Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
A hex key or hex socket is sized by the flat-to-flat width of its tip, and that width comes from two unrelated systems: whole or half millimetres, or fractions of an inch. Several sizes in the two systems sit close enough that a key from the wrong system can slide into a fastener and feel almost right. A 5/32 inch key is only 0.03 mm from a 4 mm fastener; a 5/16 inch key is only 0.06 mm from an 8 mm fastener. That gap is smaller than a human hair. Force the wrong one and it can ride up on the corners of the fastener instead of gripping the flat sides, and either the fastener or the key can be damaged before you notice the size was wrong.
This page gives a repeatable way to tell the two systems apart: read any printed size, then measure the tip, then compare the reading against the exact millimetre-per-inch figure published by NIST — arithmetic you can check yourself, not a table borrowed from somewhere else. It also gives a fit test for the moment before you turn anything, because a size can look right and still not seat flush. What it does not give is a single master chart that tells you which sizes belong to your own set: sets vary, and a low-cost set is not always cut to the nominal size in the first place, so the reader is sent to a measurement, not a guess.
Necessary equipment
| Item | Note |
|---|---|
| Caliper | Use the caliper to measure the width across two flats. |
| Millimetre ruler optional | Use the ruler if you do not have a caliper. |
Find the printed size
Estimated under a minute
- Look for a printed size on the key or the socket.<1mReport a problem with step 1, Find the printed size
- The mark sits near the long arm on an L-shaped key.
- The mark sits on the drive end of a socket.
- Wear can remove the mark from a used tool.
- A fraction, for example 3/16, shows an inch size.
- A number with no fraction, for example 5, shows a millimetre size.
Measure the key
Estimated 2 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Measure the width across two opposite flats with the caliper.<1mReport a problem with step 1, Measure the key
- Use the ruler only if you do not have a caliper.
- Record the reading in millimetres, to two decimal places.<1mReport a problem with step 2, Measure the key
- Multiply the nearest inch fraction by 25.4 to get its exact size in millimetres.<1mReport a problem with step 3, Measure the key
- One inch equals exactly 25.4 millimetres, by the 1959 international definition.
- For example, 5/32 inch converts to 3.97 millimetres.
- Compare your reading to the nearest size in millimetres and to the nearest converted size in inches.<1mReport a problem with step 4, Measure the key
- Select the size with the smaller difference from your reading.<1mReport a problem with step 5, Measure the key
- For example, a reading of 4.75 mm sits 0.01 mm from 3/16 inch and 0.25 mm from 5 mm.
- Select the closer size, 3/16 inch.
Make sure of the fit
Estimated 1 min
DO NOT FORCE THE KEY OR SOCKET ONTO THE FASTENER.
A key from the wrong system can cause damage to the corners of the fastener.
- Put the key or socket fully onto the fastener.<1mReport a problem with step 1, Make sure of the fit
- Move the key gently from side to side.<1mReport a problem with step 2, Make sure of the fit
- Do not turn the key yet.<1mReport a problem with step 3, Make sure of the fit
- Look for full contact at all six flats, with no visible gap.<1mReport a problem with step 4, Make sure of the fit
- If a gap is visible, or the key moves freely, select the other size or the other system.<1mReport a problem with step 5, Make sure of the fit
DO NOT TURN THE FASTENER YET.
THE KEY MUST SIT FLUSH ON ALL SIX FLATS FIRST. A key that slips can injure your hand.
- When all six flats have full contact, turn the fastener.<1mReport a problem with step 6, Make sure of the fit
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- hex key
- Also called: Allen wrench, Allen key, hex wrench, L-key, hex driver
- hex socket
- Also called: Allen socket, hex bit socket, socket wrench hex bit
- imperial size
- Also called: SAE, standard size, inch size, fractional size, English measure
- metric size
- Also called: millimetre size, mm size, metric mm
- across flats
- Also called: AF, wrench size, key size, width across the flats
When to use a professional
Stop if your reading does not sit close to any size on this page. Do not guess the closer size from memory. Consult the tool manufacturer or the fastener manufacturer for the exact size.
Do not turn a fastener with a key that does not sit flush on all six flats. Select a different key or a different tool instead. Take a fastener with damaged corners to a professional. Do not force it further.
Common questions
How do I know if my Allen key set is metric or standard (SAE)?
Start with any size printed on the key: a plain number like 5 is millimetres, a fraction like 3/16 is inches, but a used key can lose its marking. The reliable check is to measure the flat-to-flat width with a caliper and compare it to the nearest millimetre size and the nearest inch size, converted to millimetres by multiplying by exactly 25.4 (NIST's published inch-to-millimetre figure). Whichever size the reading is closer to is your system. If the reading sits roughly halfway between two candidates, do not guess — test the fit on the actual fastener before you apply any force.
Is a 5 mm hex key the same as a 3/16 inch hex key?
No. 3/16 inch converts to 4.7625 mm, so a 5 mm key is about 0.24 mm larger. That is a noticeable gap, wide enough to feel as looseness if you insert the wrong one, but it is still one of the pairs people search for because both sizes are common in small fastener sets.
Which metric and imperial hex sizes are easiest to mix up?
A handful of pairs sit under a tenth of a millimetre apart, close enough that a worn or slightly undersized key can be forced in. 5/64 inch (1.98 mm) and 2 mm are 0.02 mm apart. 5/32 inch (3.97 mm) and 4 mm are 0.03 mm apart. 3/4 inch (19.05 mm) and 19 mm are 0.05 mm apart. 5/16 inch (7.94 mm) and 8 mm are 0.06 mm apart. Every figure here comes from multiplying the inch fraction by exactly 25.4, the NIST-published inch-to-millimetre equivalence, so you can run the same multiplication on any size in your own set.
What actually happens if I use the wrong size hex key or socket?
An undersized key contacts the fastener only at the six corners instead of along the flat sides. Under load it can slip suddenly, which can injure your hand, and it can also damage the corners of the fastener so that no key from either system grips it afterward. A fastener that has already been rounded off often needs a specialist extraction method rather than a bigger push on the same key.
My hex key or socket has no size marking. Now what?
Measure it. A caliper reading of the width across two opposite flats, to two decimal places, is enough to place it against the nearest millimetre size and the nearest inch size once that is converted to millimetres. A steel ruler works if you have no caliper, but it is harder to read to the precision the closest pairs need.
Are hex key sizes the same as socket wrench drive sizes?
No, and this is a separate mix-up. A socket's drive size (1/4 inch, 3/8 inch, 1/2 inch) is the square fitting that connects it to a ratchet handle. The hex size covered on this page is the size of the six-sided tip that grips the fastener, which has nothing to do with which ratchet the socket fits.
Are 'SAE', 'standard' and 'imperial' the same as 'inch' sizing?
Yes, in the context of hand tools. All four words are used, often by different manufacturers or different generations, for the fractional-inch system that is not millimetres. This page uses 'inch' throughout, but the aliases mean the same set of sizes.
Related procedures
- How do I work out what size bolt this is, and what do the lines on the head mean?
- How do I set a click type torque wrench and what setting do I leave it on when I put it away
- How to clean and store hand tools so they don't go rusty
- Use a feeler gauge to measure a clearance, and find the figure it should be
Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 1SI Units – Length
Revision history
Current version 1 · updated 2026-08-04 · next review due 2027-08-04
- v1 · 2026-08-04First draft. The only specification numbers on this page are exact arithmetic (multiply an inch fraction by 25.4) resting on one tier-1 source: NIST's published inch-to-millimetre equivalence, read directly on 2026-08-04 and confirmed by a second, independent fetch of the same page. No manufacturer or third-party size chart was read or used. Across-flats manufacturing tolerance is not verified and routes to the tool manufacturer, because no source read for this page gives a tolerance figure for a consumer hex key or socket.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.