Choosing the right bolt hole size can leave you dumbfounded, especially when there’s so many different kinds of holes to use in the first place!
What’s more, bolt holes are so essential for ensuring bolts fit securely into the material being fastened.
If you don’t, selecting the wrong size can lead to a range of issues like structural failure, loosening, and damage to the bolts.
Yeah, your honey-do list or favorite hobby can turn sour in a heart-beat.
But, learning how to choose the correct size and type of bolt hole is not as complicated as it seems!
By using a bolt hole size chart, you can easily determine the appropriate size and type of bolt hole for your project.
You just need a little guidance up front before you turn to the charts…
In this article, we’ll explore everything you need to know about bolt hole sizing, charts and how they can help you get your project right.
Types of Bolt Holes
Let’s start by looking at the main types of bolt holes in the world, as they do play a big part in how to size them against your bolt diameter and thread sizes.
Standard Holes
It makes the most sense to start with the most common type: Standard holes.
These are the go-to option for most bolt applications, as they’re drilled to fit the bolt’s diameter without any excess space.
You’ll find them as the typical choice for straightforward bolted connections, from wood and steel to anchor bolt holes in concrete.
You’re no doubt already familiar with these sizings since they follow industry standards for consistency in bolt installation.
Now, let’s get on to the sizing values and bolt hole charts for these.
Clearance Holes
Clearance holes is all about wiggle room – they’re slightly larger than your bolt’s diameter.
This extra space is what allows for easier assembly, as the bolt can smoothly slide in and out of the hole.
Don’t worry, though…
Clearance holes serve their purpose well, providing a solid connection when the bolt is tightened.
Clearance hole sizes differ depending on the bolt dimensions and desired fit (close or free). More on this in a bit!
Pilot Holes
You’ll definitely want to familiarize yourself with pilot holes.
When working with materials like wood or soft metals, these smaller pre-drilled holes act as a guide, helping you position your screw or bolt with precision.
They also reduce the risk of your material splitting or a fastener like a bolt breaking off if you’re applying excessive torque to tighten it down.
Oversized Holes
Sometimes, you need a bit more room to accommodate manufacturing tolerances, alignment issues, or thermal expansion. Fancy, I know…
But that’s where oversized holes come in really handy.
These larger-than-necessary holes provide that extra space, ensuring your bolt fits, especially when there’s a need for adjustment or when multiple bolts need to be aligned before final tightening.
Slotted Holes
In the case of slotted holes, it’s all about position movement allowance between what you’re bolting together.
With an elongated shape, these holes permit slight adjustments or shifting after assembly.
Whether it’s due to load changes or the need for a more adaptable connection, slotted holes are the way to go for the greatest flexibility.
These are categorized by either long-slotted or short-slotted types.
The former is just more elongated as compared to a short-slotted hole that has a limited range of adjustment for the bolt.
Standard Bolt Hole Sizes Charts
Here’s the nominal bolt hole dimensions for standard sizes:
Imperial
Metric
As we covered above, a clearance hole is simply a hole drilled through one piece of material that allows a bolt to pass through freely without threading or binding.
You might not think you need to be precise here but correct clearance hole sizing is crucial to prevent bolt breakage, thread damage, and other issues.
Clearance hole sizes depend on factors such as thread pitch, bolt diameter, and material type.
Standard Clearance Hole Sizes for Steel Bolts
When it comes to a reference for determining the correct hole sizes, organizations like the American Institute of Steel Construction (AISC) and the European Norms (Eurocodes) provide clearance hole sizes to help.
A good chart like the one below includes both the recommended hole sizes as well as the tapping bit based on the size of the bolts.
It’s a good practice to rely on these established standards because they’ll give you the best bolt performance and that all-important structural integrity.
Imperial
| Bolt Size (in) | Clearance Hole Size (in) | Tap Drill Size (in) |
|---|---|---|
| 1/2 | 0.531 | 0.703 |
| 5/8 | 0.672 | 0.828 |
| 3/4 | 0.781 | 0.969 |
| 7/8 | 0.906 | 1.125 |
| 1 | 1.031 | 1.281 |
| 1 1/8 | 1.156 | 1.438 |
| 1 1/4 | 1.281 | 1.594 |
| 1 3/8 | 1.406 | 1.750 |
| 1 1/2 | 1.531 | 1.906 |
Metric
| Bolt Size (mm) | Clearance Hole Size (mm) | Tap Drill Size (mm) |
|---|---|---|
| M6 | 6.8 | 5.0 |
| M8 | 9.0 | 6.8 |
| M10 | 11.5 | 8.5 |
| M12 | 14.0 | 10.2 |
| M14 | 16.0 | 11.5 |
| M16 | 18.0 | 13.5 |
| M18 | 20.0 | 15.0 |
| M20 | 22.0 | 16.5 |
| M22 | 24.0 | 18.0 |
| M24 | 26.0 | 19.5 |
Minimum Clearance Hole Sizes for Different Materials and Screws
When you’re working with different materials and screws, you want to choose the right clearance hole size.
For a free fit, which allows for easy assembly and disassembly, it’s no surprise that you’ll be using larger clearance holes.
Looking further, you can check out the American Institute of Steel Construction (AISC) guidelines if you’re working with steel materials, as the AISC provides specifications tailored to steel clearance hole sizes.
Remember, the size of your clearance hole can vary based on the material and type of screw you’re using.
So, always double-check the appropriate guidelines and make sure you choose the correct size for a hassle-free project.
Maximum Clearance Hole Size for Steel Bolts
When you’re working with steel bolts, you’ll want to understand the maximum clearance hole size as well.
This ensures a proper fit and prevents issues with structural connections.
Bolt thread sizes can vary, and they’re available in both Unified National Fine (UNF) and Unified National Coarse (UNC) threads.
The thread pitch, measured in millimeters, represents the distance between thread crests.
The clearance hole size depends on both the diameter of the screw and the class of fit, which includes Close Fit and Free Fit.
Oversized and slotted holes are sometimes used in structural steel connections to accommodate tolerances and allow for adjustments during assembly.
For structural joints, ASTM A325 and A490 bolts are commonly used and have specific clearance hole requirements.
Keep in mind, the maximum clearance hole size largely depends on thread sizes, bolt diameter, and the material being connected.
So, pay attention to these factors when selecting the right clearance hole size for your steel bolt connections.
Clearance Bolt Hole Size Charts
The table above provides the clearance hole sizes for imperial bolts and screws.
It includes the nominal bolt size, as well as the close fit, medium fit, and free fit sizes.
Imperial
| Bolt Diameter (in) | Close Fit Min (in) | Close Fit Max (in) | Medium Fit Min (in) | Medium Fit Max (in) |
|---|---|---|---|---|
| 1/4 | 0.253 | 0.257 | 0.266 | 0.272 |
| 5/16 | 0.315 | 0.319 | 0.328 | 0.334 |
| 3/8 | 0.378 | 0.382 | 0.391 | 0.397 |
| 7/16 | 0.440 | 0.444 | 0.453 | 0.459 |
| 1/2 | 0.503 | 0.507 | 0.516 | 0.522 |
| 9/16 | 0.565 | 0.569 | 0.578 | 0.584 |
| 5/8 | 0.628 | 0.632 | 0.641 | 0.647 |
| 3/4 | 0.753 | 0.757 | 0.766 | 0.772 |
| 7/8 | 0.878 | 0.882 | 0.891 | 0.897 |
| 1 | 1.003 | 1.007 | 1.016 | 1.022 |
| 1 1/8 | 1.128 | 1.132 | 1.141 | 1.147 |
| 1 1/4 | 1.253 | 1.257 | 1.266 | 1.272 |
| 1 3/8 | 1.378 | 1.382 | 1.391 | 1.397 |
| 1 1/2 | 1.503 | 1.507 | 1.516 | 1.522 |
Metric
| Bolt Diameter (mm) | Close Fit Min (mm) | Close Fit Max (mm) | Medium Fit Min (mm) | Medium Fit Max (mm) |
|---|---|---|---|---|
| 6.35 | 6.429 | 6.530 | 6.759 | 6.911 |
| 7.938 | 7.938 | 8.105 | 8.331 | 8.487 |
| 9.525 | 9.606 | 9.702 | 9.931 | 10.080 |
| 11.113 | 11.176 | 11.278 | 11.506 | 11.662 |
| 12.7 | 12.776 | 12.882 | 13.116 | 13.269 |
| 14.288 | 14.326 | 14.432 | 14.664 | 14.817 |
| 15.875 | 15.875 | 15.981 | 16.278 | 16.432 |
| 19.05 | 19.119 | 19.223 | 19.461 | 19.612 |
| 22.225 | 22.301 | 22.405 | 22.636 | 22.787 |
| 25.4 | 25.403 | 25.607 | 25.806 | 25.974 |
| 28.575 | 28.654 | 28.858 | 29.013 | 29.191 |
| 31.75 | 31.826 | 31.930 | 32.132 | 32.388 |
| 34.925 | 34.925 | 35.029 | 35.339 | 35.495 |
| 38.1 | 38.102 | 38.306 | 38.507 | 38.685 |
Tips When Using a Clearance Hole Chart
When you’re working with bolts, it’s essential to get the clearance hole diameter right.
This size depends on factors like the bolt’s diameter and the material you’re working with.
Here’s how you can quickly calculate the perfect clearance hole diameter for your needs.
First, determine whether you’re working with metric or imperial bolts. They come in various sizes and correspond to different thread pitches.
For example:
Metric bolts have diameters like M6, M8, and M10, while imperial bolts are typically sized in inches such as 1/4″, 3/8″, or 1/2″.
Next, take into account the bolt dimensions and materials used.
The clearance hole diameter should be larger than the bolt’s outer diameter, providing enough space for easy assembly and minimizing stress on the any joining materials.
Don’t forget that material plays a role too.
Softer materials may require slightly larger clearance holes to prevent deformation during assembly.
Ultimately, using a chart to find the appropriate diameter for a clearance hole involves considering the bolt size, thread pitch, and material properties.
By keeping these factors in mind, you’ll ensure a smooth and secure assembly process.
Oversized Hole Sizes Chart
You might encounter situations where oversized holes are needed.
These are holes larger than the diameter of the bolt. They can be useful for fully-pretensioned bolts, but should have engineering approval.
When drilling these holes, it’s good to start with a pilot hole. It’s essentially a smaller hole you make as a guide for the final hole size.
| Bolt Diameter (in) | Hole Dimensions |
|---|---|
| Oversize (Dia) | |
| 1/8 | 5/8 |
| 5/8 | 13/16 |
| 3/4 | 15/16 |
| 7/8 | 1 1/16 |
| 1 | 1 1/4 |
| >1 1/8 | d + 5/16 |
Choosing the Right Drill Bit for Pilot and Oversized Holes
When selecting a drill bit for pilot and oversized holes, you’ll want to consider the bolt’s material and the desired fit.
Here’s how you can make the right choice for your project:
First, determine the bolt’s thread form and tolerances based on its tpi (threads per inch) using our tap drill chart.
Next, check the bolt head size and the minor diameter – this is important for proper fit.
If you’re working with a metric bolt, consult the metric clearance hole chart above.
Also, consider the pitch diameter, which affects the performance of your connection.
Now, choose a nominal drill size that’s compatible with your intended degree of fit.
When working with degrees, ensure they align with the bolt’s specifications.
By following these steps, you’ll be able to select the perfect drill bit for your pilot and oversized holes, ensuring a secure and reliable connection for your project.
Slotted Bolt Hole Sizes Chart
| Bolt Diameter (in) | Hole Dimensions | |
|---|---|---|
| Short-Slot (W x L) |
Long-Slot (W x L) |
|
| 1/8 | 9/16 x 11/16 | 9/16 x 1 1/4 |
| 5/8 | 11/16 x 7/8 | 11/16 x 1 9/16 |
| 3/4 | 13/16 x 1 | 13/16 x 1 7/8 |
| 7/8 | 15/16 x 1 1/8 | 15/16 x 2 3/16 |
| 1 | 1 1/8 x 1 5/16 | 1 1/8 x 2 1/2 |
| >1 1/8 | (d + 1/8) x (d + 3/8) | (d + 1/8) x 2.5d |
Slotting & Threading Specifications
When it comes to bolt hole sizes, you might come across two main specifications: slotted and threaded.
Let’s quickly dive into what makes them different and when you’d use each one.
Slotted specifications deal with bolt holes that have an opening or channel, allowing for some movement.
On the other hand, threaded bolt holes have interior threads.
With threaded holes, you’d screw the bolt directly into the hole itself.
Now, for use cases, you can use slotted holes when you need to adjust the positioning.
They allow for a bit of space, which can be helpful in certain applications.
But when you require a strong, fixed connection, threaded bolt holes are the way to go.
As for factors to consider, you need to think about the load you’re dealing with and the type of connection needed.
Just remember, slotted bolt holes can handle some adjustment, while threaded bolt holes provide a secure, tight fit.
In conclusion, slotting and threading both have their place depending on your specific needs.
By understanding their differences and use cases, you can choose the right bolt hole size for your project.
Examples for Sizing Bolt Holes
Here’s a few quick examples when you’re looking up what hole size you need for a particular bolt diameter.
What size hole do I need for a 1/2 inch bolt?
For a 1/2 inch bolt, you should generally use a clearance hole that’s slightly larger than the bolt itself.
This allows it to slide through easily and provides room for error.
A common recommendation is to use a hole size of 17/32 inches (or 0.531 inches) when you need a close fit or 9/16 inches (0.563 inches) when free fit is acceptable.
What’s the recommended hole size for a 3/8 inch bolt?
For a 3/8 inch bolt, consider a hole diameter of 13/32 inches (0.406 inches) for a close fit, or a slightly larger hole size of 7/16 inches (0.438 inches) for a free fit.
This provides enough clearance for secure fastening without compromising the joint’s integrity.
What is the M6 hole size in inches?
The M6 bolt has a metric diameter of 6 millimeters. To convert this to inches, divide by 25.4 (1 inch = 25.4 mm).
The M6 bolt diameter in inches is approximately 0.236 inches.
For clearance holes, add 1/64 to 1/32 inches, resulting in hole sizes of 15/64 inches (0.234 inches) for a close fit, and 1/4 inches (0.250 inches) for a free fit.
Comprehensive Charts
Below are complete charts showing size for the most common types of bolt holes.
In Inches (Imperial)
| Bolt Diameter (in) | Hole Dimensions | |||
|---|---|---|---|---|
| Standard (Dia) | Oversize (Dia) | Short-Slot (W x L) |
Long-Slot (W x L) |
|
| 1/8 | 9/16 | 5/8 | 9/16 x 11/16 | 9/16 x 1 1/4 |
| 5/8 | 11/16 | 13/16 | 11/16 x 7/8 | 11/16 x 1 9/16 |
| 3/4 | 13/16 | 15/16 | 13/16 x 1 | 13/16 x 1 7/8 |
| 7/8 | 15/16 | 1 1/16 | 15/16 x 1 1/8 | 15/16 x 2 3/16 |
| 1 | 1 1/8 | 1 1/4 | 1 1/8 x 1 5/16 | 1 1/8 x 2 1/2 |
| >1 1/8 | d + 1/8 | d + 5/16 | (d + 1/8) x (d + 3/8) | (d + 1/8) x 2.5d |
In Millimeters (Metric)
| Bolt Diameter (mm) | Hole Dimensions | |||
|---|---|---|---|---|
| Standard (Dia) | Oversize (Dia) | Short-Slot (W x L) |
Long-Slot (W x L) |
|
| M16 | 18 | 20 | 18×22 | 18×40 |
| M20 | 22 | 24 | 22×26 | 22×50 |
| M22 | 24 | 28 | 24×30 | 24×55 |
| M24 | 27 | 30 | 37×32 | 27×60 |
| M27 | 30 | 35 | 30×37 | 30×67 |
| M30 | 33 | 38 | 33×40 | 33×75 |
| >M36 | d + 3 | d + 8 | (d + 3) x (d + 10) | (d + 3) x (d + 2.5d) |