How to Use Calipers to Identify an Unknown Fastener
Digital or vernier calipers help separate nominal size from head, shank and washer dimensions.
See an error on this sheet?
Tell us what is wrong.
Text version of this sheet
0.250
QUICK RULE
Digital or vernier calipers help separate nominal size
from head, shank and washer dimensions.
Jaws on the crests: this reads the major (outside) diameter
Zero the caliper and clean burrs/dirt from the part.
Measure major diameter across thread crests; use it to identify the nearest nominal size.
Measure unthreaded body/shoulder diameter separately.
Measure head width across flats, head diameter or washer OD as appropriate.
Measure length from the product's correct datum.
Calipers do not identify TPI/pitch reliably by themselves - use a thread-pitch
gauge/checker.
IDENTIFICATION ORDER
Product -> diameter -> thread -> length -> head/drive -> grade/material -> finish -> governing specification
REFERENCE ONLY. Verify the current ASME/SAE/ASTM/ISO/OEM standard and manufacturer data. Do not use as the sole basis for critical fastener selection or installation.
Fastener Guide reference FG-482546 · thefastenerapp.com/fastener-guide/measure-and-identify/how-to-use-calipers-to-identify-an-unknown-fastener · © The Fastener App. Reference only — verify the current governing standard, drawing and manufacturer data before specifying or installing.
