Measuring Fasteners: Length & Diameter (with Diagrams)
Fastener measurement gets simple once you know which part of the screw or bolt to measure. Use the quick rules below, then grab our printable PDF for the bench.
Download: Measuring Screw Lengths (PDF)
Tip: print at Actual Size (Scale 100%) for accurate reference. If you need a ruler, use our US & Metric Printable Ruler.
How to Measure Length (by Head Type)
Imagine the fastener installed: measure the portion that will sit inside the material.
- Countersunk heads (Flat, Oval): measure the overall length (tip to head top), because the head sits flush/recessed.
- Non-countersunk heads (Pan, Button, Hex, Socket Cap): measure under the head to the tip (head height is not included).
- Set screws: measure the overall length.
- Shoulder bolts: length is the shoulder length (not counting the head or thread beyond the shoulder).
How to Measure Diameter (Major/Outer Diameter)
Diameter is the thread’s outer (major) diameter—not the head. This same nominal size pairs with matching nuts and washers.
- Metric: M6 screw → M6 nut/washer.
- US/Inch: 1/4″ screw → 1/4″ nut/washer.
- Use calipers if available, or our printable ruler to estimate.
Don’t Forget Thread Count / Pitch
Diameter alone isn’t enough—identify the thread pattern too:
- US (TPI = Threads Per Inch): count peaks across ½ inch, then double (reduces error). Example: 1/4-20 (UNC) vs 1/4-28 (UNF).
- Metric (Pitch in mm): measure across 10 threads, move the decimal left once. Example: M6-1.0 has 1.0 mm between threads.
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