When using bolts or screws, choosing the right clearance hole size helps parts fit and work properly. A clearance hole lets the fastener pass through freely, unlike a tapped hole. Metric and imperial systems use different sizes. Picking the wrong size can cause weak joints or hard assembly, so choose carefully.
Metric Clearance Hole Size Chart (MM)
| Nominal Screw Size | Normal Tolerance | Close Tolerance | Loose Tolerance | ||||||
| Nominal Drill Size | Hole Diameter | Nominal Drill Size | Hole Diameter | Nominal Drill Size | Hole Diameter | ||||
| Min. | Max. | Min. | Max. | Min. | Max. | ||||
| M1.6 | 1.8 | 1.8 | 1.94 | 1.7 | 1.7 | 1.8 | 2 | 2 | 2.25 |
| M2 | 2.4 | 2.4 | 2.54 | 2.2 | 2.2 | 2.3 | 2.6 | 2.6 | 2.85 |
| M2.5 | 2.9 | 2.9 | 3.04 | 2.7 | 2.7 | 2.8 | 3.1 | 3.1 | 3.4 |
| M3 | 3.4 | 3.4 | 3.58 | 3.2 | 3.2 | 3.32 | 3.6 | 3.6 | 3.9 |
| M4 | 4.5 | 4.5 | 4.68 | 4.3 | 4.3 | 4.42 | 4.8 | 4.8 | 5.1 |
| M5 | 5.5 | 5.5 | 5.68 | 5.3 | 5.3 | 5.42 | 5.8 | 5.8 | 6.1 |
| M6 | 6.6 | 6.6 | 6.82 | 6.4 | 6.4 | 6.55 | 7 | 7 | 7.36 |
| M8 | 9 | 9 | 9.22 | 8.4 | 8.4 | 8.55 | 10 | 10 | 10.36 |
| M10 | 11 | 11 | 11.27 | 10.5 | 10.5 | 10.68 | 12 | 12 | 12.43 |
| M12 | 13.5 | 13.5 | 13.77 | 13 | 13 | 13.18 | 14.5 | 14.5 | 14.93 |
| M14 | 15.5 | 15.5 | 15.77 | 15 | 15 | 15.18 | 16.5 | 16.5 | 16.93 |
| M16 | 17.5 | 17.5 | 17.77 | 17 | 17 | 17.18 | 18.5 | 18.5 | 19.02 |
| M20 | 22 | 22 | 22.33 | 21 | 21 | 21.21 | 24 | 24 | 24.52 |
| M24 | 26 | 26 | 26.33 | 25 | 25 | 25.21 | 28 | 28 | 28.52 |
| M30 | 33 | 33 | 33.39 | 31 | 31 | 31.25 | 35 | 35 | 35.62 |
| M36 | 39 | 39 | 39.39 | 37 | 37 | 37.25 | 42 | 42 | 42.62 |
| M42 | 45 | 45 | 45.39 | 43 | 43 | 43.25 | 48 | 48 | 48.62 |
| M48 | 52 | 52 | 52.46 | 50 | 50 | 50.25 | 56 | 56 | 56.74 |
| M56 | 62 | 62 | 62.46 | 58 | 58 | 58.3 | 66 | 66 | 66.74 |
| M64 | 70 | 70 | 70.46 | 66 | 66 | 66.3 | 74 | 74 | 74.74 |
| M72 | 78 | 78 | 78.46 | 74 | 74 | 74.3 | 82 | 82 | 82.87 |
| M80 | 86 | 86 | 86.54 | 82 | 82 | 82.35 | 91 | 91 | 91.87 |
| M90 | 96 | 96 | 96.54 | 93 | 93 | 93.35 | 101 | 101 | 101.87 |
| M100 | 107 | 107 | 107.54 | 104 | 104 | 104.35 | 112 | 112 | 112.87 |
Imperial Clearance Hole Size Chart (Inch)
| Nominal Screw Size | Normal Tolerance | Close Tolerance | Loose Tolerance | ||||||
| Nominal Drill Size | Hole Diameter | Nominal Drill Size | Hole Diameter | Nominal Drill Size | Hole Diameter | ||||
| Min. | Max. | Min. | Max. | Min. | Max. | ||||
| #0 | #48 | 0.076 | 0.082 | #51 | 0.067 | 0.071 | 3/32 | 0.094 | 0.104 |
| #1 | #43 | 0.089 | 0.095 | #46 | 0.081 | 0.085 | #37 | 0.104 | 0.114 |
| #2 | #38 | 0.102 | 0.108 | 3/32 | 0.094 | 0.098 | #32 | 0.116 | 0.126 |
| #3 | #32 | 0.116 | 0.122 | #36 | 0.106 | 0.110 | #30 | 0.128 | 0.140 |
| #4 | #30 | 0.128 | 0.135 | #31 | 0.120 | 0.124 | #27 | 0.144 | 0.156 |
| #5 | 5/32 | 0.156 | 0.163 | 9/64 | 0.141 | 0.146 | 11/64 | 0.172 | 0.184 |
| #6 | #18 | 0.170 | 0.177 | #23 | 0.154 | 0.159 | #13 | 0.185 | 0.197 |
| #8 | #9 | 0.196 | 0.203 | #15 | 0.180 | 0.185 | #3 | 0.213 | 0.225 |
| #10 | #2 | 0.221 | 0.228 | #5 | 0.206 | 0.211 | B | 0.238 | 0.250 |
| 1/4 | 9/32 | 0.281 | 0.290 | 17/64 | 0.266 | 0.272 | 19/64 | 0.297 | 0.311 |
| 5/16 | 11/32 | 0.344 | 0.354 | 21/64 | 0.328 | 0.334 | 23/64 | 0.359 | 0.373 |
| 3/8 | 13/32 | 0.406 | 0.416 | 25/64 | 0.391 | 0.397 | 27/64 | 0.422 | 0.438 |
| 7/16 | 15/32 | 0.469 | 0.479 | 29/64 | 0.453 | 0.460 | 31/64 | 0.484 | 0.500 |
| 1/2 | 9/16 | 0.562 | 0.572 | 17/32 | 0.531 | 0.538 | 39/64 | 0.609 | 0.625 |
| 5/8 | 11/16 | 0.688 | 0.698 | 21/32 | 0.656 | 0.663 | 47/64 | 0.734 | 0.754 |
| 3/4 | 13/16 | 0.812 | 0.824 | 25/32 | 0.781 | 0.789 | 29/32 | 0.906 | 0.926 |
| 7/8 | 15/16 | 0.938 | 0.950 | 29/32 | 0.906 | 0.914 | 1-1/32 | 1.031 | 1.051 |
| 1 | 1-3/32 | 1.094 | 1.106 | 1-1/32 | 1.031 | 1.039 | 1-5/32 | 1.156 | 1.181 |
| 1-1/8 | 1-7/32 | 1.219 | 1.235 | 1-5/32 | 1.156 | 1.164 | 1-5/16 | 1.312 | 1.337 |
| 1-1/4 | 1-11/32 | 1.344 | 1.360 | 1-9/32 | 1.281 | 1.291 | 1-7/16 | 1.438 | 1.463 |
| 1-3/8 | 1-1/2 | 1.500 | 1.516 | 1-7/16 | 1.438 | 1.448 | 1-39/64 | 1.609 | 1.634 |
| 1-1/2 | 1-5/8 | 1.625 | 1.641 | 1-9/16 | 1.562 | 1.572 | 1-47/64 | 1.734 | 1.759 |
What Is a Clearance Hole?
A clearance hole is a hole drilled or punched to let a bolt or screw pass through freely, without engaging threads. It’s typically used in the top component of an assembly, allowing the fastener to thread into the part beneath.
This ensures easier alignment and proper clamping force. Clearance holes are essential when using bolts, especially in metal or precision assemblies. Screws may not always need one, but in harder materials, clearance holes help avoid thread damage.
Choosing the right size involves tradeoffs—too tight, and assembly is difficult; too loose, and joint integrity suffers. Using a Clearance Hole Size Chart for metric or imperial fasteners helps balance fit, function, and material constraints, leading to stronger, more reliable connections.
Understanding Fit Types: Close, Normal, and Loose Clearance
The size of a clearance hole varies depending on the desired fit, ranging from tight to loose tolerances. Engineers typically choose from three primary fit types: close, normal, and loose.
A close fit allows minimal play between the fastener and the hole, offering precise alignment, which is ideal for critical assemblies or shear load applications. In contrast, a normal fit balances ease of assembly with moderate alignment control, making it the most commonly used option.
A loose fit provides maximum clearance, simplifying insertion in less precise applications or when components may shift due to thermal expansion. However, selecting the right fit involves tradeoffs.
Tighter fits improve alignment but can complicate manufacturing and increase costs. Looser fits ease assembly but may reduce joint strength. Therefore, consulting a Clearance Hole Size Chart—for either metric or imperial fasteners—is crucial to strike the right balance between performance, manufacturability, and cost.
Tips for Drilling and Selecting the Right Hole Size
Choosing the right drill bit size and understanding tolerances can make or break your assembly process. While clearance hole charts provide standardized guidelines, real-world results often depend on how you drill and what material you’re working with.
To begin, always match your drill bit size to the recommended clearance diameter based on the fastener’s type and desired fit. For example, use a 6.6 mm bit for a normal-fit M6 bolt—but adjust slightly if your material is prone to expansion or contraction.
Next, consider the material properties. Softer materials like aluminum may allow slight under-sizing since they deform easily, while harder materials like stainless steel demand greater precision. Additionally, drilling through layered or laminated surfaces may require pilot holes or step drilling to prevent tearing or misalignment.
However, one of the most common mistakes is drilling holes that are either too tight—causing binding or installation difficulty—or too loose, which can lead to vibration, misalignment, or reduced load capacity. Striking the right balance between fit and function often requires testing or prototyping, especially for high-tolerance assemblies.
Moreover, keep in mind the impact of drill wear, tool deflection, and operator variability. Even small deviations can alter hole size, especially in precision applications. To minimize issues, use sharp, high-quality bits, and maintain proper drilling speed and feed for the material.
In short, applying the Clearance Hole Size Chart for Metric and Imperial Fasteners is only part of the solution. You also need to account for practical drilling conditions, material behavior, and manufacturing tolerances. When you do, you ensure smoother assembly, better part alignment, and longer-lasting joints.
Standards and References
Clearance hole dimensions are often defined by international standards to ensure interchangeability and reliability. To achieve consistent results across industries and geographies, engineers and manufacturers rely on established standards such as ISO, ANSI, and ASME.
These organizations provide detailed specifications for fastener sizes, hole diameters, and fit tolerances, making it easier to design components that work together, regardless of who makes them or where they’re assembled.
For example, ISO 273 outlines preferred clearance hole diameters for metric bolts, while ANSI B18.2.8 and ASME B1.1 cover imperial dimensions and thread details. Following these standards helps reduce errors, streamline production, and ensure safety—especially in industries like aerospace, automotive, and structural engineering where failure can be costly or dangerous.
However, adhering to standards isn’t always straightforward. Sometimes, project constraints or legacy components require deviations. In such cases, it’s important to weigh the tradeoffs between standard compliance and practical considerations like material availability, tooling, or custom part design. Deviating from standards may offer short-term convenience but can create long-term compatibility or maintenance challenges.
To make informed decisions, always consult official documentation, which can be accessed through engineering handbooks, digital libraries, or the websites of standards organizations (such as iso.org or asme.org). Many CAD programs also integrate standard hole sizes directly into their design libraries, reducing guesswork.
Ultimately, using the Clearance Hole Size Chart for Metric and Imperial Fasteners in alignment with international standards enhances product quality, simplifies global collaboration, and minimizes costly redesigns down the line.
Applications and Real-World Examples
Whether you’re building furniture or designing machinery, clearance holes show up in countless applications. From mechanical assemblies and structural frames to electronic enclosures, clearance holes are a fundamental feature in engineered products.
For example, in mechanical design, they allow bolts to pass cleanly through moving parts or brackets, enabling secure yet serviceable connections. In structural engineering, clearance holes help align heavy steel members while allowing for slight positional adjustments.
Similarly, in electronics, clearance holes ensure that screws don’t interfere with circuit boards or delicate components during installation. However, failure to choose the right clearance size can have serious consequences.
Consider a real-world case where an industrial pump housing was assembled using holes drilled too tightly. The result? Bolts seized during assembly, leading to part misalignment, increased labor costs, and a complete redesign of the mounting system.
This example underscores the importance of using the correct fit and clearance values, not just for functionality, but also for long-term reliability and maintenance. Engineers routinely use clearance hole size charts during the CAD and prototyping phases to avoid such issues.
Most CAD software includes libraries of standard hole sizes based on ISO or ANSI specs, which helps streamline design and prevent errors. Still, engineers must remain vigilant—material choices, machining methods, and assembly sequence all affect how well a fastener fits.
Ultimately, applying the Clearance Hole Size Chart for Metric and Imperial Fasteners during the design phase is not just a technical detail—it’s a smart engineering practice that saves time, reduces rework, and enhances product performance across countless real-world applications.
Conclusion
Getting clearance hole sizes right the first time saves time, reduces errors, and improves assembly quality. By using standardized clearance hole charts, you balance fit, function, and manufacturability effectively. We encourage you to rely on these references and welcome your feedback or questions to help refine your fastening solutions.
FAQs
What is the difference between a clearance hole and a tapped hole?
A clearance hole is drilled larger than the fastener’s outer diameter, allowing the bolt or screw to pass freely without engaging threads. In contrast, a tapped hole has internal threads matching the fastener, allowing it to be screwed directly into the material.
How do I choose between close, normal, and loose clearance fits?
Choose a close fit when precise alignment and minimal play are critical, such as in shear load applications. Use a normal fit for general purposes, balancing ease of assembly with good alignment. Opt for a loose fit when components may shift or thermal expansion is a factor.
Why are there different clearance hole sizes for metric and imperial fasteners?
Metric and imperial fasteners use different measurement systems and standards, so their recommended hole sizes vary accordingly. Each system follows specific guidelines (ISO for metric, ANSI/ASME for imperial) to ensure compatibility and reliable fit.
Can I use the same clearance hole chart for different materials?
While clearance hole charts provide standard sizes, material properties like hardness and thermal expansion affect fit. Softer materials may allow slightly smaller holes, while harder materials might require exact sizes or even larger holes to avoid damage.
Where can I find official clearance hole standards and charts?
Official standards are available from organizations like ISO (iso.org), ANSI (ansi.org), and ASME (asme.org). Many CAD programs also include standardized hole sizes. Additionally, printable charts—like the one in this blog—offer practical quick references for daily use.