Wrench size (across the flats), across the corners, head height and thread length for square head bolts, 1/4″ to 1-1/2″. ASME B18.2.1.
A square head bolt is sized by its thread, and its wrench by the head’s width across the flats (F). From 5/16″ up a square head takes the same wrench as a hex head of that size; a 1/4″ square head is 3/8″ across the flats (a 1/4″ hex head is 7/16″). 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: F across the flats, G across the corners, H head height, LT thread length, L length.
| Size | Flats F | F max / min | Corners G max / min | Head H | H max / min | Thread LT, L ≤ 6″ | LT, L > 6″ |
|---|---|---|---|---|---|---|---|
| 1/4″ | 3/8″ | .375 / .362 | .530 / .498 | 11/64″ | .188 / .156 | .750″ | 1.000″ |
| 5/16″ | 1/2″ | .500 / .484 | .707 / .665 | 13/64″ | .220 / .186 | .875″ | 1.125″ |
| 3/8″ | 9/16″ | .562 / .544 | .795 / .747 | 1/4″ | .268 / .232 | 1.000″ | 1.250″ |
| 7/16″ | 5/8″ | .625 / .603 | .884 / .828 | 19/64″ | .316 / .278 | 1.125″ | 1.375″ |
| 1/2″ | 3/4″ | .750 / .725 | 1.061 / .995 | 21/64″ | .348 / .308 | 1.250″ | 1.500″ |
| 5/8″ | 15/16″ | .938 / .906 | 1.326 / 1.244 | 27/64″ | .444 / .400 | 1.500″ | 1.750″ |
| 3/4″ | 1-1/8″ | 1.125 / 1.088 | 1.591 / 1.494 | 1/2″ | .524 / .476 | 1.750″ | 2.000″ |
| 7/8″ | 1-5/16″ | 1.312 / 1.269 | 1.856 / 1.742 | 19/32″ | .620 / .568 | 2.000″ | 2.250″ |
| 1″ | 1-1/2″ | 1.500 / 1.450 | 2.121 / 1.991 | 21/32″ | .684 / .628 | 2.250″ | 2.500″ |
| 1-1/8″ | 1-11/16″ | 1.688 / 1.631 | 2.386 / 2.239 | 3/4″ | .780 / .720 | 2.500″ | 2.750″ |
| 1-1/4″ | 1-7/8″ | 1.875 / 1.812 | 2.652 / 2.489 | 27/32″ | .876 / .812 | 2.750″ | 3.000″ |
| 1-3/8″ | 2-1/16″ | 2.062 / 1.994 | 2.917 / 2.738 | 29/32″ | .940 / .872 | 3.000″ | 3.250″ |
| 1-1/2″ | 2-1/4″ | 2.250 / 2.175 | 3.182 / 2.986 | 1″ | 1.036 / .964 | 3.250″ | 3.500″ |
F and H in bold are the basic (nominal) sizes. Thread length is for bolts 6″ and shorter; longer bolts add 1/4″. Bolts shorter than the thread length are threaded all the way.
Threads, grades and measuring
Thread pitch and tap drill for each size are on the thread pitch chart; grade marks and strength on Bolt Grades; how to measure any bolt on How to Measure a Bolt.
Quick answers
- What size wrench fits a 1/2" square head bolt?
- 3/4″ — the head is 3/4″ across the flats (ASME B18.2.1).
- Does a 1/4" square head bolt take the same wrench as a 1/4" hex bolt?
- No — a 1/4″ square head is 3/8″ across the flats; a 1/4″ hex head is 7/16″. From 5/16″ up they match.
- How tall is the head on a 3/4" square head bolt?
- 1/2″ nominal (.476–.524″).
- How long is the thread on a square head bolt?
- On bolts 6″ and shorter: 1.250″ for 1/2″, 1.750″ for 3/4″, 2.250″ for 1″; longer bolts add 1/4″.