Nut Size Table
| Size | UNC (TPI) | UNF (TPI) | A/F (in) | A/F (mm) |
|---|---|---|---|---|
| #4 | 40 | 48 | 1/4 | 6.35 |
| #6 | 32 | 40 | 5/16 | 7.94 |
| #8 | 32 | 36 | 11/32 | 8.73 |
| #10 | 24 | 32 | 3/8 | 9.53 |
| 1/4 | 20 | 28 | 7/16 | 11.11 |
| 5/16 | 18 | 24 | 1/2 | 12.70 |
| 3/8 | 16 | 24 | 9/16 | 14.29 |
| 7/16 | 14 | 20 | 11/16 | 17.46 |
| 1/2 | 13 | 20 | 3/4 | 19.05 |
| 9/16 | 12 | 18 | 7/8 | 22.23 |
| 5/8 | 11 | 18 | 15/16 | 23.81 |
| 3/4 | 10 | 16 | 1 1/8 | 28.58 |
| 7/8 | 9 | 14 | 1 5/16 | 33.34 |
| 1 | 8 | 12 | 1 1/2 | 38.10 |
| Size | Coarse pitch (mm) | A/F main (mm) | A/F alt (mm) | Height (mm) |
|---|---|---|---|---|
| M3 | 0.5 | 5.5 | - | 2.4 |
| M4 | 0.7 | 7 | - | 3.2 |
| M5 | 0.8 | 8 | - | 4 |
| M6 | 1.0 | 10 | - | 5 |
| M8 | 1.25 | 13 | - | 6.5 |
| M10 | 1.5 | 16 | 17 | 8 |
| M12 | 1.75 | 18 | 19 | 10 |
| M16 | 2.0 | 24 | - | 13 |
| M20 | 2.5 | 30 | - | 16 |
| M24 | 3.0 | 36 | - | 19 |
Every hex nut in the table is identified by three practical numbers: the thread it takes (size and pitch), the across-flats distance a wrench must open to, and - if you dig into the second chart - how tall the nut is. The two worlds split cleanly: SAE sizes run from #4 (0.112 in thread) through 1 inch, sized in fractions with # number sizes below 1/4; metric sizes run M3 through M24 with a simple coarse-pitch column. Across flats is the column worth memorizing, because it is literally the wrench: a 1/2-13 nut needs a 3/4 wrench, an M10 nut needs a 16 mm socket, and nothing else on the nut matters at the bench.
Height follows a rule almost nobody states out loud: a standard hex nut is about 0.8 times the thread diameter tall (M6: 5 mm, 1/2 in: 0.43 in), which is the engineering compromise between thread engagement and space. Thin jam nuts run about half that, nylon-insert locknuts run about one full diameter because of the nylon collar. The thread column is the second identifier: US nuts come in coarse UNC and fine UNF twins for every size - a 1/4-20 and a 1/4-28 nut share the same wrench size but are not interchangeable - while metric nuts default to coarse pitch with fine pitches as special orders. When a loose nut rolls across the bench, measure across flats first, then count threads per inch, and the table hands you the full identity.
How to use
- Measure across the flats, not the corners: calipers on two parallel faces give the wrench size directly. US sizes are fractional inches, metric are whole millimeters - and the two systems deliberately almost-touch, which is the trap in FAQ 2.
- Count threads per inch to split the coarse/fine twins: 1/4-20 (UNC) vs 1/4-28 (UNF) take different nuts and bolts even though both need a 7/16 wrench. Metric coarse pitch is in the second chart; metric fine (e.g. M10 x 1.25) exists but is the exception.
- Read the nut's face for its grade: SAE marks 2, 5 or 8 (dash counts on some heads), ISO metric marks 8, 10 or 12. Pair the nut at or above the bolt grade - grade 8 nut on a grade 8.8 bolt, class 10 nut on a 10.9 - and the fastener system fails loudly instead of silently.
Frequently asked questions
What does across flats (A/F) mean, and why not measure corner to corner?
Across flats is the distance between two opposite parallel faces of the hexagon - exactly what a wrench jaw grips. Corner-to-corner (across corners) is larger by the geometry of a hexagon: A/C = A/F divided by cosine of 30 degrees, about 1.155 times. On a 1/2 in nut that is 0.750 in across flats versus 0.866 across corners, which is why a 12-point socket can ride on corners and round them off while a 6-point socket, which contacts flats, cannot. Wrenches and sockets are always sized by across flats for exactly this reason, so the A/F column is the only dimension that selects a tool. It is also the measurement that survives a rusty, painted nut: corners get banged round long before the flats lose their caliper-reading distance.
Why does a metric wrench almost fit a 7/16 nut - and why is that a problem?
Because the systems nearly collide: a 7/16 in nut measures 11.11 mm across flats, so an 11 mm metric wrench slips on with visible daylight and, under torque, it rounds the hexagon corners - the classic ruined-nut moment. The near-misses run through the whole table: 1/2 in is 19.05 mm (fits a 19 mm wrench loosely), 9/16 in is 22.23 mm (a sloppy 22 mm), and 5/8 in is 15/16 in = 23.81 mm. The rule is one-directional: never use a metric wrench on a US fastener or vice versa when the fit is not exact. If you cannot identify the system, measure precisely - an 11.11 mm reading is a 7/16 nut and needs a 7/16 wrench (for hex nuts through 1 in, the across-flats fraction equals the wrench fraction), while a true 11.0 mm reading is a metric size no SAE wrench should touch.
How do nut grades pair with bolt grades?
The marking system: SAE J995 grades 2, 5 and 8 (roughly 74, 120 and 150 ksi ultimate tensile), marked on the nut face; ISO 898 classes 8, 10 and 12 for metric nuts. The pairing rule is nut grade at or slightly above bolt grade: a grade 5 nut on a grade 5 bolt, a class 10 nut on a 10.9 bolt. The counterintuitive part is that the nut is deliberately the weaker link in a good design - its threads strip first, which is visible and repairable, while a snapped bolt head inside an assembly is a extraction job. A nut one grade below its bolt converts an invisible failure into a visible one. Never mix systems blindly: a hard class 12 nut on a soft grade 2 bolt just moves the failure into the bolt threads.
What is a jam nut and when does it actually work?
A jam nut is a hex nut at roughly half the standard height (M6: 3.2 mm versus 5 mm) - too thin to carry full load on its own, designed to be used in pairs with a full nut. The working arrangement: thread the jam nut on first, then the standard nut on top, and tighten the standard nut hard against it while holding the jam nut back. The two nuts then pull against each other, stretching the bolt section between them, and that tension locks both nuts - vibration cannot rotate them without overcoming the preloaded friction. The order matters: jam nut against the joint, full nut on top. Reversed, the thin nut takes the working load and its shallow thread engagement strips. For a single fastener where a second full nut is awkward, a nylon-insert locknut (about one diameter tall because of the collar) does the same job by deforming around the threads - but it is effectively single-use, losing bite after a few installations.