Imperial Bolt Size Chart: Standard Sizes, Threads & Grades

An imperial bolt size chart is a useful reference for identifying bolts measured using the inch-based system. Imperial bolts are commonly used in the United States, Canada, older machinery, automotive equipment, industrial systems, construction, agricultural machinery, and mechanical applications.

Unlike metric bolts, which are identified using millimeters, imperial bolts are generally identified using inches, fractions, or numbered sizes. Common imperial bolt sizes include #0, #1, #2, #4, #6, #8, #10, #12, 1/4 inch, 5/16 inch, 3/8 inch, 7/16 inch, 1/2 inch, 5/8 inch, 3/4 inch, 7/8 inch, and 1 inch.

Imperial bolt identification can initially seem complicated because both fractional sizes and numbered screw sizes are used. Thread standards such as UNC, UNF, and UNEF also affect the thread pitch.

This complete guide explains imperial bolt sizes, bolt diameter, thread pitch, threads per inch, bolt head sizes, UNC and UNF threads, bolt grades, wrench sizes, and how to measure an imperial bolt correctly.

Imperial Bolt Size Chart

The following chart provides common imperial bolt diameters and thread information.

The chart shows commonly encountered thread pitches. Exact thread availability can depend on the bolt specification and diameter.

What Is an Imperial Bolt?

An imperial bolt is a threaded fastener whose dimensions are specified using the inch-based measurement system.

An imperial bolt may be identified using:

• Fractional diameter
• Numbered diameter
• Threads per inch (TPI)
• Thread series
• Bolt length
• Grade or material
• Head style

For example:

1/2-13 × 2 in UNC

means:

• 1/2-inch nominal diameter
• 13 threads per inch
• UNC thread
• 2-inch bolt length

Understanding this naming system makes imperial bolt selection much easier.

Imperial Bolt Sizes in Inches

The most commonly encountered larger imperial bolt sizes include:

• 1/4 inch
• 5/16 inch
• 3/8 inch
• 7/16 inch
• 1/2 inch
• 9/16 inch
• 5/8 inch
• 3/4 inch
• 7/8 inch
• 1 inch
• 1-1/8 inch
• 1-1/4 inch
• 1-3/8 inch
• 1-1/2 inch
• 1-3/4 inch
• 2 inches

The nominal bolt diameter refers to the approximate outside diameter of the bolt’s threaded section.

Imperial Bolt Diameter Chart

The metric values are conversions of the nominal imperial diameters.

Understanding Threads Per Inch

Threads per inch (TPI) describes the number of thread peaks found along one inch of bolt length.

For example:

1/2-13

means the bolt has:

• 1/2-inch nominal diameter
• 13 threads per inch

A smaller TPI number means the threads are coarser.

A larger TPI number means the threads are finer.

For example:

• 1/2-13 UNC = coarse
• 1/2-20 UNF = fine

Both bolts have a 1/2-inch nominal diameter, but their thread pitches are different.

They are therefore not directly interchangeable.

UNC Imperial Bolt Sizes

UNC means Unified National Coarse.

UNC is one of the most common imperial thread series.

Typical UNC thread specifications include:

UNC threads are commonly used in general mechanical and structural applications.

UNF Imperial Bolt Sizes

UNF means Unified National Fine.

UNF threads have more threads per inch than the corresponding UNC thread.

Fine threads are useful in applications where thread engagement, adjustment, or particular mechanical characteristics are important.

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UNC vs UNF Bolt Threads

UNC and UNF are both part of the Unified Thread Standard, but they use different thread pitches.

For example:

1/2-13 UNC

has 13 TPI.

1/2-20 UNF

has 20 TPI.

The outside diameter is the same, but the thread spacing is different.

Therefore, a 1/2-13 nut should not be forced onto a 1/2-20 bolt.

Doing so can damage the threads and create an unsafe connection.

Imperial Bolt Head Size Chart

Bolt head size is different from bolt diameter.

A bolt can have a 1/2-inch threaded diameter but require a larger wrench across the hex head.

Common hex-head dimensions vary according to the bolt standard and product specification.

Head dimensions can vary between standards and bolt types, so these values should be treated as common reference dimensions.

Imperial Bolt Length Chart

Bolt length is measured differently depending on the bolt head style.

For most hex-head bolts, length is measured from under the head to the end of the bolt.

Common lengths include:

• 1/2 inch
• 3/4 inch
• 1 inch
• 1-1/4 inches
• 1-1/2 inches
• 2 inches
• 2-1/2 inches
• 3 inches
• 3-1/2 inches
• 4 inches
• 5 inches
• 6 inches

For example:

1/2-13 × 2

means a 1/2-inch diameter bolt with 13 TPI and a 2-inch length.

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Imperial Bolt Size Identification

To identify an unknown imperial bolt, measure the following:

1. Measure the Diameter

Use a caliper to measure the outside diameter of the threaded section.

2. Determine the Thread Pitch

Use a thread pitch gauge or count the number of threads across one inch.

3. Determine the Thread Series

Compare the measured TPI with UNC, UNF, UNEF, or another applicable thread standard.

4. Measure the Length

Measure from the underside of the head to the end of the bolt for most hex-head bolts.

5. Check the Head Markings

Head markings can provide information about bolt grade or manufacturer.

Imperial Bolt Grade Chart

Imperial bolts can be manufactured in different strength grades.

Common SAE grades include:

• Grade 2
• Grade 5
• Grade 8

Higher-strength grades generally have different head markings and mechanical properties.

Grade 2

Grade 2 bolts are commonly used for general-purpose applications where high strength is not required.

Grade 5

Grade 5 bolts are widely used in automotive and mechanical applications.

They generally have three radial lines on the head.

Grade 8

Grade 8 bolts are high-strength fasteners commonly used in demanding mechanical applications.

They generally have six radial lines on the head.

The exact mechanical properties should be confirmed using the applicable fastener specification.

Imperial Bolt Grade vs Size

Bolt size and bolt grade describe different characteristics.

For example:

1/2-13 Grade 5

contains:

• 1/2-inch diameter
• 13 TPI
• Grade 5 strength classification

A larger bolt is not automatically a stronger bolt in every application, and a higher-grade bolt does not automatically mean it is suitable for every joint.

The correct fastener should be selected based on the engineering requirements of the application.

Imperial Bolt and Nut Compatibility

An imperial bolt must be matched with a compatible nut.

For example:

1/2-13 UNC bolt → 1/2-13 UNC nut

A:

1/2-20 UNF nut

will not correctly fit a:

1/2-13 UNC bolt

even though both are nominally 1/2 inch.

The diameter and thread pitch must match.

Imperial Bolt Size to Metric Conversion

Imperial and metric bolts are not exact equivalents.

For general reference:

These are mathematical conversions, not recommendations for substituting metric fasteners.

Also Read:   How to Measure Bolt Size ( Complete Guide )

Can Imperial and Metric Bolts Be Interchanged?

Generally, imperial and metric bolts should not be treated as interchangeable.

For example:

• 1/2 inch = 12.7 mm
• M12 = 12 mm

Although these sizes are relatively close, their thread specifications are different.

Similarly, a 1/2-inch bolt may use 13 TPI or 20 TPI, while an M12 bolt may use a metric pitch such as 1.75 mm.

Forcing an almost-matching bolt or nut can damage the threads and compromise the joint.

Imperial Bolt Thread Designations

An imperial bolt specification may look like:

3/8-16 × 2 UNC

This can be interpreted as:

• 3/8 = nominal diameter
• 16 = threads per inch
• 2 = bolt length
• UNC = Unified National Coarse

Another example:

1/2-20 × 1-1/2 UNF

means:

• 1/2-inch diameter
• 20 TPI
• 1.5-inch length
• UNF thread

This format makes it easier to order the correct fastener.

Common Imperial Bolt Sizes

The following are some of the most frequently encountered imperial bolt sizes:

1/4-Inch Bolt

A common small fractional bolt used in machinery, equipment, brackets, and general assembly.

5/16-Inch Bolt

Commonly used for mechanical assemblies and medium-duty applications.

3/8-Inch Bolt

One of the most common general-purpose imperial bolt sizes.

1/2-Inch Bolt

Widely used in automotive, machinery, structural, and industrial applications.

5/8-Inch Bolt

Used for heavier mechanical and structural connections.

3/4-Inch Bolt

Common in larger mechanical and structural assemblies.

1-Inch Bolt

Used where larger fasteners and higher clamping forces are required.

Imperial Bolt Size Chart for Quick Reference

Imperial Bolt Size Chart for Wrench Selection

When working with imperial bolts, remember that bolt diameter and wrench size are different measurements.

For example, a 1/2-inch bolt commonly uses a 3/4-inch wrench or socket for a standard hex-head configuration.

Always verify the head style and applicable standard because wrench sizes can vary.

How to Choose the Correct Imperial Bolt

Choosing the correct imperial bolt requires more than selecting the diameter.

Check these specifications:

1. Bolt diameter
2. Thread pitch
3. Thread series
4. Bolt length
5. Head style
6. Strength grade
7. Material
8. Finish or coating
9. Application requirements

For example, if you need a 1/2-inch bolt, first determine whether the application requires 1/2-13 UNC or 1/2-20 UNF.

Then determine the required length and grade.

Frequently Asked Questions About Imperial Bolt Sizes

What is an imperial bolt size chart?

An imperial bolt size chart lists bolt dimensions using the inch-based system, including diameter, thread pitch, TPI, bolt length, and sometimes head dimensions.

What are the most common imperial bolt sizes?

Common sizes include 1/4, 5/16, 3/8, 7/16, 1/2, 5/8, 3/4, 7/8, and 1 inch.

What does TPI mean on an imperial bolt?

TPI means threads per inch. It indicates how many thread peaks occur within one inch of bolt length.

What does UNC mean?

UNC means Unified National Coarse, a common coarse imperial thread series.

What does UNF mean?

UNF means Unified National Fine, a finer imperial thread series.

What is the difference between 1/2-13 and 1/2-20?

Both bolts have a 1/2-inch nominal diameter, but 1/2-13 has 13 threads per inch while 1/2-20 has 20 threads per inch.

Is a 1/2-inch bolt the same as an M12 bolt?

No. A 1/2-inch bolt has a nominal diameter of 12.7 mm, while an M12 bolt has a nominal diameter of 12 mm. Their thread systems are also different.

How do I identify an imperial bolt?

Measure its diameter, determine the thread pitch or TPI, identify the thread series, measure its length, and inspect its head markings.

What are common imperial bolt grades?

Common SAE grades include Grade 2, Grade 5, and Grade 8.

Which is stronger, Grade 5 or Grade 8?

Grade 8 generally has higher specified strength than Grade 5, but the correct grade depends on the application and engineering requirements.

Final Imperial Bolt Size Reference

An imperial bolt size chart makes it easier to identify and select inch-based fasteners. The most important information is the nominal diameter, threads per inch, thread series, and bolt length.

Common imperial sizes range from small numbered fasteners such as #6, #8, #10, and #12 to larger fractional sizes such as 1/4 inch, 5/16 inch, 3/8 inch, 1/2 inch, 5/8 inch, 3/4 inch, 1 inch, and larger.

When selecting an imperial bolt, remember that diameter alone does not identify the complete fastener. A 1/2-inch bolt could have different thread specifications, such as 1/2-13 UNC or 1/2-20 UNF.

For accurate fastener selection, always verify:

• Nominal diameter
• Decimal diameter
• Threads per inch
• UNC or UNF thread
• Bolt length
• Head dimensions
• Bolt grade
• Material
• Coating
• Nut compatibility

Using the correct imperial bolt size, thread pitch, grade, and length helps ensure proper fit, clamping performance, and reliable mechanical connections.