A bolt size is a complete description, not a single diameter. To identify a hex bolt correctly, measure the shank diameter, thread pitch, length, head width across flats, head height, material/grade and the relevant standard. Those details determine whether the bolt fits the hole, nut, wrench and designed joint.
This original guide explains threaded bolt dimensions, hexagonal bolt dimensions and the practical checks used when ordering or inspecting industrial fasteners.
Read a bolt specification before measuring
An inch bolt may be marked or specified as 1/2-13 × 2 in.: 1/2 in. nominal major diameter, 13 threads per inch, and 2 in. length measured from under the head for a standard hex head. A metric format such as M12 × 1.75 × 60 means 12 mm nominal diameter, 1.75 mm pitch and 60 mm length.
| Dimension | How to measure | Why it matters |
|---|---|---|
| Bolt dia / major diameter | Measure across the crests with a caliper; do not use the worn thread valleys. | Matches the nut and clearance-hole series. |
| Thread pitch | Use a thread-pitch gauge, or count threads per inch on inch fasteners. | Same diameter can have incompatible coarse and fine threads. |
| Length | Measure from beneath the head to the end for standard hex bolts. | Controls thread engagement and grip length. |
| Head width across flats | Measure between two opposite flat faces. | Determines the correct wrench or socket size. |
| Head height | Measure from bearing face to top of head. | Confirms head style and clearance in the assembly. |
How to measure the diameter of a bolt
Clean the thread first. Place a caliper lightly across two opposite thread crests, perpendicular to the bolt axis. The result is the nominal bolt diameter or hex bolt diameter when it is close to a recognized standard size. Do not identify a bolt solely from a hole size: clearance holes are intentionally larger than the bolt.
For threaded portions, a pitch gauge is the fastest reliable method. On inch fasteners, count the thread peaks across one inch to establish TPI. On metric fasteners, match the gauge leaf to the spacing between threads. A 1/2 in. bolt can be 1/2-13 UNC or 1/2-20 UNF; both have the same diameter but different threads.
Hex head dimensions and wrench sizes
The nominal bolt diameter is not the same as the head width. Across-flats dimensions are standardized by the applicable fastener specification, but may differ by product standard and series. Always confirm the drawing or standard before selecting a tool for controlled tightening.
| Common nominal bolt size | Typical hex head / wrench size* | What to verify |
|---|---|---|
| 3/8 in. | 9/16 in. across flats | Thread series and bolt grade |
| 1/2 in. | 3/4 in. across flats | Whether the request means bolt size or wrench size |
| 3/4 in. | 1-1/8 in. across flats | Heavy-hex vs standard-hex configuration |
| 1-1/2 in. | 2-1/4 in. across flats | Governing standard and head style |
| M12 | 18 mm across flats (common) | ISO/DIN product standard |
*Typical values are a quick identification aid only. Use the applicable standard or drawing for purchasing and acceptance.
How to read bolt specs without mixing systems
- Inch series: diameter – threads per inch × length, followed by grade/material and finish.
- Metric series: M diameter × pitch × length, followed by property class/material and finish.
- Head style: hex head, flange head, socket head and countersunk screws use different length reference points.
- Thread length: a partially threaded bolt needs both its overall length and thread length checked.
1/2 bolt hole size: bolt size is not hole size
A “1/2 bolt hole” needs clarification. It may mean a hole for a 1/2 in. bolt, a 1/2 in. tapped thread, or an actual 1/2 in. diameter hole. These are different requirements. Define whether the hole is clearance, close-clearance, threaded, reamed or oversized, and call out the governing drawing and tolerance.
Inspection checklist for hex bolts
- Identify the standard, material, grade/property class and finish.
- Measure shank major diameter and confirm pitch or TPI.
- Measure length from the correct head reference plane.
- Measure head across flats and head height; check for damage.
- Verify thread length for partially threaded bolts.
- Review markings, lot traceability and required certificates.
Final takeaway
To measure a hex bolt accurately, record more than its diameter. Combine the bolt dia, thread form, length, head dimensions and applicable standard. This makes purchasing clearer, prevents mismatched nuts and tools, and helps inspection teams compare parts to the correct specification.
Songhai supports industrial fastener enquiries. Send the drawing, standard, material, grade, dimensions and documentation requirements for a technical review.