is not the default choice for breaking loose a tight fastener or applying maximum torque. Seat a standard flat-end key squarely whenever access permits, then use the manufacturer’s tool and torque guidance.

Related size charts

• Bolt Size to Wrench & Socket Size Chart — compare external wrench sizes for common bolts and nuts.
• Metric Thread Pitch Chart — identify common metric thread diameters and pitches.
• Socket Head Cap Screw Dimensions — review head and socket dimensions when the screw family is known.

Frequently asked questions

Can I substitute an inch key for a close metric size?

No. A 5/16-inch key is close to an 8 mm key, but the dimensions are not equal. Use the key size specified for the fastener and stop if it does not seat fully by hand.

Does this chart give the key size for every screw with an internal hex?

No. Its fastener mapping covers selected socket-head cap screw rows. Set screws and other head styles can use different socket sizes. Check the exact fastener designation and manufacturer specification.

Sources and limitations

The metric socket-head cap screw scope is based on ISO 4762:2004, which ISO lists as current after its 2023 review. ISO 2936:2014 is the current ISO standard for hexagon socket screw keys after its 2025 confirmation. The inch-series scope is based on ASME B18.3-2026. The selected public rows are cross-checked against the STANLEY Camcar Socket Screw Catalog and its 2024 technical manual.

The full ISO and ASME standards are copyrighted and their complete dimensional tables require licensed access. This page does not reproduce the complete standards, set-screw families, gaging limits, manufacturing tolerances, torque values, strength data, or every possible key/fastener combination. Verify the current licensed standard and the exact product specification for production-critical work.