Fasteners reference
Where fastener length starts: under the head or over it?
The practical length convention for external hex, socket cap, button, and flat countersunk fasteners.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Follow the installed bearing surface
The useful length is the portion that enters the assembly. A regular external hex head, socket-head cap screw, and button head remain above the work, so their head thickness is excluded.
A flat countersunk head is designed to sit in a recess with its top at or near the work surface. Its head is therefore included in the nominal length.
| Head profile | Start measurement |
|---|---|
| External hex | Flat underside of head |
| Socket-head cap | Flat underside of cylindrical head |
| Button head | Bearing surface under the dome |
| Flat / countersunk | Top face of head |
| Headless set screw | One end to the other |
On-screen versus manual length
An on-screen height gauge is convenient for short fasteners: place the selected head reference on zero and move the tip jaw to the far end. When the fastener is longer than the usable screen range, measure it with a ruler, tape, or caliper and enter the number directly.
Manual entry is not a lower-quality fallback. It is the correct method when the display cannot physically contain the part.
Small details that change the reading
- Measure to the physical end, including a dog point or cone point when that feature enters the assembly.
- Do not add a captive washer unless the fastener specification explicitly includes it.
- Hold a rounded or damaged tip square to the scale and record sensible precision.
- Keep units attached to the number; 1 inch and 25 mm are close, but not identical.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standard- ISO 4014:2022 — Hexagon head boltsInternational Organization for Standardization (opens in a new tab)
- ISO 4762:2004 — Hexagon socket head cap screwsInternational Organization for Standardization (opens in a new tab)
- ASME B18.2.1-2012 (R2021) — Inch-series hex bolts and screwsAmerican Society of Mechanical Engineers (opens in a new tab)
Use it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.