How to Measure Screw Length: A Complete Guide

You would think measuring a screw is straightforward. Put it next to a ruler, note the number, done. But if you have ever ordered screws online and received ones that were slightly too long or too short, there is a good chance the measurement was taken from the wrong point. The starting point changes depending on the type of screw you are measuring, and getting it wrong leads to mismatches, assembly headaches, and wasted orders. This guide explains exactly where to start and end your measurement for each common screw type.

The General Rule

For most screws, length is measured from the point where the head sits flat against the surface to the tip of the screw. The logic behind this is practical: you want to know how much of the screw actually goes into the material, not how much of it exists in total.

The head of most screws sits on top of the surface, so the length that matters is everything below it. But a few screw types are designed to sit flush with or below the surface, which changes where you start measuring. Let us go through each type.

Pan Head, Round Head, Truss Head, Button Head and Socket Cap Screws

These all have raised heads that sit on top of the surface. The measurement starts from the underside of the head, where it meets the material, all the way down to the tip of the screw.

A simple way to remember this: wherever the head ends and the shaft begins, that is your starting point for these screw types.

Flat Head (Countersunk) Screws

Countersunk screws are designed to sit flush with the surface of the material. Because the entire head gets recessed into the material, the full length of the screw — head included — goes into the hole. So the measurement starts from the top of the head and goes all the way to the tip.

This is the one exception where you include the head in your measurement. If you measure only from the underside of the head, you will end up ordering a screw that is shorter than what you actually need.

Common countersunk screw types include flat head machine screws, flat head wood screws, and self-countersinking screws used in metalwork and joinery.

Hex Head and Hex Washer Head Screws and Bolts

Hex heads are flat on both the top and the bottom of the head. Even so, they are measured from the underside of the head, not the top. The hex head sits on the surface, and only the shank goes in, so the standard rule applies: measure from where the bearing surface meets the material, down to the tip.

Oval Head Screws

Oval head screws are partially countersunk — they sit partially below the surface and partially above it, with a rounded dome visible at the top. Since there is no clean flat bearing point, the measurement starts from the widest point of the head, which is where it meets the surface of the material, down to the tip.

Self-Drilling Screws

Self-drilling screws have a drill-point tip that does the job of both drilling and fastening in one step. Because the drill point is part of how the screw functions, it is included in the length measurement. You measure from the underside of the head all the way to the end of the drill tip — not just to where the threads end.

Headless Screws and Set Screws (Grub Screws)

Screws with no head — such as socket set screws or grub screws — go entirely into the material. The full body of the screw is the length. Measure from the very top to the tip. Browse our grub screw range: Grub Screws

Quick Reference Summary

Screw Type	Measure From	Measure To
Pan / Round / Button / Socket Cap	Underside of head	Tip
Flat head (countersunk)	Top of head	Tip
Hex head / Hex washer head	Underside of head	Tip
Oval head	Widest point of head	Tip
Self-drilling	Underside of head	End of drill tip
Grub screw / Set screw	Top of screw	Tip

What Tool Should You Use?

For most purposes, a digital or vernier caliper is the best tool. It gives you a precise reading in either millimetres or inches and removes the guesswork that comes with eyeballing a ruler. For larger screws where precision is less critical, a steel ruler works fine — but for anything under 10 mm, use a caliper.

Place the jaws at the correct starting point for your screw type, extend them to the tip, and read the measurement. That is your length. Cross-reference it with a standard screw size chart if you are trying to identify an existing screw, or use it directly when placing an order.

A Few Things Worth Keeping in Mind

Metric vs Imperial

Metric screws are measured in millimetres (e.g. M4 × 20 mm). Imperial screws use inches or gauge numbers. Double-check the unit before ordering, especially if you are sourcing from different suppliers.

Length and Diameter Are Not the Same

Length tells you how deep the screw goes into the material. Diameter tells you the thickness of the threaded shaft. Both matter when placing an order. If you are unsure how to measure diameter, we have a separate guide covering that.

Partial Thread vs Full Thread

Some screws have a smooth unthreaded shank below the head and threads only on part of the shaft. The length measurement still follows the same rules above, but it helps to also note the thread length separately if your application requires it.

Now that you know how to measure screw length, finding the right one should be straightforward. Browse OnlyScrews to find exactly what you need.