Imperial Bolt Sizes - What You Need to Know

An understanding of both imperial and metric measurements is essential when considering bolts and screws. In our latest blog we focus on imperial bolt sizes and measurements.

Here in the UK we have become accustomed to using both imperial and metric units of measurement. On our roads, for example, speed limits and distances are generally defined using miles, rather than the metric equivalent - kilometres. And when we buy our groceries they are generally packaged and sold based on metric measurements.

What are Imperial Measurements?

When talking about imperial measurements we are referring to miles, feet and inches as units of length. Yards, furlongs, hands and chains are also imperial units of length along with fathoms, cables and nautical miles, used in maritime applications. Square miles and acres are commonly used imperial measurements for area.

Imperial measurements for mass and weight include pounds, ounces, stones and tons. And imperial units for volume include pints, gallons, gills, quarts and fluid ounces.

Brief History of Imperial Measurements

It was the British Weights and Measures act of 1824 that clearly defined the imperial system of measurement for the United Kingdom. This was also known as the British Imperial System or Exchequer Standards of 1826, when it became fully adopted.

Since the late 15th century a system known as the Winchester standards had been used. The name refers to the ancient capital of Britain (Winchester) where, in the 10th century, King Edgar the Peaceful kept a royal bushel, known as the Winchester bushel. This was the reference standard for measurement of grain and agricultural produce.

Other Winchester standard units of measurement included gallons, quarts, yards and inches. These measurements were derived from historic units including the mile, which was based on the Roman measurement of 1000 paces. This length was then subdivided into furlongs, chains, yards and inches. The term ‘furlong’, for example, refers to ‘a furrow long’ which was the distance that could be ploughed by an ox without taking a rest.

Introduction of Metric Measurements

The metric system of measurement was first described in 1668 and adopted by France in 1795. The system is based on length measurement in metres with all other metrics, including mass or weight, related to this measurement. A gram, for example, is defined as the mass of one cubic centimetre of water. It was the French Revolution of 1789 that provided the opportunity for France to move away from the preceding collection of confusing units of measurement and adopt the far more sensible metric system, based on multiples of 10.

The adoption of metric measurements in the UK effectively began when an initial proposal was submitted to parliament in 1790. The weights and measures act of 1864 permitted the use of metric units for trade and then, throughout the 19th and 20th centuries, the metric system was progressively adopted. But it wasn’t until 1965 that the British Government formally switched to the metric system.

Which Countries Use the Imperial System?

As of 2023 there are just three countries that officially use imperial units of measurement: the United States, Liberia and Myanmar. But some countries, such as Canada and the UK, use both metric and imperial units.

Although the UK completed it’s transition to the metric system in 1995, imperial units continue to be legal for certain applications including speed limits, road signs and as primary indications of measurement for various items including draught beer and cider, precious metals (by troy ounce) and milk in returnable containers.

Metric and Imperial Nuts and Bolts

Nuts and bolts have the same physical characteristics, regardless of whether they are metric or imperial. The primary differences are the threads and the way in which size measurements are defined.

For more information on metric bolt sizes, please refer to our previous post: Get to Know Metric Bolt Sizes. You can see that a key measurement for metric bolts is the thread pitch. Metric fasteners are available with coarse, fine or even extra fine thread pitch options.

Imperial nuts and bolts use the ‘threads per inch’ (TPI) measurement to describe how close the threads are. Imperial thread sizes typically range from 6 to 40 threads per inch.

Regardless of whether a bolt is either imperial or metric, important bolt measurements include the ‘thread diameter’, both outside and inside. The outside diameter or major diameter of an imperial bolt is measured across the bolt at the crest of the threads. The inside diameter of a bolt, also called the minor diameter or nominal diameter, is the distance across the bolt at the dips between the threads.

When referring to the size of a bolt its important to remember how this refers to the dimensions of the bolt’s shank (threaded or partially-threaded shaft). The size and style of the bolt head determines the size and type of wrench that works with the bolt, along with its application.

Unified Thread Standards

Established in 1949 the Unified Thread Standard was adopted by the UK, USA and Canada to define standards for fastener threads. Two of the most commonly used designations are UNC (unified coarse pitch) and UNF (unified fine pitch).

UNF threads have more threads per unit length (TPI) than UNC. Each form of fastening has optimum applications. For general purpose, lower-tolerance applications, versatile UNC bolts are often used. Whereas UNF bolts, which have higher tensile strength, are more suitable for machine part assembly and higher precision applications.

Another bolt thread standard that is sometimes encountered, especially for vintage vehicles and machinery, is the British Standard. British Standard Whitworth (BSW) refers to coarse threads, British Standard Fine (BSF) describes fine threads.

ISO Standards for Bolts

ISO Standards are internationally agreed standards that define the dimensions, properties and methods of testing for fasteners including bolts, screws and nuts. These standards are developed by the International Organisation for Standardisation (ISO), and they differ from DIN numbers on bolts which is a metric standard established by the Deutsches Institut für Normung (DIN).

The most common bolt ISO standards include:

• ISO 898-1:2013 - Specifies the mechanical and material properties of bolts, screws and studs made of carbon steel and alloy steel used in bolting assemblies. It also specifies the methods for verifying these properties.
• ISO 898-2:2009 - Standard applies to bolts, screws and studs made of austenitic or austenitic-ferritic stainless steel used in bolting assemblies. It specifies the mechanical and material properties of these fasteners, as well as the methods for verifying these properties.
• ISO 3506-1:1997 - Standard applies to stainless steel bolts, screws and studs used in bolting assemblies. It specifies the mechanical and material properties of these fasteners, as well as the methods for verifying these properties.
• ISO 3506-2: Corrosion-resistant stainless steel fasteners
• ISO 965-1: General table of tolerances
• ISO 1042: Chemical analysis of steels

Bolts that conform with these standards are often marked. Look for a raised or indented indication on the bolt head.

Imperial Bolt Sizes

Imperial bolt sizes are generally defined using two key dimensions, the diameter and length of the bolt shank. The bolt head size and style are also important, but the bolt shaft dimensions are primary bolt size attributes. For example a ¼ inch bolt refers to a bolt with a minor diameter or nominal diameter of ¼ inch.

Other important bolt dimensions and attributes include:

• Length (the length of the bolt shank or shaft)
• Type (bolt, nut or screw)
• Strength
• Material and finish
• Tensile properties
• Thread length (full or partial).
• Threads per inch (TPI)

Imperial bolts are sometimes described using a fraction, followed by a hyphen, followed by a number. E.g. ¼-20. The fraction refers to the diameter of the bolt shaft as a fraction of an inch and the number after the hyphen defines the threads per inch. So a ¼-20 bolt is a quarter inch bolt shaft nominal diameter with 20 threads per inch.

Its worth noting that UNF bolts are often described by their spanner size rather than the diameter of the bolt shaft. Please see the table below.

Bolt Head Types

Bolts and screws are available with a huge variety of head shapes and types. The bolt head type and size determines the tool necessary to tighten and loosen the bolt. Hex head bolts are the most common but you will also sea square head bolts, Allen bolts, coach bolts and a wide variety of specialised bolt types used for specific applications.

We’ve previously described many of the most common bolt types in our bolt selection guide.

Are Imperial and Metric Bolts Interchangeable?

The question of whether an imperial bolt can be replaced with a metric one often arises. The thread profiles and pitches differ between imperial and metric fasteners so metric bolts should not be used to replace imperial fasteners and imperial bolts should never be used to replace metric ones.

Metric and imperial fasteners are not interchangeable and thread damage is likely to occur if they are mixed.

How to Measure Bolt Size

Standard or imperial bolt sizes are measured in inches while metric bolts are measured in millimetres. To accurately measure the size of a bolt you can use a calliper along with a useful bolt size gauge.

This comprehensive bolt size guide, produced by Sealy, is a good example. Some bolt size guides offer both metric and imperial measurements. To determine the bolt shaft outer or major diameter you simply find the smallest hole through which the bolt shaft will pass. The calliper can be used to determine both the threads per inch (TPI), the thread pitch and the size of spanner that works with the bolt head. 

Where are Imperial Bolts Used?

Imperial bolts continue to be widely used in the USA and the UK along with some specialised applications worldwide. Aircraft, agricultural machinery, vintage vehicles and equipment are all examples of where you will find standard imperial fastenings, reliably holding things together.

Imperial Bolt Threads and Spanner Sizes

As previously noted, UNF (unified fine pitch) bolts can often be described by their required spanner size and not by the bolt shaft diameter. Here’s a useful reference for both BSF (British Standard Fine) and UNF imperial bolt dimensions and associated spanner sizes.

Imperial Bolt Thread Lengths

Here are the standard thread lengths for imperial bolts - in inches.

 Clearance Hole Sizes for Imperial Bolts

Clearance holes need to be drilled large enough to allow a bolt to pass through. Clearance holes might be close fit or free fit, depending on requirements. Close fit is appropriate for tighter tolerances while a free, looser fit can simplify assembly.

The following table provides clearance hole sizes for imperial bolts.

 Conversion Charts - Metric to Imperial - Imperial to Metric

Follow this link to find metric to imperial conversion charts along with a downloadable PDF version.

For more information on Imperial Bolt Sizes - What You Need to Know talk to Insight Security