Allen Wrench Size Chart

Key size is measured across flats. Closest sizes are numerical comparisons, not approved substitutes.

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Scroll across for the closest size; scroll down for more rows.

Coverage: 22 inch sizes (0.028–1 in) and 28 metric labels (0.7–36 mm). This is a selected reference range, not every manufactured size. Small rounded labels such as 1.3 mm may represent a 1.27 mm tool; verify the product specification. Conversion factor: exactly 25.4 mm per inch.

Allen Wrench Size Chart | Turn2Engineering | See the printed scope and sources before using the values.

Enter a nominal key size or an across-flats measurement. Inch fractions such as 5/32 are accepted. The closest result is limited to the sizes listed above.

M6 socket-head cap screw → 5 mm key. M8 socket-head cap screw → 6 mm key. 1/4 in socket-head cap screw → 3/16 in key. Check the head style first: the same thread can use a different drive.

Metric screw-to-key chart

All key sizes below are in millimeters. Socket-head cap screws: ISO 4762. Button heads: ISO 7380. Socket set screws (grub screws): ISO 4026/4027/4028/4029. A dash means no value is supplied here, not that the product cannot exist. Bossard reference [3].

SAE socket-head cap screw chart

For the regular socket-head cap screws listed by Holo-Krome to ASME B18.3; not button, flat, low-head or set screws. Both coarse and fine thread series use these drive sizes within the cited scope. Holo-Krome reference [4].

For countersunk, low-head, shoulder, proprietary or unlisted screws, use the exact manufacturer drawing. A driver’s 1/4 in bit-holder shank is not the hex tip size.

Many compact metric sets include 1.5, 2, 2.5, 3, 4, 5, 6, 8 and 10 mm. Inch sets commonly span 0.050, 1/16, 5/64, 3/32, 7/64, 1/8, 9/64, 5/32, 3/16, 7/32 and 1/4 in; larger sets may add 5/16 and 3/8 in. Set contents vary. The main chart also includes intermediate and larger sizes. Manufacturer size reference [2].

Metric chart: select Metric above. SAE chart: select Inch / SAE. With scripts disabled, the static chart lists inch sizes first and metric sizes second.

A hexagon has six flat sides and six corners. The key’s nominal size refers to the distance between opposite parallel flats. A corner-to-corner reading is larger and will point toward the wrong tool. On a small recess, caliper alignment, dirt and edge wear can make a direct reading unreliable; the specified drive size is a better reference.

For an ideal regular hexagon, across-corners distance equals across-flats distance divided by cos 30°, or about 1.1547 times the flat-to-flat dimension. A theoretical 6 mm hex therefore measures about 6.928 mm across opposite corners. That geometry explains the difference; it is not a machining tolerance for the actual recess.

The larger or smaller result is not automatically a usable replacement. A loose key may concentrate contact and damage the recess; an oversized key may not seat. Use the size specified for the fastener, with full appropriate engagement, rather than forcing the nearest number from a conversion chart.

Choose a tool form that can engage the recess as intended. A straight tip, ball-end tip, L-key and hex-bit socket are not interchangeable in every access or torque situation. Follow the tool and fastener instructions for angled use and tightening; this nominal-size chart supplies no torque rating.

For tiny tools, AFT’s nominal 1.3 mm entry illustrates the danger of treating a rounded label as a precision limit: some small tools are also described by a 0.050 in label, which converts to 1.27 mm. Check the actual tool specification rather than inferring a 0.03 mm manufacturing discrepancy from the label alone. Source [1]

Why does my Allen wrench feel loose?

Check for a mismatched metric/inch size, a worn key, a rounded recess, or dirt and paint preventing full seating. Clean and inspect the socket, then use the specified size. Stop if the key slips; repeated slipping can worsen the damage. A ball end is for angled access within the tool’s instructions; use the straight end for full engagement when appropriate.

References checked September 30, 2026. Nominal unit conversions are calculated with 1 in = 25.4 mm. Listed sizes are selected manufacturer/reference sizes, not a claim to reproduce the complete ISO or ASME tables. Standards overview pages establish scope and edition; the screw mappings come from the product/reference tables below.

1. AFT Fasteners — Allen wrench sizes and dimensions. Base inch and metric labels; rounded small-tool labeling. Arm lengths and tool tolerance tables are not reproduced.
2. Bondhus — 2025 catalog (PDF), hex-end L-wrenches. Intermediate metric size availability. Larger drive sizes are also supported by the Bossard table.
3. Bossard — Assembly tools (PDF), January 2025, page 94. Metric thread/head-style assignments. Special products may differ.
4. Holo-Krome — inch alloy socket-head cap screw product standard (PDF), Rev. 01, 2011. Nominal key assignments; the source rounds inch key values to three decimals. Fractions here identify the nominal tool.
5. ISO 2936:2014 — hexagon socket screw keys. Published edition confirmed in 2025; covers key dimensions and testing.
6. ASME B18.3-2026 — inch socket screws and keys. Current listing; full normative dimensional tables were not independently checked against this edition.

Browse all engineering charts and tables for related dimensional references.