Bolt Hole Clearance Calculator
This bolt hole clearance calculator works out the recommended clearance hole diameter for a metric bolt or screw to pass through, based on the standard close, medium and free fit classes given in ISO 273.
Select the bolt nominal diameter and the required fit class, and the calculator returns the standard clearance hole diameter along with the resulting diametral and radial clearance.
Fit classes
Close fit (H12) gives the smallest clearance and is used where bolt position needs to be well controlled, for example in fitted or reamed holes. Medium fit (H13) is the general-purpose choice for most structural and mechanical assemblies. Free fit (H14) gives the most clearance and is used where holes may not line up precisely, such as in fabricated brackets or where quick assembly is more important than fit accuracy.
This calculator uses the standard ISO 273 clearance hole sizes for common metric bolt diameters. It does not replace checking the relevant fastener standard or drawing requirements for a specific application.
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