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Metric or imperial socket set

Updated: October 2026

A 19 mm socket may slide over a 3/4-inch nut, but that does not make the two sizes interchangeable. When choosing a metric or imperial socket set, use the system in which the fastener was made and reject any socket that rocks on the flats. This page explains metric, AF/SAE and Whitworth fittings, then identifies which options in the [socket set comparison](/hub/tool-accessories/socket-set) explicitly include inch sizes. The practical UK answer is usually metric for a recent car, AF for older or American equipment, and Whitworth only where the machinery actually uses Whitworth heads. A qualifying purchase can earn a commission at no extra cost to you.

A 19 mm socket may slide over a 3/4-inch nut, but that does not make the two sizes interchangeable.

Our top picks

Bahco Bahco SL25 25-piece

Best overall

Bahco Bahco SL25 25-piece
9.0/10about £21.59
View offer
Amazon Basics Amazon Basics 201-piece

Alternative

Amazon Basics Amazon Basics 201-piece
8.3/10about £70.07
View offer
EGOFINE EGOFINE 46-piece 1/4 inch

Best value

EGOFINE EGOFINE 46-piece 1/4 inch
7.6/10about £16.99
View offer
On this page
  1. 1.Three systems, not two
  2. 2.What a UK car is likely to use
  3. 3.What this batch actually contains
  4. 4.Near sizes that still round the nut

Three systems, not two

Metric sockets are marked in millimetres. They dominate modern European and Japanese vehicle work and most current household equipment. A run such as 8, 10, 13, 17 and 19 mm is a set of exact sizes, not a conversion ladder for inch nuts. AF means across flats. SAE is the retail name for inch sizes such as 1/2, 9/16 and 3/4 inch. An AF/SAE set can be necessary for American equipment and older vehicles, and it does not replace metric sockets on newer machinery. Whitworth is a separate historic British system: the marking follows the bolt thread, not an AF size. BSW and BSF heads remain on some classic British vehicles. None of the listings in this batch specifies Whitworth. Drive size is a separate choice, covered in the socket set buying guide.

What a UK car is likely to use

Halfords’ buyer guidance uses the early 1990s as a broad clue: modern cars are generally metric, while imperial tools may appear on UK cars from that earlier period. It is a starting point, not a guarantee. A restored classic can mix original Whitworth or AF heads with later metric replacements, and an American component can add SAE sizes to a newer car. For a recent mainstream UK car, begin with metric sockets and the workshop information. Add AF only when the fastener or the manual says so. On a classic, identify AF or Whitworth before you buy. Do not diagnose the system by forcing tighter sockets onto a corroded head. Wheel and impact limits are in the socket set for car repairs.
Bahco Bahco SL25 25-piece

Testsieger

Bahco Bahco SL25 25-piece
9.0/10about £21.59
View offer

What this batch actually contains

Most sets here are metric. WORKPRO 39-piece, listed at £19.99, explicitly states metric and SAE. Amazon Basics 201-piece, then £70.07, also states both. Those are the clearer single-case choices for known metric and inch fasteners. Still read the itemised sizes. Bahco SL25, both EGOFINE sets, WORKPRO 34-piece, WORKPRO mini, Ginour and Mannesmann are metric in their attributes. WORKPRO 108-piece bullets also describe metric coverage. One customer review mentions SAE. A review does not upgrade the stated specification. None claims Whitworth. The best socket set shortlist names who a set suits. Prices on amazon.co.uk are a snapshot.

Near sizes that still round the nut

Three-quarters of an inch equals 19.05 mm. A 19 mm opening is close and still a different nominal size. On a stubborn nut the wrong socket can load the corners and round the head. A metric case can skip sizes too. Newer applications may use 16 and 18 mm beside 17 and 19 mm. A stored Mannesmann review describes a 1/2-inch sequence of 15, 17, 19 and 20 mm with no 18 mm. Seat the socket on clean flats and check for rocking before you load the ratchet. Choose metric for a modern UK vehicle, AF/SAE for confirmed inch fasteners, and Whitworth only for confirmed Whitworth heads.

Common questions

Does a recent UK car need AF sockets?
Usually not for routine original fasteners. A recent European or Japanese car is generally metric. Imported accessories, replacement components or an American vehicle can introduce inch sizes. Check the vehicle documentation and the actual fastener rather than buying AF purely because the car is sold in the UK.
Are SAE and AF the same idea?
For consumer socket shopping, both refer to inch-sized openings measured across the fastener flats, and retailers often use the terms together. That does not make either term equivalent to Whitworth. Read the fractional sizes listed in the case and match them to the equipment specification.
Will a Whitworth socket fit an AF nut?
Do not assume it will. Whitworth markings follow a different historic convention, and a nearby physical opening is not evidence of correct fit. Identify the fastener standard and use its matching socket, especially where age, corrosion or high tightening force increases the risk of rounding.
Can I use 19 mm on a 3/4-inch nut?
No as a general rule. Although 3/4 inch converts to 19.05 mm, 19 mm is a distinct nominal socket size. Use a 3/4-inch socket on a 3/4-inch fastener so the flats are supported as intended. A near fit can damage the corners under load.
Why do sets include both 17 mm and 19 mm?
They are separate metric sizes used by different fasteners, not alternatives for one nut. A useful set may also need 16 and 18 mm depending on the equipment. Check for an unbroken run around your known jobs rather than assuming the two familiar sizes cover everything.
Which listings here include inch sockets?
The WORKPRO 39-piece and Amazon Basics 201-piece listings explicitly state metric and SAE coverage. Other products in the batch are described as metric, and none specifies Whitworth. Listing contents can change, so confirm the current itemised sizes on amazon.co.uk before purchasing.

Related pages

How to use a socket set

Socket set

Best socket set UK: picks for drawer, bits or a case

Socket set buying guide: choose the useful sizes first

Socket set for car repairs: where a general kit stops

Sources

Public UK and standards sources named for drive size, metric and AF, impact sockets, and satisfactory quality. amazon.co.uk prices, stars, and review counts are the figures stored for these ten listings. Prices move.

  1. RS, socket sets guide (retrieved October 2026)
  2. Toolden, choosing a socket set (retrieved October 2026)
  3. Auto Express, socket set sizes for a workshop (retrieved October 2026)
  4. ISO 1174-1:2011, assembly tools for screws and nuts, driving squares (retrieved October 2026)
  5. ISO 3315:2018, assembly tools, driving parts for hand-operated square-drive socket wrenches (retrieved October 2026)
  6. Chicago Pneumatic UK, which socket to use on an impact wrench (retrieved October 2026)
  7. Consumer Rights Act 2015, section 9 (retrieved October 2026)
  8. Halfords, imperial vs metric tools (retrieved October 2026)