Measuring ToolsMeasuring instrument
Vernier & Dial Caliper
Jaws that slide on a graduated beam, reading outside, inside and depth dimensions to a hundredth of a millimetre. The instrument the identifier on this site expects you to have.
What it does
- Reading the outside diameter of a screw or bolt
- Measuring a bore or a slot with the inside jaws
- Checking a dimension against a drawing
What to look forthe judgement calls, before the spec sheet
- Resolution is the smallest increment the scale reads: 0.02 mm on most vernier calipers, 0.01 mm on a dial or digital one. Choose by the tolerance you are checking, not by the largest number.
- Range is usually 150 mm for the common instrument, which covers the fastener sizes in this database with room to spare.
- Jaw geometry matters at the small end: a heavy beam and long jaws will not enter a tight recess or measure a thread crest cleanly.
Misuse and safetythe ways this tool causes rework
- Calipers are a transfer instrument, not a metrology standard. Resolutions of 0.01 mm are quoted, but the practical accuracy is around 0.05 mm — fine for identification, not for accepting a close-tolerance part.
- Measuring over the crests of a thread reads slightly under the major diameter because the jaws land on the rounded flank. Around 0.1 mm of mismatch on a plated fastener is normal, which is why the identifier treats a close match as confirmation rather than proof.
Projects that call for itand what it is doing on each one
All projectsFrequently asked questions
How do I measure a screw to identify it?
Take the outside diameter across the thread crests, then the pitch — crest to crest for a metric thread, or the thread count in one inch for a unified one. Two readings are usually enough.
Why does my caliper read differently from a micrometer?
A micrometer has a friction thimble and a rigid frame, so it measures with consistent force. A caliper depends on how hard you close the jaws. On a soft or thin part that difference is the whole disagreement.