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When you’re shopping for bolts or preparing for a big project, understanding bolt measurements can be confusing. One of the most common questions I get is: Does the length of a bolt include the head? Let’s tackle that in this article and clear things up for good.
For most bolts, the length is measured from under the head to the tip, excluding the head itself. However, for countersunk bolts and socket screws, the head thickness is included in the length. Carriage bolts are measured from under the head, not the square neck.
You might be asking, “How do I measure bolts the right way?” Don’t worry, it’s easier than it sounds! In this guide, I’ll walk you through the basics of bolt sizes and how to measure them. Whether you’re an expert or just getting started, you’ll be able to measure bolts accurately every time.
How do you measure the length of a hex bolt?
Hex Bolts: For hex bolts, the length is measured from under the head to the tip. This excludes the head, giving you only the usable shank length.
How do you measure the length of a countersunk bolt?
Countersunk Bolts: For countersunk bolts, the length measurement includes the head, as these bolts are designed to sit flush with the surface.
How do you measure the length of a carriage bolt?
Carriage Bolts: Carriage bolt length is measured from the flat under the head to the end, including the square neck portion. This ensures accuracy in applications requiring a flush finish.
Bolt Dimensions Explained
Understanding bolt dimensions is crucial for selecting the right fastener for any job. It’s not just about choosing the correct length, but also understanding the other critical measurements that affect the bolt’s fit and function. Here’s a rundown of the key bolt dimensions.
- Bolt Diameter
The diameter of a bolt refers to its shank diameter or major diameter. This is measured from the outer edge of the threads on one side to the outer edge on the opposite side. You can use calipers or a bolt gauge to measure it accurately. For bolts like hex bolts, this measurement is key to ensuring it fits into the right hole or fixture. - Bolt Length
For most bolts, like hex bolts, length is measured from the underside of the head to the tip of the bolt. However, for countersunk bolts (such as carriage bolts), you measure from the top of the head to the end of the threaded portion. The idea is that countersunk bolts are designed to sit flush with the surface, so including the head’s thickness is important. - Bolt Thread Pitch
Thread pitch refers to the distance between threads, usually given as threads per inch for standard bolts, or in millimeters for metric bolts. For instance, a bolt might be described as having a thread pitch of 6 threads per inch or 1.0 mm for metric sizes.
- Bolt Head Size and Shape
The head size and shape can vary widely. Common shapes include hex, square, and dome-shaped heads. The size and shape affect the torque you can apply to the bolt and how well it fits into a wrench or socket. - Bolt Grade
The grade of a bolt indicates its strength and material properties. For example, the markings on the head of the bolt will tell you what material it’s made from and its tensile strength. This helps you determine which bolt to use based on the specific demands of your project. - Bolt Thread Length
This specifies how long the threaded portion of the bolt is. You’ll need to know whether you want a fully threaded bolt or a partially threaded one for your application.
Example Specifications
For metric bolts, the dimensions are often provided like this:
M8-1.25 x 20
Where:
- M8 is the nominal diameter of 8 mm
- 1.25 mm is the thread pitch
- 20 mm is the length
For standard bolts, the dimensions might look like this:
1/2″-13 x 3
Where:
- 1/2″ is the diameter
- 13 is the threads per inch
- 3″ is the length
What Tool Do You Use to Measure Bolts?
Measuring bolts is not as easy as just using a ruler. To get the right measurements, you need the right tools. There are a few important tools that can help you measure bolts accurately.
- Calipers
Calipers are a go-to tool when it comes to measuring the diameter of a bolt, as well as the length of its shank. They can measure both internal and external dimensions, and digital calipers make the process even easier by giving you an accurate reading. - Bolt Gauge
A bolt gauge is a specialized tool that helps you quickly determine the size of a bolt. It comes with a range of holes, each corresponding to a different diameter of bolt. Simply insert the bolt into the holes to find the correct size. Some bolt gauges even include a thread pitch gauge. - Thread Pitch Gauge
If you need to measure the thread pitch of a bolt, a thread pitch gauge is essential. This tool has blades or notches that match different thread pitches. It’s particularly useful for determining the threads per inch (for standard bolts) or the distance between threads (for metric bolts). - Ruler or Tape Measure
In a pinch, a ruler or tape measure can do the job for measuring the length and diameter of bolts. However, it won’t give you the precision of calipers or a bolt gauge, so use these only when the other tools aren’t available. - Specialized Tools
There are also all-in-one tools, like the Gorilla Tool, which combines bolt sizing, thread pitch gauging, and grade identification into one device. This can be really helpful if you need to measure multiple bolts quickly without needing several separate tools.
How to Measure the Size of a Bolt?
Measuring a bolt is easy once you have the right tools. Here’s a simple, step-by-step guide to help you get the correct measurements every time.
- Measure the Bolt Shank Diameter
Start by measuring the shank diameter of the bolt, which is the smooth part without the threads. Use calipers or a bolt gauge to do this accurately. - Determine the Bolt Thread Pitch
Next, measure the thread pitch. You can use a thread pitch gauge to determine how many threads per inch (for standard bolts) or the distance between threads (for metric bolts).
- Measure the Bolt Length of the Shank
For most bolts, you measure from where the shank meets the head to the tip of the bolt. For countersunk bolts, you measure from the top of the head to the end of the threaded portion. - Determine Bolt Grade
Check the markings on the bolt’s head to determine its grade. This tells you the material and strength properties, which are crucial for selecting the right bolt for your application. - Identify Bolt Head Shape
The shape of the bolt’s head is another important factor. Common shapes include hex, square, and dome-shaped heads. Knowing the shape helps ensure you’re using the right tools for installation. - Measure Bolt Thread Length
Finally, measure the length of the threaded portion of the bolt. This is essential if you need a bolt with specific thread engagement.
Conclusion
Measuring bolts might seem like a small detail, but it’s really important for making sure everything fits and works correctly in your project. Whether you’re using hex bolts, countersunk bolts, or any other type, knowing how to measure bolts properly can save you a lot of time and hassle. If you’re ever unsure about the size, use the right tools or ask an expert for help. And remember, the length of the bolt usually doesn’t include the head—except for countersunk bolts.
If you want to get more detailed insights into bolts and fasteners, feel free to reach out to me directly at info@hrfastener.com.
