How is the Length of a Screw Measured?

This is one of our most commonly asked questions. Remember these two rules and you won’t be disappointed!

The general rule for measuring screws

For almost all screws, the rule is that you measure from the underside of the head to the end of the thread, as illustrated below;

MEASURING SCREW LENGTH

The rule for measuring countersunk screws

For countersunk screws you measure the total length of the screw including the head.

MEASURING COUNTERSUNK SCREW LENGTH

The one exception to the countersunk rule is that raised countersunk heads (the type you find in electrical fittings such as light switches) are measured from the tip of the thread to the top of the countersunk but without including the slightly raised domed top.