Switch Plate Screws:
Switch plate screws are generally 6-32 (thread diameter of #6 and 32 threads per inch). Depending on the device you are covering, you'll need different lengths of screws to hold your cover plate in place.
5/16" (rocker, Despard, vintage low voltage) for strap mount plates
1/2" (toggle, duplex, blank, simplex) for box or device mount plates
UNC (Unified Course Threads) Threads According to ANSI B1.1
6-32 screw diameter oval head: #6 equals .138" (a little over 1/8", 3.505 mm). The pitch of the screw head is 0.794" and diameter is 1/4". This is used in the majority of electrical handy or masonry boxes.
8-32 screw diameter oval head: #8 equals .164" (a little over 5/32", 4.166 mm). The pitch of the screw head is 0.794" and diameter is 19/64". This larger size is used for diagonal screws in 4x4 square boxes, 1900 boxes, round or octagon boxes when mounting mudrings, etc.
Types of Screws:
Screws can have a flat, round oval or pan head that's Phillips, slotted, combination or tamper-proof. They can also be self-tapping, which means they can tap their own hole as they are driven in, eliminating the need to first drill a hole in your material.
1. Phillips/slotted combination pan head screw: This is an example of the type of screw used to mount an electrical device into a handy box. Note that the bottom of the screw is tapered. This is an example of a self-tapping screw.
2. Phillips flat head screw: Flat head electrical screws are for use where the screw's head must be flush with the final surface.
3. Slotted flat head screw: Another example of a flat head screw, but made for a slotted screwdriver rather than Phillips.
4. Slotted oval head screw: Standard switch plate screws like this and are typically 5/16" or 1/2" in length.
5. Round slotted head screw: Used in specialty plates without round screw holes, such as these dryer outlet covers.
6. Plastic slotted screw: Plastic screws can be used to mount switch plates when heat transfer from metal screws may cause plastic plates to warp.
7. Spanner head screw: Special tamper proof screws that requires a special screwdriver to screw and unscrew.
8. Rosette head screw: Another type of fancy tamperproof screw used with a specialized screwdriver for installation.
How to Measure Wall Plate Screws
1/4" Screws (Nominal)
Quarter inch screws for light switch plates have a 5/16" length without the screw head.
The length, including the standard slotted oval head, is 5/16", although the length can vary +/- a 1/16".
These shortest wall plate screws are the standard for covers with strap mount screw spacing, such as Decora rocker plates.
Half inch screws can be measured including the screw head.
This size is the standard for device mount and box mount covers.