Choose an Allen key by the fastener’s specified hex-drive size—not by the bolt’s outside diameter. Match the key’s nominal size and system: millimetres for metric fasteners, inches or fractions for inch-series fasteners. A nearby converted size is not a dependable substitute.
What does an Allen key size measure?
An Allen key, also called a hex key or hex wrench, is sized by the distance across its parallel flat sides. That dimension must match the width of the hexagonal socket in the fastener. It is not the diameter of the bolt or screw.
Look for the drive size in the fastener specification, product documentation, or equipment manual. Bolt diameter alone is not enough to identify the correct key for every fastener style.
How metric and inch-series sizing differ
Metric hex keys are designated in millimetres. Inch-series keys are designated in inches, often as fractions. The two systems are covered separately: ISO 2936:2014 specifies hexagon socket screw keys in the metric system, while ASME B18.3 covers inch-series hex keys and specified socket screws. ASME also cautions that its dimensional tables govern over recalculation by formula when product acceptance questions arise.
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#1 Best Overall
- 【Sturdy Allen Key Set】Our allen wrenches set made from hardened chrome vanadium and S2 alloy steel for rust resistance and long-lasting strength. The set includes color-coded, clearly marked sizes for easy identification
- 【Ball Head Design】The ball head design of the hex key set allows up to 25° entry for easy access in tight spaces. Surface oxidation treatment for corrosion resistance
- 【Sizes of AMTOVL' Hex Wrenches】Includes metric wrenches (1.5, 2, 2.5, 3, 4, 5, 6, 8, 10 mm), covering common sizes for various tasks. Perfect for installing or removing socket cap screws in automobiles, furniture, bikes, and equipment
- 【Widely Used】The ball end allen key set has clear visible size mark for fast size selection, good for diy repairs of electronics, bikes, electrical work, machinery, automobile and motorcycle
- 【Distinctive Color Coding】Every allen key heat-treated, chrome-plated, and uniquely color-coded for easy size identification. The textured, slip-resistant surface ensures a secure grip, even with oily hands. A perfect present for any tool enthusiast
For metric-series keys and bits, ASME B18.3.2M gives nominal sizes from 0.7 mm to 36 mm. That stated range describes the standard’s coverage; it is not a conversion table between metric and inch sizes.
ISO reports that ISO 2936:2014 was reviewed and confirmed in 2025 and remains current. Its scope includes hexagon socket screw keys for tightening hexagon socket screws up to property class 12.9 under ISO 898-1 and socket set screws under ISO 898-5. See the ISO 2936:2014 standard page.
Rank #2
- Allen wrenches (aka hex keys) for tightening or loosening screws
- Includes 36 hex keys in SAE and metric sizes and with long and short arm lengths
- Made of durable chrome vanadium steel with a black oxide finish for rugged strength and rust resistance
- 2 hinged cases included, complete with dedicated slots for key storage and clear size markings for fast locating
- Ideal for working on bicycles, motorcycles, vehicles, furniture, machinery, and more
Examples for common socket-head cap screws
The examples below apply to the named socket-head cap screw type, not automatically to every screw with the same nominal thread diameter. Confirm the fastener standard and drive specification for the item in hand.
| Fastener example | Hex key example | Qualification |
|---|---|---|
| M4 socket-head cap screw | 3.0 mm | Secondary sizing chart example; verify the exact fastener specification. |
| M6 socket-head cap screw (ISO 4762) | 5 mm | Secondary sizing chart example; applies to the named screw type. |
| M8 socket-head cap screw | 6 mm | Secondary sizing chart example; verify the exact fastener specification. |
| M10 socket-head cap screw | 8 mm | Secondary sizing chart example; verify the exact fastener specification. |
| M12 socket-head cap screw | 10 mm | Secondary sizing chart example; verify the exact fastener specification. |
| #6 inch screw | 7/64 inch | Secondary sizing chart example; verify the exact fastener specification. |
These examples are not a universal bolt-diameter-to-key chart. Head style and fastener specification matter, and a complete socket-by-socket table is not established by the cited official standard-page summaries.
Rank #3
- 【PREMIUM MATERIAL】 Heat-treated steel with black-oxide finish, featuring 15 long reach keys and 15 high leverage short keys for durability and strength.
- 【HIGH LEVERAGE】 Conventional drive end on both long and short keys allows for high leverage on stubborn fixings, ensuring efficient operation.
- 【TORQUE HANDLE】 Comes with a torque handle for added force during loading and unloading fasteners, increasing reach and enhancing torque performance.
- 【COMPLETE COVERAGE】 31-piece Allen wrench set with both inch and metric measurements, covering all hexagon socket screws for comprehensive use.
- 【EASY ORGANIZATION】 High-visibility size markings on each wrench and a sturdy carrying case for quick size selection and convenient storage.
Can a metric key fit an inch-series screw?
Sometimes a nearby size may seem to fit, but that does not establish a safe or acceptable substitution. The metric and inch series have separately standardized nominal sizes, and the cited standards do not define a universal substitution tolerance. A loose or forced fit can impair engagement and risk damaging the socket or tool.
- Identify the fastener’s stated drive size from its specification or documentation.
- Confirm whether the designation is metric or inch-series.
- Choose a key with that same nominal size and system.
- Seat it fully and check that it fits snugly before applying force.
Choosing a set for mixed-system work
If your work regularly includes both metric and inch-series fasteners, a set containing both systems can be practical. Catalogs list separate metric and inch sets as well as combined fractional-and-metric sets; no single set should be assumed to cover every fastener. Check the included sizes against the equipment and fasteners you actually service.
Rank #4
- 【NOTE】In this hex wrench set, only some large-size wrenches are equipped with magnetic adsorption functionality; small-size wrenches do not support it due to structural limitations. To balance user experience and magnetic convenience, large-size models feature a weak magnetic design. The magnetism will gradually diminish after prolonged use, which is a normal phenomenon and does not affect the wrench's primary function.
- 【Premium Material】DISEN 13pc Allen Wrenches are forged from high quality heat treated CR-V steel for great hardness and durability, and finished with black oxide for anti-corrosion and rust prevention
- 【Multiple Size Selection】DISEN 13 Pieces Metric Allen Key Sets provide Metric Sizes including: 1.27, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 8, 10mm. Every hex key is labelled with specific sizes on the surface so that you can easily get the size you want
- 【Long Arm Design】DISEN Allen Key Set has a long-arm design that can provide a leverage for stable and strong torque strength. The chamfered edges and the long arm ball head design provide a 25-degree entry to the hard-to-access space
- 【Useful Tools】DISEN Allen Wrench Key Sets are useful hand tools and small enough to go into the tight space where traditional hex key can not reach. You can use them to easily tighten or loosen the bolt and nut, suitable for auto repair, DIY project, and household appliance maintenance
When comparing sets, check the size range and markings, whether the keys are short-arm or long-arm, the format (such as L-key, T-handle, or bits), and the storage or holder. These are fit-for-task considerations rather than signs that one format is universally best. A catalog also states the basic conversion 25.4 mm = 1 inch, but converting units does not make adjacent standardized key sizes interchangeable.
Quick Recap
Best Value
- Folding Hex Keys with two key positions (straight-out or right angle) for extra leverage
- Hex ends are square-cut for a secure full-depth fit and maximum contact with the hex socket to prevent slippage and stripping of the socket
- Handy sets fit in your pocket or tool box easily and help eliminate the lost key problem
- Includes 7 hex key sizes: 1-1/2 mm, 2 mm, 2-1/2 mm, 3 mm, 4 mm, 5 mm and 6 mm
- Made in USA
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