Metric spanners are sized in millimetres (like 10mm or 13mm), while AF (“Across Flats”) spanners are sized in imperial fractions of an inch (like 1/2″ or 9/16″). They measure the same thing — the distance across a bolt head’s flat sides — but in different unit systems, so the two rarely swap in cleanly.
What does “AF” actually stand for?
AF stands for “Across Flats.” It describes how the spanner size is measured — the distance from one flat face of a hex bolt or nut to the opposite flat face, not the diameter of the bolt shaft itself. AF sizes are given in imperial fractions of an inch, such as 7/16″ or 5/8″.
Metric spanners measure the same distance — across the flats — but state it in millimetres instead. A “metric” spanner and an “AF” spanner are measuring the identical feature of a fastener; they’re just using two different rulers to do it.
Why don’t metric and AF sizes just convert directly?
They don’t line up neatly because millimetres and inches aren’t round multiples of each other, and fastener standards were developed independently on each system. A 1/2″ AF spanner is close to 13mm, but not identical to it, and “close” is exactly the problem with a fastener.
Some sizes land near enough that a spanner from the wrong system will appear to fit — it slips onto the nut and even seems to grip. That near-fit is worse than an obvious mismatch, because it’s easy to miss until the spanner slips under load.
- A slightly undersized spanner rounds off the corners of the bolt head over repeated use.
- A slightly oversized spanner can slip mid-turn, especially when breaking a tight fastener loose.
- Rounded bolt heads are harder to remove with any tool afterward, sometimes requiring specialty extraction methods.
A few sizes are close enough that they’re often used interchangeably in practice by people who know the specific pairing well, but relying on “it’s basically the same” as a general rule is how corners get rounded.
How do I know which system a fastener uses?
There’s no visual signal on the bolt head itself — a hex head looks the same whether it was made to a metric or imperial standard. The system is determined by where the part came from and what it was designed to.
- Vehicles, appliances and equipment sourced from regions on the metric system typically use metric fasteners throughout.
- Older machinery, some plumbing fittings, and equipment built to historical imperial standards often use AF (or the related Whitworth/BSF sizing) fasteners.
- Mixed-origin equipment — a machine assembled with parts from multiple suppliers — can genuinely use both systems on different fasteners, which is where confusion happens most.
When in doubt, the fastener’s own documentation, a parts diagram, or the equipment’s manual is the reliable source — not a guess based on how a bolt head looks.
What happens if I use the wrong spanner anyway?
Using a spanner that’s slightly the wrong size for a fastener risks rounding the bolt head’s corners, slipping off under load, or both. A rounded fastener becomes harder to grip with any tool afterward and may need to be drilled out or otherwise removed by more invasive methods.
A spanner that slips suddenly under force can also send knuckles into whatever is nearby — a chassis edge, an adjacent tool, a sharp casting. Working in a confined space with a fastener that’s borderline sized is a common way people scrape or cut a hand, not because the spanner failed, but because it let go without warning.
Eye protection is worth having on hand too when working under a vehicle or piece of equipment where rust, old grease or paint flakes can drop as a stubborn fastener finally breaks free.
How can I tell metric and AF spanners apart in a mixed toolbox?
The size is stamped on the spanner itself, but the format differs: metric spanners show a plain number like 10 or 13 (understood to mean millimetres), while AF spanners show a fraction like 1/2 or 9/16, sometimes followed by “AF” explicitly.
In a toolbox where both sets have mixed together over the years, that stamped marking is the only reliable way to sort them — spanners don’t otherwise look different from each other, and a “13” laid next to a “1/2” gives no visual clue that they’re different systems.
- Keep metric and AF sets on separate rails or in separate trays, so a hand doesn’t reach for the nearest close-enough size out of habit.
- Label the tray or roll, especially if the set gets used by more than one person.
- When buying a new set, note which system it is on the packaging — sets are rarely mixed within one product, but it’s easy to end up with two AF sets and no metric ones by accident.
Do I need both a metric and an AF set?
Whether both are worth owning depends on what equipment gets worked on. Anyone maintaining only late-model, metric-designed equipment may never need an AF set at all; anyone with a mix of older machinery, imported parts and modern gear likely will end up needing both eventually.
When comparing spanner sets to buy, the meaningful differences are less about metric-versus-AF and more about:
- Size range covered — whether the set includes the specific sizes the equipment actually uses, rather than just “a lot of sizes.”
- Open-end vs combination vs ratcheting — a combination spanner has an open end and a ringed end of the same size on one tool, useful when access is tight on one side.
- Material and finish — chrome vanadium is common and durable for general use; finish affects grip and corrosion resistance more than strength in most home-workshop use.
- Drop-forged construction versus cast — forged tools generally hold up better to repeated torque without deforming, which matters more for stubborn or rusted fasteners than for light assembly work.
None of these specifications replace checking that the actual sizes in the set match the fasteners on the job — a beautifully made spanner in the wrong size is still the wrong spanner.
Are there other measuring systems besides metric and AF?
Yes — Whitworth (BSW) and British Standard Fine (BSF) are older imperial standards sized differently again from AF, sometimes appearing on antique machinery, vintage vehicles and some plumbing fittings. A spanner marked with a Whitworth size won’t match an AF-marked fastener of a similar apparent size either.
For anyone restoring older equipment, it’s worth confirming which of these older standards is in play before assuming “imperial” automatically means AF — the two aren’t the same thing, and a close-but-wrong Whitworth spanner has the same rounding risk as a mismatched metric/AF pairing.
What’s the safest way to check a size before starting a job?
The most reliable check is fitting the spanner to the fastener by eye and feel before applying force: a correct-fit spanner sits flush on all flats with no visible gap and no rocking. If it needs to be forced on, or it wobbles once seated, it’s the wrong size.
A digital or dial caliper, covered in more depth in the site’s measuring and layout section, can confirm a bolt’s across-flats measurement precisely when the size isn’t obvious or isn’t marked anywhere accessible. This is a useful check before ordering replacement fasteners for unfamiliar equipment, rather than guessing from a nearby spanner that “looks about right.”
For anyone building out a first set of spanners or deciding between metric, AF, or both, the site’s buying tools guide covers how to weigh set size, material and completeness against the equipment actually being worked on. The broader tool guide hub also covers how hand tools like these are specified and compared more generally.
Spanners sit alongside other gripping and turning tools that often get confused with each other — the piece on pliers vs pincers vs nippers covers a similar kind of “these look alike but aren’t interchangeable” mix-up. For fastener work that also involves screws rather than bolts, what a screwdriver’s tip type actually changes is a useful companion read. Readers comparing tool categories more broadly can browse the hand tools archive or the comparisons archive for related write-ups, and the buying guides archive for more on choosing sets and kits rather than single tools.
Find Tool publishes general information, not professional advice. Power tools, mains electricity and workshop machinery are genuinely dangerous, and the safe settings, guards and procedures differ between models — always follow the manual and the markings on the tool in front of you rather than a general guide. Never remove or defeat a guard or safety feature. Anything involving fixed wiring, gas or structural work belongs to a qualified tradesperson. Wear the eye, ear and respiratory protection the task calls for.
Related reading
What Is a Bench Vice’s Jaw Width Telling You?, What Is a Pipe Wrench, and Why That Shape?.