Head diameter and height, square neck size and minimum thread length for round head square neck carriage bolts, #10 to 1″. ASME B18.5.
A carriage bolt has a smooth, domed head and a square neck under it. The square keeps the bolt from turning as the nut is tightened — there’s nothing on the head to put a wrench on. It’s measured like any bolt with a head that sits on top: from under the head to the end.
Letters match the table columns: A head diameter, H head height, O square width, P square depth, LT thread length, L length.
| Size | Head dia A max / min | Head H max / min | Square O max / min | Square depth P max / min | Thread LT min: to 6″ long / longer |
|---|---|---|---|---|---|
| #10 | .469 / — | .114 / .094 | .199 / .185 | .125 / .094 | .630″ / .880″ |
| 1/4″ | .594 / .563 | .145 / .125 | .260 / .245 | .156 / .125 | 3/4″ / 1″ |
| 5/16″ | .719 / .688 | .176 / .156 | .324 / .307 | .187 / .156 | 7/8″ / 1-1/8″ |
| 3/8″ | .844 / .782 | .208 / .188 | .388 / .368 | .219 / .188 | 1″ / 1-1/4″ |
| 7/16″ | .969 / .907 | .239 / .219 | .452 / .431 | .250 / .219 | 1-1/8″ / 1-3/8″ |
| 1/2″ | 1.094 / 1.032 | .270 / .250 | .515 / .492 | .281 / .250 | 1-1/4″ / 1-1/2″ |
| 5/8″ | 1.344 / 1.219 | .344 / .313 | .642 / .616 | .344 / .313 | 1-1/2″ / 1-3/4″ |
| 3/4″ | 1.594 / 1.469 | .406 / .375 | .768 / .741 | .406 / .375 | 1-3/4″ / 2″ |
| 7/8″ | 1.844 / 1.719 | — / .438 | .895 / .865 | .469 / .438 | 2″ / 2-1/4″ |
| 1″ | 2.094 / 1.969 | .531 / .500 | 1.022 / .990 | .531 / .500 | 2-1/4″ / 2-1/2″ |
Thread length is a minimum: twice the diameter plus 1/4″ for bolts 6″ and shorter, plus 1/2″ for longer ones — shorter bolts are often threaded all the way. Threads are UNC. A dash: our two sources differ — give us a call at 405-670-1234.
The square neck
The square under the head keeps the bolt from turning while the nut is tightened; its size across the flats is O and its depth P in the table. Thread pitch and tap drill for each size are on the thread pitch chart; how to measure any bolt is on How to Measure a Bolt.
Quick answers
- How is a carriage bolt measured?
- From under the head to the end of the bolt — the domed head sits on top of the surface, so it isn’t part of the length.
- How big is the square neck on a 3/8" carriage bolt?
- The square neck is .368–.388″ across the flats and .188–.219″ deep (ASME B18.5).
- How big is the head on a 1/2" carriage bolt?
- 1.032–1.094″ across and .250–.270″ tall.
- How long is the thread on a carriage bolt?
- At least twice the diameter plus 1/4″ on bolts 6″ and shorter (a 1/2″ carriage bolt: 1-1/4″), and twice the diameter plus 1/2″ on longer ones.