Allen Wrench Sizes: Metric & SAE Hex Key Conversion Chart
Metric to SAE Hex Key Calculator
- Closest SAE hex key
- 3/16"
- 0.188"
- Decimal inches
- 0.197"
- mm ÷ 25.4
- Difference
- +0.009"
- metric − SAE
A 5mm hex key is closest to 3/16" (0.188"), a difference of +0.009", rated Use with Caution. Hex recesses round far sooner than bolt heads, so treat anything below Highly Interchangeable as a last resort.
Quick sizes
Hex recesses round far sooner than bolt heads, so this calculator applies the tight wrench band rather than the looser socket band. Almost every pairing lands short of a free swap.
Metric Allen keys run from 1.5mm to 19mm and SAE hex keys from 1/16" to 1/2", and the two are not interchangeable. That last point separates hex keys from every other tool on this site. A socket wraps a bolt head that stands proud of the work, gripping six external faces with plenty of metal behind them; a hex key twists inside a small recess sunk into the head itself, where the engaged surface is a fraction of the area and there is nothing behind it. The same 0.009" mismatch that a 6-point socket tolerates comfortably will start rounding a hex recess on the first hard pull. Where the socket conversion chart can offer genuinely safe substitutions, this page mostly documents why you should not attempt them.
Metric Allen Key Sizes
Thirteen metric hex key sizes, each with its decimal inch value, the closest SAE key, the gap between them, and what the size is normally used for.
| Metric key | Decimal (in) | Nearest SAE | Difference | Typical use |
|---|---|---|---|---|
| 1.5mm | 0.0591 | 1/16" | −0.0034 | Small grub screws, RC and hobby hardware. |
| 2mm | 0.0787 | 5/64" | +0.0006 | Electronics enclosures and instrument knobs. |
| 2.5mm | 0.0984 | 3/32" | +0.0047 | Bicycle accessories and cabinet hardware. |
| 3mm | 0.1181 | 1/8" | −0.0069 | Bicycle derailleur and brake fixings, flat-pack furniture. |
| 4mm | 0.1575 | 5/32" | +0.0012 | Bicycle stems and seat posts, furniture assembly. |
| 5mm | 0.1969 | 3/16" | +0.0094 | Bicycle cranks, appliance panels, larger furniture bolts. |
| 6mm | 0.2362 | 1/4" | −0.0138 | Bottom brackets, automotive brake caliper guide bolts. |
| 8mm | 0.3150 | 5/16" | +0.0025 | Automotive drain plugs and suspension fixings. |
| 10mm | 0.3937 | 3/8" | +0.0187 | Heavy machinery and large socket head cap screws. |
| 12mm | 0.4724 | 1/2" | −0.0276 | Industrial machine hardware. |
| 14mm | 0.5512 | 1/2" | +0.0512 | Large industrial socket head cap screws. |
| 17mm | 0.6693 | 1/2" | +0.1693 | Heavy plant and structural machine screws. |
| 19mm | 0.7480 | 1/2" | +0.2480 | The largest common metric hex key size. |
Read the Difference column against a much tighter standard than you would for sockets. Nothing in that column is comfortably inside ±0.004", which is the band a hex recess really wants. The closest pairings, 5mm to 3/16" and 10mm to 3/8", both sit near 0.009" and will feel loose in the recess. The Typical use column is the more useful half of this table: if you know the job, you know the size, and you never have to substitute at all.
SAE Hex Key Sizes
| SAE key | Decimal (in) | Millimetres | Typical use |
|---|---|---|---|
| 1/16" | 0.0625 | 1.59mm | Miniature set screws. |
| 5/64" | 0.0781 | 1.98mm | N/A |
| 3/32" | 0.0938 | 2.38mm | Small set screws on pulleys and collars. |
| 7/64" | 0.1094 | 2.78mm | N/A |
| 1/8" | 0.1250 | 3.17mm | The most common SAE set screw size. |
| 9/64" | 0.1406 | 3.57mm | N/A |
| 5/32" | 0.1563 | 3.97mm | Furniture and light machinery. |
| 3/16" | 0.1875 | 4.76mm | Machine guards and light structural fixings. |
| 7/32" | 0.2188 | 5.56mm | N/A |
| 1/4" | 0.2500 | 6.35mm | Heavy set screws and shaft collars. |
| 5/16" | 0.3125 | 7.94mm | Industrial socket head cap screws. |
| 3/8" | 0.3750 | 9.52mm | Large industrial fixings. |
| 1/2" | 0.5000 | 12.70mm | The largest common SAE hex key size. |
SAE hex keys survive mainly in set screws, pulleys, shaft collars, door handles and machine hardware built to inch standards, rather than in vehicle assembly. 1/8" is the size you meet most, and it sits at 3.18mm, close enough to 3mm to be tempting and 0.18mm away, which in a set screw recess is plenty to strip it. The millimetre column here is the exact conversion; converting arbitrary values in either direction is what the millimetre to inch chart is for, and the fraction to decimal reference covers the fractions on their own.
Allen Key vs Hex Socket
The tip is the same hexagon in both tools. Everything else about how they behave is different.
| Property | Allen key | Hex bit socket |
|---|---|---|
| Drive | Hardened hex bar, turned by hand | Socket on a 1/4" or 3/8" ratchet |
| Torque available | Limited by the short arm and your grip | Full ratchet leverage |
| Access | Reaches into recesses a socket cannot | Needs clearance for the socket body |
| Engagement depth | Full length of the key can enter the recess | Limited by the bit length |
| Best use | Assembly, low torque, awkward angles | Breaking loose and final torque |
| Failure mode | Key twists or rounds the recess | Bit shears, recess usually survives |
The row that decides most jobs is the last one. When an Allen key fails it twists, and the energy goes into rounding the recess you were trying to turn; when a hex bit socket fails the bit usually shears first and the fastener survives to be tried again. That asymmetry is why torque-critical hex fasteners belong on a ratchet with a proper hex bit socket, sized from the impact and drive socket range, rather than on a folding key from the bottom of the toolbox.
Identifying and Recovering a Stripped Hex Bolt
Spotting it early
A healthy recess takes the key with a definite stop and no rotational play. A failing one lets the key rock slightly before it bites, and the corners of the recess look softened rather than sharp when you shine a light into them. The moment you feel that rock, stop pulling. Every further turn removes more of the corner. Clean the recess out first; packed grit is a surprisingly common cause of a key that will not seat fully and then rounds under load.
Once it is rounded
Try the next size up from the other standard, a 5mm key driven into a rounded 3/16" recess, or a Torx bit one size over, hammered in to bite fresh metal. If that fails, use a hex bolt extractor with reverse-spiral flutes, which grips harder the more you turn anticlockwise. Last resorts are drilling the head off entirely or welding a nut to the top, after which the drill bit size chart and tap drill sizes cover cleaning up and re-cutting the thread.
The prevention is duller than the cure but far cheaper. Seat the key to its full depth before applying any load, use the long arm for reach and the short arm for torque rather than the other way round, and replace keys with visibly rounded tips. A worn key destroys every fastener it touches. Above all, match the standard: check the vehicle or equipment origin, and if it was built anywhere outside older US industry it is metric. The two-way conversion calculator will confirm how far apart any specific pair actually sits before you commit a tool to a fastener.
Frequently Asked Questions
The questions that come up most around hex keys and rounded recesses.
Are metric and SAE Allen keys interchangeable?
No, and hex keys are far less forgiving than sockets. A hex recess is a small feature, so the same absolute error is a much larger proportion of the engaged surface. A mismatch a 6-point socket would shrug off will round a hex socket head almost immediately. The closest pairing on the whole chart is 5mm to 3/16", which is 0.0091" apart and still not a free swap. Treat metric and SAE hex keys as two separate systems.
What is the closest SAE size to a 5mm Allen key?
5mm is closest to 3/16", but the two are not equivalent. 5mm is 0.1969" and 3/16" is 0.1875", a gap of 0.0094 inches. In a bolt head that would be borderline; in a hex recess it is enough that the key will feel loose and start to round the corners under load. If a 5mm key is what the fastener needs, use a 5mm key.
What size Allen key do bicycles use?
Almost all modern bicycles are metric, and four sizes cover most of the bike: 3mm for derailleur and brake adjustment, 4mm for stems and seat post clamps, 5mm for cranks and larger clamp bolts, and 6mm for bottom brackets and some disc brake mounts. A 2.5mm key covers accessory hardware. SAE hex keys have no place on a modern bicycle.
How do I remove a stripped Allen bolt?
Start by trying the next size up in the other standard. A slightly larger key hammered into a rounded metric recess sometimes bites enough to break the fastener loose. If that fails, a hex bolt extractor with reverse-spiral flutes cuts into the damaged recess as it turns anticlockwise. Beyond that the options are drilling the head off or welding a nut to it. Prevention is cheaper: seat the key fully before applying any load.
What is the difference between an Allen key and a hex socket?
The tip geometry is identical. Both are hexagonal bars sized across the flats. The difference is the drive. An Allen key is a bent hardened bar turned by hand, limited to the torque your grip can produce; a hex socket is that same tip on a ratchet drive, which delivers far more torque and far better control. For anything with a torque specification, use the socket.
More Conversion Tools
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.