Clearance Hole Calculator
Calculate the correct clearance hole size for bolts, screws, and fasteners based on the selected diameter and fit.
What Is a Clearance Hole?
A clearance hole is a hole drilled slightly larger than the diameter of a bolt or screw. It allows the fastener to pass through freely without threading into the material. The clearance hole is typically drilled in the part that will be clamped, while the threaded hole or nut is in the mating part. Getting the clearance hole size right ensures proper assembly, prevents binding, and avoids damaging threads during installation.
How Clearance Hole Sizes Are Determined
Clearance hole sizes follow standard fit classifications. The most common are:
- Normal fit — Provides enough clearance for most general-purpose bolted connections. Suitable for standard assembly where alignment is straightforward.
- Close fit — Tighter clearance for applications requiring precise alignment. Used when minimal fastener movement is acceptable.
- Loose fit — Extra clearance for situations where alignment is difficult or where fasteners need to be inserted quickly. Common in field assembly or when using slotted holes.
Each fit class corresponds to a specific hole diameter based on the nominal fastener size. The calculator applies these standard values so you don't have to look up tables manually.
How to Use the Clearance Hole Calculator
Select the nominal diameter of your bolt or screw from the dropdown menu. Then choose the desired fit type — normal, close, or loose. The calculator will display the recommended clearance hole diameter for that combination.
Always verify that the calculated hole size matches your material thickness and application requirements. For critical structural connections, consult the relevant engineering standard for your region.
Understanding the Results
The output shows the recommended drill bit size or hole diameter for the selected fastener. This value represents the minimum hole size that allows the fastener to pass through freely while maintaining adequate bearing surface for the bolt head or washer.
If the calculated size falls between standard drill bit sizes, choose the next larger bit to ensure proper clearance. For tapped holes, remember that the clearance hole applies only to the unthreaded part — the threaded portion requires a different hole size.
Common Mistakes When Selecting Clearance Holes
- Using the same hole size for all fits — Different applications require different clearances. Using a loose fit where a close fit is needed can cause misalignment.
- Confusing clearance holes with tap drill sizes — A clearance hole is larger than the fastener diameter. A tap drill is smaller and creates threads. Mixing them up leads to assembly failures.
- Ignoring material thickness — Very thin materials may require different clearance considerations to avoid deformation under clamping load.
- Assuming all standards are identical — Clearance hole standards vary between ISO, ANSI, and other systems. Always confirm which standard applies to your fasteners.
Practical Applications
Clearance holes are used in nearly every mechanical assembly. Common examples include:
- Mounting brackets to machinery frames
- Attaching covers and panels with machine screws
- Assembling structural steel connections with bolts
- Installing hardware in woodworking projects
- Securing components in automotive and equipment repairs
Using the correct clearance hole size reduces installation time, prevents thread damage, and ensures the joint performs as intended.
FAQ
What is the difference between a clearance hole and a tap hole?
A clearance hole is drilled larger than the fastener so the bolt or screw passes through freely. A tap hole is drilled smaller and then threaded to accept the fastener. Clearance holes are used in the part being clamped; tap holes are used in the part receiving the threads.
Can I use the same clearance hole size for metric and imperial fasteners?
No. Metric and imperial fasteners have different diameter increments and corresponding clearance hole standards. Always select the correct measurement system for your fasteners.
What happens if my clearance hole is too small?
A clearance hole that is too small will not allow the fastener to pass through freely. This can cause binding, make assembly difficult, and potentially damage threads or the fastener head during installation.
What happens if my clearance hole is too large?
An oversized clearance hole reduces the bearing surface under the bolt head or washer. This can lead to localized deformation of the clamped material, reduced clamping force, and potential joint loosening under vibration.
Do I need a washer with a clearance hole?
Washers are recommended, especially with oversized clearance holes. A washer distributes the clamping load over a larger area and prevents the bolt head from pulling through the hole in softer materials.