How do spanner sizes work?

How do spanner sizes work? - Fix It Cape Town

Spanners and wrenches are essential tools used in various industries and for a wide range of tasks. Whether you are a professional mechanic or a DIY enthusiast, understanding the measurement system used to size these tools is crucial. Knowing how spanner sizes relate to the dimensions of nuts and bolts they can accommodate will help you work efficiently and avoid any mishaps during your projects. In this article, we will dive deeper into this topic, providing you with valuable insights and knowledge.

The Basics of Spanner Sizing

Before we delve into the measurement system, let’s start with the basics. Spanners, also known as wrenches in some regions, are hand tools designed to provide leverage and grip for turning fasteners such as nuts, bolts, and screws. They come in various sizes and shapes to accommodate different types of fasteners.

The Measurement System: Metric vs. Imperial

Spanner sizes are divided into two main systems: metric and imperial. The metric system is predominant globally, while the imperial system is commonly used in the United States and a few other countries. Each system has its own set of measurements and standards.

Metric System

In the metric system, spanner sizes are measured in millimeters (mm). The size of a spanner refers to the distance across two opposite flat sides of the jaws, also known as the “opening size.” For example, a spanner labeled as 10mm means it can accommodate nuts or bolts with a width of 10mm.

The most common metric spanner sizes range from 6mm to 32mm, with each size incrementally increasing by 1mm.

Imperial System

In the imperial system, spanner sizes are measured in inches. Just like the metric system, the size of a spanner refers to the opening size. However, instead of millimeters, the measurement is expressed in fractions of an inch or using decimal inches.

Common imperial spanner sizes range from 1/4 inch to 2 inches. The increments between sizes can vary, typically being either 1/16 or 1/32 of an inch.

Understanding the Relationship between Spanner Size and Nuts/Bolts Dimensions

The size of a spanner directly correlates with the dimensions of the nuts and bolts it can accommodate. If you have the wrong size spanner, it may not fit correctly, leading to inefficiency and potential damage to the fasteners.

Metric System Compatibility

In the metric system, the opening size of a spanner should match the width across the flats of the nut or bolt. For example, if you have a 10mm spanner, it will fit a nut or bolt with a width of 10mm across the flats.

Imperial System Compatibility

In the imperial system, it’s important to consider the compatibility between the spanner size and the nut or bolt’s width across the flats. A 3/8 inch spanner, for instance, fits a nut or bolt with a width of 3/8 inch across the flats.

It’s worth noting that spanners are typically designed with a slightly larger opening size to accommodate variations in fastener dimensions. This allows for a better fit and reduces the risk of rounding off the nuts or bolts.

Tips for Selecting the Right Size Spanner

Choosing the correct size spanner is crucial to ensure smooth operations and prevent damage to your fasteners. Here are some tips to help you select the right size:

  1. Know your fasteners: Take note of whether your nuts and bolts use metric or imperial sizing, as this will determine the compatible spanner measurement.

  2. Use measurement tools: If you’re uncertain about the size of a nut or bolt, use a vernier caliper or a ruler to measure the width across the flats accurately. This will help you determine the appropriate spanner size.

  3. Try gradually larger spanners: If you’re unsure about the exact size, start with a smaller spanner and work your way up gradually until you find the one that fits precisely.

  4. Consider adjustable spanners: Adjustable spanners, also known as crescent wrenches, can be a versatile option as they offer a range of sizes within a single tool. However, be cautious when using them on delicate or high-torque applications, as they may not provide the same level of grip and stability as fixed-size spanners.


Q: Can I use a metric spanner on imperial-sized nuts and bolts?

A: It is generally not recommended to use a metric spanner on imperial-sized fasteners, as the sizes will not match accurately. Doing so could lead to damaged nuts or bolts.

Q: What about mixing and matching metric and imperial spanners?

A: While it’s not optimal, in some situations, you may need to use a spanner from one system on a fastener from the other. In such cases, try to find the closest matching size and use caution while applying torque to avoid damage.

Q: Are there spanners available in both metric and imperial sizes?

A: Yes, there are some spanners available with dual sizing, catering to both metric and imperial dimensions. These can be useful if you work with a mix of fasteners from different systems.


Understanding the measurement system used to size spanners and wrenches is fundamental for anyone working with nuts and bolts. Whether you’re dealing with metric or imperial fasteners, matching the correct spanner size is essential to ensure effective operations and prevent damage to your tools and materials. Remember to always choose the appropriate spanner size by considering the system of measurement, and if in doubt, use measurement tools to accurately determine the width across the flats. With the right size spanner, you can confidently tackle your projects and achieve optimal results.

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