Hex (wrench) size by thread size and length for hex coupling nuts, 1/4″ to 1″ and number sizes.
A coupling nut (rod coupling) is a long hex nut that joins two pieces of threaded rod, or a rod and a bolt, of the same thread. It is sold by thread size and length. Coupling nut sizes differ between standards and makers: the same thread and length is made with more than one hex size, so check the wrench against the part. The table lists only the combinations two or more of our sources agree on.
Letters match the table columns: F across flats (the wrench), L length. Not to scale.
| Size | Length L | Wrench F | F max / min |
|---|---|---|---|
| #6 | 1/2″ | 5/16″ | — / — |
| #8 | 5/8″ | 5/16″ * | — / — |
| #10 | 3/4″ | 5/16″ * | — / — |
| 1/4″ | 7/8″ | 3/8″ * | .375 / .365 |
| 1/4″ | 1-3/4″ | 7/16″ | — / — |
| 5/16″ | 7/8″ | 1/2″ * | .500 / .489 |
| 5/16″ | 1-1/8″ | 1/2″ | .500 / .489 |
| 5/16″ | 1-3/4″ | 1/2″ | — / — |
| 3/8″ | 1-1/8″ | 1/2″ * | .500 / .489 |
| 3/8″ | 1-3/4″ | 9/16″ * | .562 / .551 |
| 1/2″ | 1-3/4″ | 11/16″ * | .688 / .663 |
| 5/8″ | 2-1/8″ | 13/16″ | .813 / .782 |
| 3/4″ | 2-1/4″ | 1″ * | 1.000 / .963 |
| 7/8″ | 2-1/2″ | 1-1/4″ | 1.250 / 1.212 |
| 1″ | 2-3/4″ | 1-3/8″ | 1.375 / 1.325 |
* Our sources also list another hex size for this thread and length — check the part. Basic sizes in inches; F max / min where two sources agree. A thread + length that isn’t here (or a different hex for one that is) is a maker’s own size — give us a call.
Good to know
One thread, two pieces. A coupling nut joins two pieces of threaded rod (or a rod and a bolt) of the same size and thread.
Not on this chart: heavy hex couplings and reducing couplings (two different threads) — give us a call for those.
Quick answers
- What size wrench fits a 3/8 coupling nut?
- A 3/8″-16 × 1-3/4″ coupling nut is 9/16″ across the flats on our chart; a 3/8″ × 1-1/8″ is 1/2″. Some makers use another hex — check the part.
- What size socket for a 1/2-13 coupling nut?
- A 1/2″ × 1-3/4″ coupling nut takes an 11/16″ wrench on our chart. Some makers use another hex — check the part.
- What hex is a 5/8 coupling nut?
- A 5/8″ × 2-1/8″ coupling nut is 13/16″ across the flats.
- What wrench fits a 3/4 coupling nut?
- A 3/4″ × 2-1/4″ coupling nut takes a 1″ wrench on our chart. Some makers use another hex — check the part.
- Do coupling nuts come in more than one hex size?
- Yes: makers sell the same thread and length in more than one hex size. Our chart lists the combinations two or more of our sources agree on, and marks with * the ones also sold in another hex.