What Size Back Box Do I Need?

A back box is the metal or plastic box fitted into the wall, which the socket or switch faceplate then screws onto. The right one depends on your wall type and the depth needed for your chosen faceplate — standard plates need less depth than flat, screwless, or slim designs.

Which back box material do I need — metal or plastic?

There are two main materials used for back boxes, and which one you need depends on the wall itself, not personal preference.

Plastic back boxes

Plastic back boxes are used in stud partition walls — a wooden stud frame finished with plasterboard. The hole is cut out of the plasterboard, and two lugs clasp onto the plasterboard tightly as the faceplate screw is tightened.

Metal back boxes

Metal back boxes are used in solid walls — stone, brick, or breeze block. The cut-out is normally chiselled out of the wall material, and the metal lugs are fixed in place, pulling the faceplate tight against the wall as the screws are tightened.

Single or double — which back box do I need?

The two most common types are single and double back boxes, which cover the majority of installations. There are also less common types, including twin singles and round back boxes, used for specific plate styles or accessories.

What depth back box do I need?

If you're replacing an existing back box like-for-like, matching the current depth is usually the simplest option — unless you're also changing to a different plate style. If you're fitting new back boxes, it's worth going slightly deeper than the minimum: it makes installation considerably easier, and your electrician will thank you for it.

Standard sockets and switches

Recommended depths for standard faceplates:

Switches: 16mm
Sockets & dimmers: 25mm
USB sockets: 35mm
Cooker switches: 32mm
Shaver switches: 47mm

Note that a cooker switch technically only needs 32mm, but we'd still recommend fitting a deeper box where possible — it makes the installation itself considerably easier, even though 32mm is sufficient.

Flat, screwless, and slim sockets and switches

Flat, screwless, and slim plates sit closer to the wall and need a deeper back box as a result:

Switches: 25mm
Sockets & dimmers: 35mm
USB sockets, shaver & cooker switches: 47mm

Please note: for flat plate installations, we'd recommend sitting metal back boxes slightly further back in the wall. This allows the flat plate to sit tight against the wall — if the box isn't set back enough, the plate can sit slightly proud of the wall, which spoils the clean look flat plates are chosen for.

What if I can't fit a back box into the wall?

Where a back box can't or shouldn't be recessed into the wall, a surface-mounted back box — also called a pattress box — is the alternative. These are often available in a finish that matches your sockets and switches, but they aren't a universal size, so it's worth checking that your chosen plate will sit well on top before buying, particularly with larger plates.

Because most pattress boxes have a decorative finish, the cable entry point is usually in the back of the box rather than the sides, which is where standard back boxes take their cable.

Fitting a back box safely

A competent DIYer can fit a back box, but take care when cutting or drilling into a wall — watch for hidden cables and pipes, wear appropriate safety equipment, and consult a qualified electrician if you're in any doubt.

Frequently Asked Questions

What is a back box?

A back box is the metal or plastic box recessed into a wall, onto which a socket or switch faceplate is fixed. Plastic back boxes are used in stud partition walls; metal back boxes are used in solid walls such as brick, block, or stone.

What depth back box do I need for a standard socket?

A standard socket or dimmer needs a 25mm back box. Standard switches need 16mm, USB sockets need 35mm, cooker switches need 32mm, and shaver switches need 47mm.

Do flat and screwless plates need a different back box depth?

Yes. Flat, screwless, and slim plates sit closer to the wall and need a deeper back box — 25mm for switches, 35mm for sockets and dimmers, and 47mm for USB sockets, shaver switches, and cooker switches.

What is a pattress box?

A pattress box is a surface-mounted back box, used where a recessed back box isn't possible or practical. These are often available in a finish to match your sockets and switches, though sizing isn't universal, so it's worth checking fit before buying — particularly for larger plates.

Can I fit a back box myself?

A competent DIYer can fit a back box, but care is needed when cutting or drilling into walls to avoid hidden cables and pipes. If you're in any doubt, consult a qualified electrician.