Hex nut dimensions
Metric
(ISO 4032)
| Thread Designation | Width Across Flats (mm) | Nut Height (mm) | Thread Pitch (mm) |
|---|---|---|---|
| M2 | 4.0 | 1.6 | 0.40 |
| M2.5 | 5.0 | 2.0 | 0.45 |
| M3 | 5.5 | 2.4 | 0.50 |
| M4 | 7.0 | 3.2 | 0.70 |
| M5 | 8.0 | 4.7 | 0.80 |
| M6 | 10.0 | 5.2 | 1.00 |
| M8 | 13.0 | 6.8 | 1.25 |
| M10 | 16.0 | 8.4 | 1.50 |
| M12 | 18.0 | 10.8 | 1.75 |
| M16 | 24.0 | 14.8 | 2.00 |
| M20 | 30.0 | 18.0 | 2.50 |
| M24 | 36.0 | 21.5 | 3.00 |
Imperial
(ANSI/ASME B18.2.2)
| Thread Designation | Width Across Flats (in) | Nut Height (in) | Threads Per Inch (TPI) |
|---|---|---|---|
| #0-80 | 5/32" | 3/64" | 80 |
| #2-56 | 3/16" | 1/16" | 56 |
| #4-40 | 1/4" | 3/32" | 40 |
| #6-32 | 5/16" | 7/64" | 32 |
| #8-32 | 11/32" | 1/8" | 32 |
| #10-24 | 3/8" | 1/8" | 24 |
| 1/4"-20 | 7/16" | 7/32" | 20 |
| 5/16"-18 | 1/2" | 17/64" | 18 |
| 3/8"-16 | 9/16" | 21/64" | 16 |
| 7/16"-14 | 11/16" | 3/8" | 14 |
| 1/2"-13 | 3/4" | 7/16" | 13 |
| 5/8"-11 | 15/16" | 35/64" | 11 |
| 3/4"-10 | 1-1/8" | 41/64" | 10 |
| 1"-8 | 1-1/2" | 55/64" | 8 |
Design Parameters
Hexagon nuts (ISO 4032 / DIN 934) are the standard internal-thread fastener used to secure bolts or screws. They provide the clamping force necessary to maintain a secure joint.
- Thread Designation: The nominal internal thread size (e.g., M10 or 3/8"-16).
- Width Across Flats: The size of the wrench or socket required for tightening.
- Nut Thickness (Height): The total vertical thickness of the nut.
- Thread Pitch / TPI: The distance between internal threads (Metric) or threads per inch (Imperial).
Engineering Note: To achieve full rated strength, a nut must be thick enough to prevent thread stripping before the bolt reaches its breaking point. A general rule of thumb is that at least two full threads of the bolt should extend beyond the face of the nut once tightened.
Technical Guidance for Hex Nut Assemblies
The hex nut is more than a simple threaded collar; it is the anchor that allows a bolt to act as a spring. When a nut is tightened, it stretches the bolt, creating the preload necessary to hold a joint together against external loads and vibration.
Strength Matching: The Golden Rule
In mechanical design, the nut should always be "stronger" than the bolt. The engineering goal is for the bolt to fail in tension (snap) before the nut threads strip. Stripping is a catastrophic and often hidden failure, whereas a snapped bolt is immediately obvious.
- Metric: Pair a Class 8 nut with an 8.8 bolt; a Class 10 nut with a 10.9 bolt.
- Imperial: Pair a Grade 5 nut with a Grade 5 bolt.
Thread Engagement and Load Distribution
While the "two-thread" rule is standard for visual inspection, the physics of thread engagement is non-linear. The first three threads of a hex nut typically carry nearly 70% of the entire load. This is why the height of a standard nut (ISO 4032) is calculated specifically to prevent thread shear. Using "Thin" or "Jam" nuts (ISO 4035) for primary load-bearing is dangerous, as they lack the height to develop the bolt's full tensile strength.
Torque, Friction, and the "K-Factor"
When tightening a nut, roughly 90% of the torque applied is used simply to overcome friction—partly in the threads and partly on the nut's bearing surface. Only 10% actually stretches the bolt.
- Dry Threads: High friction leads to inconsistent preload.
- Lubricated Threads: Reduces friction significantly. If you lubricate a nut that was designed for "dry" torque, you risk over-stretching and breaking the bolt at the same torque value.
Wrench Size Awareness
Similar to hex bolts, be mindful of ISO vs. DIN wrench sizes. For M10, M12, and M14 nuts, the Width Across Flats (AF) can differ by 1mm between standards. Always specify the standard clearly in assembly manuals to ensure the correct tools are used and to avoid rounded corners.
Common Standards Reference
| Standard | Description | Notes |
|---|---|---|
| ISO 4032 | Regular Hex Nut (Style 1) | Most common metric standard |
| DIN 934 | Older Metric Standard | Slightly different heights/AF for some sizes |
| ASME B18.2.2 | Imperial Hex Nut | Standard finished hex |
| ISO 4035 | Thin Hex Nut (Jam Nut) | For locking or low-load only |
Note: For high-vibration environments, standard hex nuts should be supplemented with a prevailing torque feature or chemical thread-locker.