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Clearance Hole Calculator

Construction

Calculate the correct clearance hole diameter to drill for a given bolt size and fit class. Get close, normal, or loose fit drill measurements instantly.

0.0632

Clearance Hole Diameter

0.266

This calculator computes your Clearance Hole Diameter from the values you enter.

Inputs
Bolt DiameterFit Class
Outputs
Clearance Hole Diameter

What is a Clearance Hole?

A Clearance Hole Calculator determines the correct diameter to drill when a bolt needs to pass freely through a hole rather than thread into it. Clearance holes are used any time a bolt goes through one piece of material and threads into a nut, a tapped hole in another piece, or is secured by a washer and nut on the far side. Getting the size right — not too tight, not too loose — is essential for smooth assembly and proper load distribution.

The calculator applies one of three standard fit classes (close, normal, loose) as an added-clearance allowance over the bolt's nominal diameter, matching the fastener charts used in machining and construction. If you're also sizing structural fasteners for a project, the Rebar Calculator and Framing Calculator cover related material-quantity planning.

How to use this Clearance Hole calculator

  1. Enter your Bolt Diameter in inches using the input field or slider.
  2. Select the appropriate Fit Class — Close, Normal, or Loose — based on your assembly's alignment and tolerance needs.
  3. Review the Clearance Hole Diameter result.
  4. Round up to the nearest available drill bit size from your set.
  5. Drill a test hole in scrap material first to confirm the bolt slides through smoothly before committing to your final workpiece.

Formula & Methodology

The calculator adds a fit-class allowance to the bolt's nominal diameter:

Clearance Hole Diameter = Bolt Diameter + Fit Class Allowance

Where the allowance is approximately 0.004 in for close fit, 0.016 in for normal fit, and 0.031 in for loose fit.

Worked example: For a 3/8 in (0.375 in) bolt with a normal fit:

Clearance Hole Diameter = 0.375 + 0.016 = 0.391 in

Rounding up to the nearest standard drill bit gives a 25/64 in (0.390625 in) or 13/32 in (0.40625 in) bit, depending on what's available.

Frequently Asked Questions

A clearance hole is a drilled hole sized slightly larger than a bolt's diameter so the bolt shank passes through freely without engaging threads in the material itself. It's the opposite of a tapped or pilot hole, which is sized smaller than the fastener so threads can cut or self-tap into the material.
Close fit adds the least clearance (about 0.004 in over bolt diameter) and is used when precise alignment matters, such as dowel-pinned joints. Normal fit (about 0.016 in) is the standard choice for most general-purpose bolted connections. Loose fit (about 0.031 in) adds the most clearance and is used when parts need to be assembled quickly or when some misalignment tolerance is required.
The calculator adds a fit-class clearance allowance to your bolt's nominal diameter. You enter the bolt diameter and select a fit class, and it returns the diameter you should drill for that hole.
For a standard 1/4 inch (0.25 in) bolt with a normal fit clearance of 0.016 in, the target clearance hole diameter is 0.266 in, which rounds to the nearest standard drill bit — commonly a 17/64 in (0.2656 in) or 9/32 in (0.28125 in) bit depending on what's available in your set.
Drilling a hole at the exact bolt diameter creates a tight interference fit that makes assembly difficult and can bind or gall threads and shanks during insertion. A small amount of clearance ensures the bolt slides through smoothly while still providing adequate bearing surface for the bolt head or washer.
The same fit-class logic applies to metric bolts — you simply enter the metric bolt diameter converted to inches, or use the equivalent metric clearance values from a fastener reference chart. The added-clearance principle (close, normal, loose) is the same regardless of measurement system.
Loose fit clearance is generally fine for non-critical assemblies but structural or load-bearing bolted connections often specify a particular fit class in engineering drawings — check your project specifications before defaulting to loose fit on structural work.
An oversized hole reduces the bearing surface under the bolt head or nut, can allow excessive play or misalignment, and in some applications may require a larger washer to properly distribute clamping force. It's best to stay within the standard fit-class allowances rather than drastically oversizing.
Yes, clearance hole sizing principles apply to both wood and metal fastening — the calculator gives you the target hole diameter regardless of material, though you should pair it with an appropriate drill bit and, for wood, consider pilot holes for the fastener threads separately if using lag screws instead of through-bolts.
Also known as
bolt clearance hole calculatordrill size for bolt calculatorclearance hole size chart calculatorbolt hole diameter calculatorfastener clearance fit calculator