Online RulerHow to measure a small screw
For a screw with a flat countersunk head, measure from the top of the head to the tip. For a screw with a pan, round, or other head that sits above the surface, measure from the underside of the head to the tip. Look at the head from the side to choose your starting point.
- A flat countersunk head has a tapered underside that fits into a matching recess.
- The underside of a raised head is the surface that bears against the part when the screw is seated.
- Length runs along the screw's centerline, from the appropriate head surface to the end of the tip.
- Lay the screw on its side on a flat surface. Turn it until you can see the profile of the head and the shape of the tip.
- Identify the head shape. Use the top face of a flat countersunk head or the bearing surface under a raised head.
- Set the ruler parallel to the shank and put zero in line with the starting surface. Hold the screw still without hiding the head or tip.
- Look straight down at the tip and read where its farthest point falls on the scale. If it falls between marks, write a range or an approximate value.
- Record the length and unit with the head shape. If you measured from a numbered line, subtract that starting reading before you write the length.
Read the head from the side
A drive slot or recess tells you what turns the screw. The side profile tells you where to put zero. Look for a taper that slopes inward from the rim toward the shank. On a flat countersunk screw, that taper sits in a recess and the top face is the reference. With a raised head and a flat bearing surface, look for the flat ledge where the head meets the shank.
Keep the screw still
Work on a surface where you can see the screw's outline. If it rolls, steady the middle of the shank with a fingertip or brace it lightly from the side with folded paper. Leave both ends visible. Check that the shank still runs alongside the ruler. If the head lifts the shank, bring the scale close and view the head and tip from directly above their respective marks.
Describe a worn end
If the tip is bent, chipped, or worn, record the length you can measure and describe the condition. When you need to match an original screw, measure an intact screw of the same type if you have one. For a head shape you cannot identify, record where you placed zero. A supplier's dimension drawing can show which surfaces its listed length refers to.
Do I measure only the threads?
Measure to the end of the tip from the head reference. That length includes any smooth portion of the shank between the head and the threads.
Where do I read a pointed tip?
Read the point farthest along the screw's centerline. With a blunt end, use the end face at the far side of the shank.
What if the screw has a domed top?
Check the underside. A domed head with a flat bearing surface is measured from that surface. If the underside tapers into a countersink, check the dimension drawing for that head style.