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25mm or 35mm back box

Updated: October 2026

You already know whether the box is flush metal, dry lining, or a surface pattress, and the plate is either a plain socket or something with a USB module behind it. Choosing a 25mm or 35mm back box is the difference between terminals that sit in clear air and a plate that will not screw home. The usual bands are a check. The accessory’s own minimum is the figure that counts. This page stays on depth. The [Wall box comparison](/hub/electrical-equipment/wall-box) holds the three amazon.co.uk listings as examples of 25 mm, 32 mm, and 35 mm, with prices in pounds. If the wall type is still open, start with the [buying guide](/hub/electrical-equipment/wall-box/sub/wall-box-buying-guide).

You already know whether the box is flush metal, dry lining, or a surface pattress, and the plate is either a plain socket or something with a USB module behind it.

Our top picks

Evoges Evoges EDB1G35 dry lining box

Best overall

Evoges Evoges EDB1G35 dry lining box
9.6/10about £8.99
View offer
Lyvia Lyvia 25 mm metal wall box

Best value

Lyvia Lyvia 25 mm metal wall box
9.4/10about £3.68
View offer
LEENUE LEENUE 32 mm surface wall box

Rank 3

LEENUE LEENUE 32 mm surface wall box
9.4/10about £6.99
View offer
On this page
  1. 1.The four depths and what usually fits
  2. 2.25 mm or 35 mm depends on the plate, not on a slogan
  3. 3.Where the three boxes sit
  4. 4.Deeper is not automatically safer

The four depths and what usually fits

Trade boxes are sold in nominal depths of 16 mm, 25 mm, 35 mm, and 47 mm. Socket Store’s help page, which is a retailer’s guide rather than a British Standard, lines them up like this. A standard switch often fits 16 mm. A standard socket or a dimmer often fits 25 mm. A shaver supply unit is often 47 mm. Flameport makes the same caution in metal-box terms: 16 mm is a poor general choice, and 47 mm is there for larger conductors and bulky accessories, not as a default “deeper is safer” buy.

25 mm or 35 mm depends on the plate, not on a slogan

A plain UK socket or a simple dimmer is the 25 mm case on the Socket Store table, provided the conductors are not piled in behind a shallow plate. A USB socket often needs 35 mm because the charger module stands behind the fascia. It is not a rule that every USB socket needs 35 mm. MK’s USB brochure says Logic Plus integrated USB sockets fit a 25 mm box, while the Aspect and Edge ranges need 35 mm. Hamilton’s Hartland G2 screwless USB states a minimum 35 mm box for that one plate. If your plate is a Hartland G2, 25 mm is too shallow even though a Logic Plus USB would have been happy there.
Evoges Evoges EDB1G35 dry lining box

Testsieger

Evoges Evoges EDB1G35 dry lining box
9.6/10about £8.99
View offer

Where the three boxes sit

The Lyvia listing is the 25 mm example, a galvanised 1-gang flush box. The bullets say 25 mm. The technical details also say 7 × 7 × 7 cm. Both figures are on the amazon.co.uk page, and they cannot both be the depth, so measure the box. The LEENUE listing is 32 mm, and it is a surface pattress, 86 mm square, not a flush 35 mm box with 3 mm missing. Reviewers describe 32 mm as enough for a Cat6 keystone without a tight bend. That is a data-plate comment, not proof that 32 mm satisfies a USB socket specified at 35 mm. The Evoges EDB1G35 is the 35 mm example, and it is a dry lining box for plasterboard, not a deeper metal flush box. If you still do not know whether you should be in metal or in a lug box, that decision is on metal or dry lining.

Deeper is not automatically safer

A deeper box gives the conductors and the module more room. It also asks for a deeper chase in masonry, or more clear space behind plasterboard, and the plate screws still have to reach the lugs. Screwless plates are the usual example: they need the depth, and they also need the box set to the finished face, not to the brick before the plaster. If the wall and the depth now agree and you are ready to fit, the sequence is on how to fit a back box. If you want the row that matches a plasterboard wall rather than the millimetre, that is the best back box page.

Common questions

Is 25 mm enough for a standard double socket?
For a standard double socket, 25 mm is the usual rule of thumb, as long as the box is 2-gang and the conductors are not forced in behind a flat plate. This comparison’s boxes are all 1-gang, so the Lyvia 25 mm single will not take a double plate at any depth. If the plate is screwless or includes USB, read its minimum. Many of those leave 25 mm behind.
Do all USB sockets need 35 mm?
No. Many do, because the charger stands behind the plate, and Hamilton’s Hartland G2 states 35 mm for that screwless USB socket. MK specifies 25 mm for Logic Plus integrated USB sockets and 35 mm for Aspect and Edge. Match the plate in your hand. Do not apply 35 mm to every socket with a USB mark, and do not apply 25 mm to a plate that prints 35 mm.
When is 16 mm actually enough?
A slim switch plate with little behind it is the case Socket Store allows at about 16 mm. Flameport does not recommend 16 mm as a general box, because the next owner often fits a socket or a dimmer and finds the box short. If you might later want a dimmer or a USB plate at the same point, 16 mm is the choice you will regret. None of the three listings here is a 16 mm box.
Is 32 mm the same job as 35 mm?
Not quite. Three millimetres can be the difference between a USB module rated at 35 mm and a box that is officially short. The LEENUE box is also a surface pattress, so its 32 mm stands proud of the wall. Reviewers found 32 mm workable for a Cat6 keystone. That does not clear a flush USB plate specified at 35 mm. The Evoges box is the 35 mm listing, and it is a dry lining box.
What if the listing depth and the carton size disagree?
Do not pick the number that suits a hole you have already cut. The Lyvia page says 25 mm in the bullets and 7 × 7 × 7 cm in the technical details. Measure the metal when it arrives and chase to that, or send it back if it is not the depth the plate needs. The Evoges conflict is quantity, pack of 5 against number of items 1, not depth. Depth on that listing stays 35 mm.

Related pages

Wall box

Best back box for plasterboard, brick, or a surface

What size back box do I need for a UK socket plate?

How to fit a back box once the circuit is proved dead

Metal or dry lining back box for the wall you have

Sources

Public UK sources are listed below. Shelf prices and star lines are the amazon.co.uk figures checked on 21 October 2026.

  1. BS 4662: boxes for flush mounting of electrical accessories (retrieved 21 October 2026)
  2. BEAMA Technical Bulletin: selection of boxes (retrieved 21 October 2026)
  3. Approved Document P, electrical safety (retrieved 21 October 2026)
  4. IET: Part P frequently asked questions (retrieved 21 October 2026)
  5. IET: Part P in Scotland and Northern Ireland (retrieved 21 October 2026)
  6. Electrical Equipment (Safety) Regulations 2016 (retrieved 21 October 2026)
  7. Socket Store: what is a back box (retrieved 21 October 2026)
  8. NHBC: earth bonding of metal back boxes (retrieved 21 October 2026)