A correctly sized driver should “fill up” the screw head, utilizing both the full depth and width of the socket.

Beginners commonly make the mistake of using a driver that’s too small. An undersized driver slips easily, damaging the screw head and making it difficult to turn afterward—even with a correctly-sized driver. This is known as a stripped screw.

An oversized driver fits into the screw only shallowly, if at all.

When you find a driver that fits about right, compare the next size up as well as the next size down before you proceed.

In general, use the largest driver size that fits snugly into the screw.