Hex nuts are among the most widely used fasteners in construction, automotive repairs, machinery, plumbing, and general engineering. Their six-sided shape allows them to be tightened or loosened with a wrench or socket, making them practical for securing threaded bolts and screws. However, choosing the correct hex nut requires more than matching its appearance to a bolt. You must consider thread diameter, thread pitch, wrench size, nut thickness, material, and applicable standards.
A hex nut dimensions chart helps you identify the correct nut size by comparing its nominal thread diameter with its across-flats width and height. Understanding these measurements reduces installation problems, improves fastening reliability, and helps prevent thread damage.
This guide explains standard hex nut dimensions, metric and imperial sizing, measurement methods, common nut types, and tips for selecting the right fastener for your project.
What Is a Hex Nut?
A hex nut is a six-sided metal fastener with an internally threaded hole. It is designed to work with a compatible externally threaded fastener, such as a bolt or machine screw. When tightened, the nut creates a clamping force that helps hold components together.
Hex nuts are available in various sizes, materials, thicknesses, and thread configurations. Standard versions are commonly used for everyday fastening, while specialized versions are designed for high-strength applications, vibration resistance, corrosion protection, or limited installation space.
The dimensions of a hex nut generally include:
Nominal thread diameter: The approximate outside diameter of the bolt thread the nut fits.
Thread pitch: The distance between adjacent thread peaks.
Across-flats width: The distance between two opposite flat sides of the nut.
Across-corners width: The distance between two opposite corners.
Nut thickness: The height of the nut measured parallel to its threaded axis.
Chamfer diameter: The diameter of the beveled area around the nut’s top face, where applicable.
These measurements help determine whether a nut will fit the intended bolt and whether the correct wrench can be used.
Standard Hex Nut Dimensions Chart in Metric Sizes
Metric hex nuts are measured using millimeters. Their thread sizes are commonly identified by the letter M followed by the nominal thread diameter, such as M6, M8, M10, or M12.
The following table provides representative dimensions for common standard metric hex nuts. Values are approximate reference dimensions and may vary depending on the governing standard, product grade, and manufacturer.
| Metric Nut Size | Nominal Thread Diameter | Typical Thread Pitch | Across-Flats Width | Approx. Nut Thickness |
|---|---|---|---|---|
| M3 | 3 mm | 0.50 mm | 5.5 mm | 2.4 mm |
| M4 | 4 mm | 0.70 mm | 7 mm | 3.2 mm |
| M5 | 5 mm | 0.80 mm | 8 mm | 4 mm |
| M6 | 6 mm | 1.00 mm | 10 mm | 5 mm |
| M8 | 8 mm | 1.25 mm | 13 mm | 6.5 mm |
| M10 | 10 mm | 1.50 mm | 17 mm | 8 mm |
| M12 | 12 mm | 1.75 mm | 19 mm | 10 mm |
| M14 | 14 mm | 2.00 mm | 22 mm | 11 mm |
| M16 | 16 mm | 2.00 mm | 24 mm | 13 mm |
| M18 | 18 mm | 2.50 mm | 27 mm | 15 mm |
| M20 | 20 mm | 2.50 mm | 30 mm | 16 mm |
| M22 | 22 mm | 2.50 mm | 32–34 mm | 18 mm |
| M24 | 24 mm | 3.00 mm | 36 mm | 19 mm |
| M27 | 27 mm | 3.00 mm | 41 mm | 22 mm |
| M30 | 30 mm | 3.50 mm | 46 mm | 24 mm |
| M36 | 36 mm | 4.00 mm | 55 mm | 29 mm |
The dimensions in this chart are intended for general identification, not as a substitute for engineering specifications. For example, the across-flats dimension of an M22 nut can depend on the applicable product standard, so verify the required specification before ordering or machining.
How to Read Metric Hex Nut Sizes
An M10 hex nut has a nominal thread diameter of 10 mm. A typical standard nut has a 17 mm across-flats width and approximately 8 mm of thickness.
The M10 designation does not mean that the entire nut measures 10 mm across. It identifies the nominal thread size. The wrench size and nut height are separate dimensions.
Metric nuts may also have different thread pitches. For example, M10 × 1.5 is a common coarse-thread size, while M10 × 1.25 and M10 × 1.0 are fine-thread options. The nut’s pitch must match the bolt’s pitch for proper engagement.
Imperial Hex Nut Dimensions Chart
Imperial hex nuts use inch-based thread sizing. Common designations include 1/4 inch, 5/16 inch, 3/8 inch, 1/2 inch, and 3/4 inch.
Imperial threads are commonly specified using Unified National Coarse (UNC) or Unified National Fine (UNF) standards. UNC threads generally have fewer threads per inch than UNF threads of the same nominal diameter.
The following table provides representative dimensions for common standard imperial hex nuts. Across-flats dimensions and thickness can vary with the applicable ASME, ANSI, SAE, or product-specific standard.
| Nominal Nut Size | Nominal Thread Diameter | Common Coarse Thread | Typical Across-Flats Width | Approx. Standard Nut Thickness |
|---|---|---|---|---|
| 1/4 in | 0.250 in | 20 TPI | 7/16 in | 7/32 in |
| 5/16 in | 0.3125 in | 18 TPI | 1/2 in | 17/64 in |
| 3/8 in | 0.375 in | 16 TPI | 9/16 in | 21/64 in |
| 7/16 in | 0.4375 in | 14 TPI | 5/8 in | 3/8 in |
| 1/2 in | 0.500 in | 13 TPI | 3/4 in | 7/16 in |
| 9/16 in | 0.5625 in | 12 TPI | 7/8 in | 31/64 in |
| 5/8 in | 0.625 in | 11 TPI | 15/16 in | 35/64 in |
| 3/4 in | 0.750 in | 10 TPI | 1-1/8 in | 41/64 in |
| 7/8 in | 0.875 in | 9 TPI | 1-5/16 in | 3/4 in |
| 1 in | 1.000 in | 8 TPI | 1-1/2 in | 7/8 in |
| 1-1/8 in | 1.125 in | 7 TPI | 1-11/16 in | 31/32 in |
| 1-1/4 in | 1.250 in | 7 TPI | 1-7/8 in | 1-3/32 in |
| 1-1/2 in | 1.500 in | 6 TPI | 2-1/4 in | 1-5/16 in |
TPI means threads per inch. A value of 13 TPI indicates that 13 thread intervals occur over one inch of thread length.
Important: The table is a practical reference for common dimensions, not a universal specification for every hex nut. Heavy hex nuts, finished hex nuts, structural nuts, and nuts manufactured to different standards may have different dimensions. Always check the required standard and product grade for critical applications.
Key Hex Nut Dimensions Explained
Knowing what each dimension represents makes it easier to identify, compare, and purchase compatible fasteners.
1. Nominal Thread Diameter
The nominal thread diameter identifies the bolt size for which the nut is designed. It is typically expressed in millimeters for metric fasteners and fractions of an inch for imperial fasteners.
For example:
- An M8 nut is intended for a compatible M8 threaded fastener.
- An M12 nut is intended for a compatible M12 threaded fastener.
- A 3/8-inch nut is intended for a compatible 3/8-inch bolt.
- A 1/2-inch nut is intended for a compatible 1/2-inch bolt.
The nominal diameter does not describe the nut’s outside width. It identifies the thread system and size.
2. Across-Flats Width
Across-flats width, often abbreviated as AF, is the distance between two opposite flat faces of the hex nut. It determines the wrench or socket size needed for installation.
For example, a typical M8 standard hex nut uses a 13 mm wrench, while a typical M10 standard hex nut uses a 17 mm wrench. A common 1/2-inch imperial hex nut has a 3/4-inch across-flats dimension.
Across-flats width is one of the easiest nut dimensions to measure because the two opposite faces provide clear contact points for calipers.
3. Across-Corners Width
Across-corners width is the maximum distance between two opposite corners of the hexagonal nut. It is larger than the across-flats measurement.
For an ideal regular hexagon, across-corners width is approximately 1.155 times the across-flats width.
This relationship is useful for estimating the outer size of a nut, but actual measurements can differ slightly because of chamfers, manufacturing tolerances, and corner rounding.
4. Nut Thickness
Nut thickness, also called nut height, is measured parallel to the threaded hole from one face to the opposite face.
A standard nut, thin nut, and heavy hex nut of the same nominal thread diameter may have different thicknesses. This affects thread engagement, available clearance, and the amount of material supporting the threads.
Thin nuts are useful where space is limited or where a jam-nut arrangement is needed. They should not automatically replace standard or heavy nuts in structural applications.
5. Thread Pitch
Thread pitch describes the spacing between adjacent thread peaks. Metric pitch is measured in millimeters, while imperial thread spacing is commonly expressed as threads per inch.
For instance, an M8 × 1.25 nut has a nominal thread diameter of 8 mm and a pitch of 1.25 mm. A 3/8-16 UNC nut has a nominal diameter of 3/8 inch and 16 threads per inch.
A nut can have the correct diameter but still be incompatible with a bolt if the pitch or thread series differs.
6. Chamfer and Bearing Surface
Many hex nuts have a beveled edge, known as a chamfer, around one or both faces. The chamfer provides a finished edge and may help the nut seat properly against the mating surface.
The bearing surface is the face that contacts the washer or clamped component. Its condition and geometry can affect how the load is distributed.
Chamfer dimensions vary according to nut design and manufacturing standard, so they are generally checked against the applicable specification rather than a basic sizing chart.
How to Measure Hex Nut Dimensions Correctly
You can measure a hex nut using a digital caliper, micrometer where appropriate, thread-pitch gauge, and suitable measuring reference.
Step 1: Measure the Across-Flats Width
Place the caliper jaws against two opposite flat faces of the nut. Keep the jaws square to the faces and record the measurement.
Compare the result with a metric or imperial sizing chart. If the measurement is close to a standard wrench size, use the nominal thread diameter and thread pitch to confirm the identification.
Step 2: Measure the Nut Thickness
Position the caliper across the two flat faces at the top and bottom of the nut. Measure parallel to the threaded hole.
Avoid including a protruding washer, attached flange, or other separate component in the measurement. If the nut has a pronounced chamfer, measure the full face-to-face thickness rather than the beveled area alone.
Step 3: Determine the Thread Diameter
For a loose nut, measure the internal thread diameter carefully with suitable internal measuring equipment, or identify the nut using a thread gauge and known compatible fastener.
Internal thread measurements can be difficult because the thread grooves and crests affect the reading. The measured internal diameter is normally smaller than the nominal bolt diameter, so do not identify a nut solely from a simple internal caliper reading.
Step 4: Check the Thread Pitch
Use a thread-pitch gauge to identify the distance between threads. Select the metric leaf that fits the thread profile, or use the appropriate imperial gauge to count threads per inch.
If a gauge is unavailable, compare the nut with a known compatible bolt. The nut should turn smoothly by hand without binding or excessive looseness.
Step 5: Confirm the Standard and Nut Type
Check whether the nut is standard, thin, heavy hex, flange, lock, or another specialized design. Verify the material grade and applicable standard if the fastening application requires a specific load capacity or corrosion resistance.
Never force a nut onto a bolt that does not engage smoothly. Mismatched threads can damage both components and reduce joint reliability.
Standard Hex Nut vs. Heavy Hex Nut Dimensions
Standard hex nuts and heavy hex nuts may share the same nominal thread diameter, but their external dimensions and intended applications can differ.
| Feature | Standard Hex Nut | Heavy Hex Nut |
|---|---|---|
| General shape | Six-sided | Six-sided, usually more robust |
| Across-flats width | Depends on standard and size | Commonly larger for the same thread diameter |
| Thickness | Standardized for the specified product | Often greater than a comparable standard nut |
| Typical applications | General machinery, maintenance, everyday fastening | Specified structural and heavy-duty assemblies |
| Material options | Steel, stainless steel, brass, and others | Commonly steel grades selected for demanding applications |
| Selection priority | Compatibility and specified strength | Applicable structural or engineering specification |
A heavy hex nut is not automatically stronger in every application. The complete fastening system must be compatible, including the bolt grade, nut grade, washer requirements, thread engagement, and installation procedure.
For structural work, follow the applicable design documents and fastener specifications rather than selecting a nut based on appearance alone.
Also Read:
Common Types of Hex Nuts and Their Dimensions
Hex nuts are available in several designs. Their external dimensions may be similar, but their thickness, bearing surfaces, locking features, and intended uses differ.
Standard Hex Nuts
Standard hex nuts are used for general fastening applications. They have six flat sides and a threaded central opening. Their dimensions are determined by the applicable product standard and nominal thread size.
They are commonly used in furniture assembly, equipment maintenance, light fabrication, and general mechanical work.
Heavy Hex Nuts
Heavy hex nuts have a larger overall profile than many standard hex nuts of the same thread diameter. They are selected for applications requiring a particular heavy-duty fastener specification, including certain structural assemblies.
Always verify the required nut standard, grade, and matching bolt specification.
Thin Hex Nuts
Thin hex nuts have a reduced thickness compared with standard hex nuts. They are often used as jam nuts or in assemblies with restricted axial space.
Because they provide less thread engagement, their suitability depends on the application and the manufacturer’s specifications.
Nylon Insert Lock Nuts
Nylon insert lock nuts include a polymer insert that increases resistance to loosening. The insert grips the mating threads, helping the fastener resist rotation under suitable service conditions.
The insert can affect installation torque, and its temperature limitations must be considered. Dimensions can differ from those of standard hex nuts.
Flange Hex Nuts
Flange nuts have an integrated flange at one end, providing a wider bearing surface. Some versions feature serrations that help resist rotation when installed against suitable materials.
Their overall outside diameter is larger than that of a comparable standard hex nut, and their flange dimensions should be checked separately.
Acorn or Cap Nuts
Acorn nuts have a closed, domed end that covers the exposed end of a bolt or threaded rod. They are used where a finished appearance or protection from exposed threads is desirable.
Their overall height includes the domed cap, so it should not be compared directly with the thickness of an open-ended standard hex nut.
Also Read:
Metric vs. Imperial Hex Nut Sizes
Metric and imperial nuts use different measurement systems and thread specifications. Although some sizes may appear similar, they are not interchangeable unless the thread specifications are explicitly compatible.
| Feature | Metric Hex Nuts | Imperial Hex Nuts |
|---|---|---|
| Measurement system | Millimeters | Inches |
| Thread identification | M6, M8, M10, M12 | 1/4 in, 3/8 in, 1/2 in |
| Thread pitch | Millimeters between threads | Threads per inch |
| Common coarse-thread example | M10 × 1.5 | 3/8-16 UNC |
| Wrench sizing | Usually millimeter sockets | Usually inch-based sockets |
| Common applications | Metric machinery and equipment | Equipment using Unified inch threads |
For example, an M10 bolt and a 3/8-inch bolt have relatively similar nominal diameters, but their threads are different. An M10 coarse thread commonly uses a 1.5 mm pitch, while a 3/8-16 UNC thread has 16 threads per inch.
Using an incompatible nut can cause cross-threading, poor engagement, and fastening failure. Always match the nominal diameter, pitch, and thread series.
Also Read:
How to Choose the Correct Hex Nut Size
Selecting the right nut involves more than matching its nominal diameter. Consider the following factors before purchasing or installing a fastener.
Match the Bolt Diameter
Start with the bolt’s nominal thread size. An M12 bolt generally requires an M12 nut, while a 1/2-inch bolt requires a compatible 1/2-inch nut.
Do not assume that similar-looking metric and imperial sizes will fit each other.
Match the Thread Pitch
Verify that the thread pitch or threads-per-inch specification matches the bolt. Fine-thread and coarse-thread nuts of the same diameter are not interchangeable.
Check the Wrench Size
Confirm the across-flats measurement so you can select a properly fitting socket or wrench. An incorrect tool can round the nut’s corners or make tightening difficult.
Select the Correct Thickness
Choose standard, thin, or heavy hex nuts according to the required specification and available clearance. Do not use a thin nut as a substitute for a specified structural nut.
Consider Material and Corrosion Resistance
Steel nuts are common for general-purpose fastening. Stainless steel is often selected for corrosion resistance, while brass and other materials are used for particular electrical, decorative, or mechanical applications.
The material should suit the operating environment and be compatible with the mating fastener. Dissimilar metals can introduce galvanic corrosion under certain conditions.
Verify Strength and Application Requirements
For load-bearing, structural, automotive, pressure-related, or safety-critical applications, confirm the required nut grade, bolt grade, coating, and governing standard.
A correct physical fit does not guarantee adequate strength. Follow the manufacturer’s installation instructions and any applicable engineering requirements.
Common Hex Nut Measurement Mistakes
Small measurement errors can result in the wrong fastener or an unreliable connection. Avoid these common problems.
Confusing thread diameter with wrench size: An M10 nut does not typically require a 10 mm wrench; a common standard version uses 17 mm across flats.
Ignoring thread pitch: Nuts with the same nominal diameter may have different thread pitches.
Mixing metric and imperial threads: Similar diameters do not establish compatibility.
Measuring across corners instead of flats: Across-corners width is larger than across-flats width.
Ignoring nut type: Heavy, thin, flange, and locking nuts can have different dimensions.
Assuming every standard is identical: Product standards and manufacturing tolerances can change specific dimensions.
Using an incorrect socket: A loose or oversized socket can damage the nut’s corners.
Choosing by appearance alone: Verify the marking, thread size, material, and grade when these details matter.
A reliable identification process checks several dimensions and the applicable specification rather than relying on a single measurement.
Hex Nut Dimensions Chart: Quick Reference
Use this compact table to review several common metric sizes and their approximate wrench widths.
| Nut Size | Typical Across-Flats Width | Approx. Standard Thickness | Common Coarse Pitch |
|---|---|---|---|
| M4 | 7 mm | 3.2 mm | 0.70 mm |
| M5 | 8 mm | 4 mm | 0.80 mm |
| M6 | 10 mm | 5 mm | 1.00 mm |
| M8 | 13 mm | 6.5 mm | 1.25 mm |
| M10 | 17 mm | 8 mm | 1.50 mm |
| M12 | 19 mm | 10 mm | 1.75 mm |
| M16 | 24 mm | 13 mm | 2.00 mm |
| M20 | 30 mm | 16 mm | 2.50 mm |
| M24 | 36 mm | 19 mm | 3.00 mm |
| M30 | 46 mm | 24 mm | 3.50 mm |
These values are general reference dimensions for common metric hex nuts. Check the applicable standard before using them for manufacturing, engineering calculations, or critical fastening work.
Frequently Asked Questions About Hex Nut Dimensions
What Does M10 Mean on a Hex Nut?
M10 identifies a metric nut designed for a nominal thread diameter of 10 mm. A common coarse-thread version has a pitch of 1.5 mm. Its wrench size and thickness are separate dimensions.
How Do I Know What Wrench Size a Hex Nut Needs?
Measure the distance between two opposite flat faces of the nut. This is the across-flats width and usually corresponds to the required socket or wrench size. Confirm the measurement against the applicable nut standard.
Are Metric and Imperial Hex Nuts Interchangeable?
No, not by default. Metric and imperial fasteners use different thread systems, diameters, and pitches. Always match the nut to the exact bolt thread specification.
What Is the Difference Between Across-Flats and Across-Corners?
Across-flats is the distance between two opposite flat sides of the nut. Across-corners is the distance between opposite vertices. Across-corners is greater than across-flats for a regular hexagon.
Do All M12 Hex Nuts Have the Same Dimensions?
No. Different standards, nut styles, strength classes, and manufacturing tolerances can affect dimensions. A typical standard M12 nut often has a 19 mm across-flats width and approximately 10 mm thickness, but the required product specification should always be checked.
Are Heavy Hex Nuts Stronger Than Standard Hex Nuts?
Not necessarily in every application. Heavy hex nuts have different external dimensions and are designed for specific fastening requirements. Actual performance depends on the material, grade, mating bolt, thread engagement, and applicable standard.
Can I Use a Thin Hex Nut Instead of a Standard Nut?
Only when the application permits it. Thin nuts have less thickness and may provide less thread engagement. They are often used as jam nuts but should not replace specified load-bearing nuts without approval.
How Do I Measure the Thread Pitch of a Hex Nut?
Use a compatible thread-pitch gauge. For metric nuts, the gauge identifies the distance between adjacent thread peaks in millimeters. For imperial nuts, it identifies threads per inch.
Conclusion
A hex nut dimensions chart makes it easier to identify the correct fastener for construction, machinery, automotive maintenance, and general engineering. The most important measurements are nominal thread diameter, thread pitch, across-flats width, and nut thickness.
Metric and imperial nuts must be matched to compatible bolts, and different nut styles can have different external dimensions even when their thread sizes are identical. For general reference, a sizing chart can help narrow down the correct option. For critical applications, always verify the governing standard, material grade, and manufacturer’s specifications before installation.
