What size back box do I need?
Updated: October 2026
You are replacing a single socket or a switch and the plate will not sit flat. What size back box you need is the wall, then the depth the plate needs, then 1-gang or 2-gang. Miss one of those and the screws miss, the terminals crush, or the plate rocks. This page is that order for a UK accessory. The [Wall box comparison](/hub/electrical-equipment/wall-box) holds the three amazon.co.uk listings still in this set, with prices in pounds. The Products Compared editorial team read those listings on 21 October 2026 and did not lab-test the boxes.
You are replacing a single socket or a switch and the plate will not sit flat. What size back box you need is the wall, then the depth the plate needs, then 1-gang or 2-gang.
You are replacing a single socket or a switch and the plate will not sit flat. What size back box you need is the wall, then the depth the plate needs, then 1-gang or 2-gang. Miss one of those and the screws miss, the terminals crush, or the plate rocks. This page is that order for a UK accessory. The Wall box comparison holds the three amazon.co.uk listings still in this set, with prices in pounds. The Products Compared editorial team read those listings on 21 October 2026 and did not lab-test the boxes.
Our top picks
| Badge | Product | Mounting | Depth | Gang | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Evoges Evoges EDB1G35 dry lining box | Dry lining, spring lugs | 35 mm | 1-gang | 9.6 | about £8.99 | View offer |
| Best value |  Lyvia Lyvia 25 mm metal wall box | Flush, screw-in | 25 mm (bullets); listing also says 7 x 7 x 7 cm | 1-gang | 9.4 | about £3.68 | View offer |
 LEENUE LEENUE 32 mm surface wall box | Surface, screw-in | 32 mm | 1-gang | 9.4 | about £6.99 | View offer |
On this page
Start with the wall, not the brand
The first cut is what the wall is made of, because a box that is the right depth still fails if it cannot grip. A galvanised flush box is chased into brick or block and screwed into the masonry. A dry lining box uses spring lugs in hollow plasterboard, and it has nothing to bite on if you push it into a solid chase. A surface pattress screws to the face of the wall and stands proud by its full depth, which is the route when a lease, concrete, or a finished surface means you must not cut. BEAMA’s selection bulletin, summarising BS 7671 Regulation 530.4.2, expects a socket-outlet or a plate switch to sit on a box to BS 4662 or BS 5733, with safety performance in the BS EN 60670 series. If you are still choosing between a chase and a stud wall, use metal or dry lining before you look at depth.
Then match depth and gang
Once the mounting is settled, match the plate. This comparison is 1-gang only. A 2-gang plate is a wider box, and Deta cites centres of about 60.3 mm for a single and about 120.6 mm for a double, so a single metal box will not take a double socket however deep it is. Socket Store’s guide, which is a retailer’s rule of thumb and not a British Standard, puts ordinary switches around 16 mm, sockets and dimmers around 25 mm, many USB sockets around 35 mm, and shaver switches around 47 mm. The plate maker can override the table. Hamilton’s Hartland G2 screwless USB listing states a minimum 35 mm box for that one plate, while some integrated USB sockets are sold for 25 mm. The full bands, including the 32 mm surface case, are on 25 mm or 35 mm.
Read the listing before you trust the title
The title often says “back box” and then contradicts the technical details, so read both before you order. A metal box needs a sound earth terminal. The Lyvia bullets state an earth fixing connection, and one UK review says that brass point was thin and sheared on first use. The LEENUE specification does not state an earth terminal, and one UK review says the samples had none. Only the Evoges bullets say UKCA. The Lyvia bullets say 25 mm and the technical details also print 7 × 7 × 7 cm, so do not chase a 70 mm hole from the cube.
What this search often gets wrong
A search for “wall box” on amazon.co.uk also returns products that will not take a British socket plate. Key safes, key cabinets, cable tidies for extension leads, a US in-wall media enclosure, and an IP65 rubber-membrane junction box are different objects. Thread-forming screws into a European box are not the UK method for a BS 1363 socket. If the wall may be a fire compartment, a flue, or asbestos, stop before you cut. Once the box in your hand matches the wall and the plate, the sequence is on how to fit a back box.
Common questions
What size back box do I need for a normal socket?
For a standard UK single socket, the usual rule of thumb is a 25 mm box. Keep 16 mm for a slim switch, and move to 35 mm or more when the plate maker asks for it. Socket Store and Flameport publish those bands as guidance, not as a British Standard. A 2-gang socket needs a 2-gang box. Every listing in this set is 1-gang, so a double plate is outside the table even when the depth looks right.
Metal, dry lining, or surface: which question comes first?
Start with the wall. Chase a galvanised flush box into masonry, clip a dry lining box into hollow plasterboard the lugs can grip, and use a surface pattress when you must not cut. Depth comes after that. A 35 mm dry lining box does not become a brick box because 35 mm is a common socket depth, and a 25 mm metal box does not clip into a stud wall.
Does a USB socket change the depth?
Often, and not always. Many USB sockets need 35 mm because the charger sits behind the plate, and Hamilton’s Hartland G2 screwless USB states a 35 mm minimum. Some integrated USB sockets, including MK Logic Plus in that maker’s 2021 brochure, are specified for 25 mm. Read the plate, then use the depth page if you are choosing between those two figures.
How do I tell a real accessory box from a key safe or a junction box?
An accessory box carries a socket, switch, or data plate on UK fixing centres, with a cable entry and, on metal, an earth point. A key safe has a keypad or a shackle and no faceplate lugs. A junction box joins cables and is not a pattress for a BS 1363 plate. If the title says wall box and the pictures show a safe door or rubber membranes, it is the wrong product for this job.
What if the listing gives two different sizes?
Keep both figures and do not average them. The Lyvia bullets say 25 mm and the technical details also say 7 × 7 × 7 cm, which cannot both be the depth. Measure the box when it arrives, before you chase. The Evoges title says a pack of 5 and the item count says 1, which is a quantity conflict rather than a depth. Do not cut the wall to the more convenient number.
Related pages
Sources
Public UK sources are listed below. Shelf prices and star lines are the amazon.co.uk figures checked on 21 October 2026.
- BS 4662: boxes for flush mounting of electrical accessories (retrieved 21 October 2026)
- BEAMA Technical Bulletin: selection of boxes (retrieved 21 October 2026)
- Approved Document P, electrical safety (retrieved 21 October 2026)
- IET: Part P frequently asked questions (retrieved 21 October 2026)
- IET: Part P in Scotland and Northern Ireland (retrieved 21 October 2026)
- Electrical Equipment (Safety) Regulations 2016 (retrieved 21 October 2026)
- Socket Store: what is a back box (retrieved 21 October 2026)
- NHBC: earth bonding of metal back boxes (retrieved 21 October 2026)