Allen Key Sizes — Which Hex Key Do I Need?
Allen keys (hex keys) are used with hex socket screws — the type with a hexagonal recess in the head. You’ll encounter them in flat-pack furniture, bicycles, machine bolts, and countless other applications. This guide covers every common Allen key size, what each is used for, and answers the most frequent question: which Allen key for IKEA furniture?
Allen Key Size Chart — Metric (UK Standard)
The UK uses metric Allen keys. Imperial sizes (measured in inches) are American and rarely encountered here unless you’re working on American imports or older motorcycles.
| Allen Key Size | Bolt Size (Metric) | Common Applications |
|---|---|---|
| 1.5mm | M2 | Small electronics, grub screws in door handles, precision instruments |
| 2mm | M2.5 | Grub screws, small set screws, model-making, electronics enclosures |
| 2.5mm | M3 | Small machine screws, laptop internals, RC models, 3D printers |
| 3mm | M4 | Drawer handles, small furniture bolts, light switch blanking plates |
| 4mm | M5 | IKEA furniture (most common size), bicycle bolts, cable clamps |
| 5mm | M6 | Furniture bolts (heavier items), bicycle components, gym equipment, IKEA beds |
| 6mm | M8 | Structural furniture bolts, automotive, motorcycle bolts, heavy brackets |
| 8mm | M10 | Heavy-duty bolts, industrial equipment, large structural fixings |
| 10mm | M12 | Industrial machinery, large structural bolts, heavy plant equipment |
Which Allen Key for IKEA Furniture?
IKEA includes a small L-shaped Allen key with most flat-pack products. The size varies by product, but the most common sizes are:
4mm — The most common IKEA Allen key size. Used for most shelving units (KALLAX, BILLY), desks (MALM, MICKE), and TV units (BESTA). This is the one that comes in the hardware bag with most IKEA products.
5mm — Used for bed frames (MALM, HEMNES), larger wardrobes (PAX), and heavier furniture where the bolts need to be larger for structural strength.
3mm — Occasionally used for smaller items like drawer handles, shelf pegs, and some lighting fixtures.
If you’ve lost the IKEA Allen key, a 4mm or 5mm replacement from any hardware shop will work. Better yet, use a hex bit in a drill/driver — it’s much faster than hand-tightening dozens of bolts.
Allen Keys vs Hex Bits — Which to Use?
L-shaped Allen keys are fine for occasional furniture assembly, but if you’re driving hex socket screws regularly, hex screwdriver bits are much faster and more comfortable. Hex bits fit into any 1/4″ hex driver, drill/driver, or impact driver. They come in the same metric sizes (2mm–10mm) and work with the same screws.
For power driving, use a ball-end hex bit — the ball-shaped tip allows you to drive at an angle (up to about 25°), which is extremely useful in tight spaces where you can’t approach the bolt head straight on.
Metric vs Imperial — How to Tell
If an Allen key doesn’t quite fit — it goes in but wobbles or feels too tight — you might be using the wrong measurement system. In the UK, virtually everything is metric. Imperial hex sizes (measured in fractions of an inch) are standard in the US and are found in American imports, some Harley-Davidson motorcycles, and older British machinery.
| Metric Size | Closest Imperial Size | Difference |
|---|---|---|
| 4mm | 5/32″ (3.97mm) | 0.03mm — very close, but not identical |
| 5mm | 3/16″ (4.76mm) | 0.24mm — noticeably different, will strip |
| 6mm | 15/64″ (5.95mm) | 0.05mm — close enough to seem OK but risks damage |
| 8mm | 5/16″ (7.94mm) | 0.06mm — close but not interchangeable |
Never force a metric Allen key into an imperial bolt or vice versa. Even a fraction of a millimetre difference will round off the bolt head, making it extremely difficult to remove later.
Recommended Allen Key Sets
Wera 950/9 Hex-Plus L-Key Set (Metric)
Wera’s Hex-Plus Allen keys feature a unique profile that makes contact with the flat faces of the bolt rather than the corners. This reduces wear on both the key and the bolt, and allows up to 20% more torque before the bolt rounds. The set covers 1.5mm–10mm in a compact clip holder. These are the best-in-class L-key set for anyone who uses Allen keys regularly.
Wera Hex Bit Set for Drills & Impact Drivers
If you prefer using hex bits in a power driver, Wera’s hex bit range covers 2mm–8mm in both standard and ball-end versions. The Impaktor-rated bits are suitable for impact drivers. These are dramatically faster than L-keys for furniture assembly and repetitive bolt-driving tasks.
For a complete overview of every screw type and the matching bit, see our complete screw-to-bit reference table.