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Metric or imperial spanners: which system fits the nut

Updated: 22 October 2026

A 13 mm spanner almost drops onto a 1/2 inch nut, and that is how the corners get rounded. Metric or imperial spanners is the choice between across-flats systems, not between brands, and a near size is not a fit. This page separates metric, AF, Whitworth and BA so you can match the fastener in front of you. The [spanner comparison](/hub/hand-tools/spanner) is a metric-labelled amazon.co.uk shelf, which is part of the point. Peter F. used UK maintenance guides and size tables, plus the measurement lines on these listings. No nut was gauged here, and this page does not print a full conversion chart. A purchase through our links may earn a commission at no extra cost to you.

A 13 mm spanner almost drops onto a 1/2 inch nut, and that is how the corners get rounded. Metric or imperial spanners is the choice between across-flats systems, not between brands, and a near size is not a fit.

Our top picks

Stanley Stanley MaxSteel 200 mm adjustable

Best overall

Stanley Stanley MaxSteel 200 mm adjustable
6.9/10about £7.99
View offer
Hilka Hilka 11-piece metric combination set

Rank 2

Hilka Hilka 11-piece metric combination set
7.3/10about £8.68
View offer
SOL SOL 6-piece double-ended metric set

Best value

SOL SOL 6-piece double-ended metric set
6.2/10about £4.79
View offer
On this page
  1. 1.Metric for a modern UK car, bike, or flat-pack
  2. 2.AF is inches across the flats, not a thread size
  3. 3.Whitworth, BSF, and BA are separate
  4. 4.What to buy first

Metric for a modern UK car, bike, or flat-pack

A metric mark is millimetres across the flats, not the thread diameter. Modern UK cars, bikes and flat-pack furniture are that system. Hagerty puts the shift after the move towards ISO metric in the 1970s. Hilka lists 6, 8, 10, 11, 12, 13, 14, 15, 17, 18 and 19 mm. SOL's open pairs run from 6 mm to 17 mm. A metric scale on an adjustable, such as Navegando's 6–19 mm bullets, is still one moving jaw. If the jaw rocks, stop. Moving jaw or ring is on adjustable or combination.

AF is inches across the flats, not a thread size

AF means across the flats, not the thread diameter. A 1/2 inch AF head is half an inch flat to flat. Older British equipment and American fasteners use it. A modern French or German car usually does not. EICAC and the IME table give 1/2 inch AF as 12.70 mm. 13 mm is 13.00 mm, a gap of 0.30 mm. Hagerty's pair is the same: close, not exact. Do not cross them. This shelf has no AF set. Stanley and the Barpre print metric and SAE as scales on one worm, and Stanley's own jaw fields already disagree.
Stanley Stanley MaxSteel 200 mm adjustable

Testsieger

Stanley Stanley MaxSteel 200 mm adjustable
6.9/10about £7.99
View offer

Whitworth, BSF, and BA are separate

1/2 W is not 1/2 inch AF. Whitworth and BSF mark the bolt diameter. Wheels Alive puts a 1/2 inch Whitworth head at about 0.919 inches across the flats, so an AF half-inch spanner will not fit, and a 1/2 W spanner swallows an AF nut. Read the stamp. BA is a small numbered series for older electrics and classic wiring, not a short Whitworth set. A 0–6 BA range is a classic-car rule of thumb, and no BA tool is here. Classic Car Hub's rule of thumb is Whitworth or BSF plus AF before about 1971, and metric after about 1973. Those years are not a statute. BSP is a pipe thread, not a spanner size.

What to buy first

Buy metric first for a modern UK car, bike or flat-pack. Hilka if you want rings and will count the packet. It is the comparison's top pick. SOL is the best value row, open-ended metric, not AF. A Stanley marked metric and SAE is one adjustable with two scales. Buy AF only for heads you have measured. Buy Whitworth, BSF or BA only when that fastener is in front of you. 13 mm is 0.30 mm larger than 1/2 inch AF, and 1/2 W is about 0.919 inches across the flats. The shape of the tool is the spanner buying guide.

Common questions

Do I need metric or imperial spanners for a modern UK car?
Start with metric. Modern UK cars, bikes and flat-pack furniture use millimetre sizes across the flats. Imperial AF, Whitworth and BA belong to older British or American fasteners, not to a recent service job. This amazon.co.uk set's combination and open-ended tools are metric. An imperial set is a second purchase after you have found an imperial head, not a substitute for 10 mm, 13 mm and 17 mm.
Will 13 mm fit a 1/2 inch nut?
No. Half an inch AF is 12.70 mm across the flats, and 13 mm is 13.00 mm, a difference of 0.30 mm. The spanner feels close and sits loose enough to round a tight nut. Hagerty and the size tables agree on that pair. Do not pull a near size because a chart prints them on neighbouring rows.
What is AF?
AF means across the flats. The number is the width of the nut, in inches, from one flat to the opposite flat. It is not the bolt's thread diameter. A 1/2 inch AF head is half an inch across the flats. A Whitworth stamp of 1/2 inch is a different and larger head. If the tool says AF, measure or match the head, not the thread you can see on the bolt.
What is a Whitworth spanner size actually measuring?
It is measuring the bolt diameter in the Whitworth series, not the head. Wheels Alive puts a 1/2 inch Whitworth head at about 0.919 inches across the flats, so a 1/2 inch AF spanner will not fit it. BSF is the same kind of thread-based marking, with its own widths. Read the stamp before you reach for the nearest AF or metric size.
Do I still need BA spanners?
Only if you are working on older electrical parts or classic-car wiring that uses the BA series. BA is a small numbered system of its own. A 0–6 BA range is a common rule of thumb for that work, not a legal requirement, and no BA tool is in this comparison. Do not buy BA for a modern bike or a flat-pack bolt.
Does a metric-and-SAE adjustable replace an AF set?
No. Stanley's listings and the Barpre stubby print metric and SAE as scales on one moving jaw. That helps you set an opening. It does not give you a 1/2 inch AF spanner, a 9/16 inch AF spanner, or any Whitworth size. The jaw can still slip, and the Stanley fields already disagree about the opening. An AF set is a set of fixed sizes.

Related pages

Best spanner

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Adjustable vs combination spanner: when each one wins

Spanner buying guide: length versus the jaw opening

How to use a spanner: seat, pull, and when to stop

Sources

Public UK and standards sources listed below, as used in the digest. Shelf prices and star lines are the amazon.co.uk figures dated 19 October 2026, not a fresh check on 22 October 2026.

  1. HSE, PUWER overview (retrieved 19 October 2026)
  2. University of Essex, Using hand tools safely (retrieved 19 October 2026)
  3. ISO 1711-1:2019, hardness and test torque (retrieved 19 October 2026)
  4. ISO 3318:2016, maximum head widths (retrieved 19 October 2026)
  5. RS, adjustable spanners guide (retrieved 19 October 2026)
  6. RS PRO 541-6573 adjustable wrench (retrieved 19 October 2026)
  7. GOV.UK trade tariff, heading 8204 (retrieved 19 October 2026)
  8. Trading Standards, Consumer Rights Act goods guidance (retrieved 19 October 2026)