Clearance Hole Calculator

Find a practical hole diameter for metric fasteners using close, normal, or loose clearance.

Clearance Hole Diameter Calculator
Enter the nominal fastener diameter and select the assembly fit.

About clearance holes

A clearance hole lets the body of a bolt, screw, pin, or rivet pass through a component without its threads engaging that component. Its diameter must exceed the fastener diameter by enough to accommodate manufacturing tolerances, drilling accuracy, coatings, alignment, and assembly conditions. This calculator adds a selected diametral allowance to the nominal metric fastener diameter. It provides a transparent workshop estimate rather than silently substituting a nearby standard drill. A close fit adds 0.10 millimeter and is useful when alignment is controlled and positional accuracy matters. A normal fit adds 0.20 millimeter, offering a balanced choice for general fabrication and routine assembly. A loose fit adds 0.50 millimeter to make installation easier where parts may be less accurately positioned, field assembly is expected, or coatings consume some clearance. The result is the theoretical minimum target generated by those allowances. Select the next suitable available drill size when the exact diameter is unavailable. Real engineering standards use size-dependent clearance tables, so one fixed allowance is not a universal substitute for an applicable drawing, code, or standard. ISO 273 and company-specific design rules may prescribe different fine, medium, and coarse holes for a given nominal thread. Aerospace, structural steel, precision machinery, and safety-critical products can also impose tolerances, surface finish, edge distance, and inspection requirements. Always use the specification on the approved drawing when it exists. Measure the unthreaded shank or confirm the nominal thread designation before entering a value. Do not confuse a clearance hole with a tapping drill: a tapping drill must be smaller so the tap can form internal threads, whereas a clearance hole must let the fastener pass freely. Likewise, a press-fit or reamed dowel hole follows an interference or transition-fit tolerance and should not be sized with this tool. Drilling behavior can enlarge or distort a hole. Tool runout, worn cutting edges, thin material, burrs, and thermal effects all influence the finished diameter. Make a test hole in scrap when fit is important, deburr both faces, and verify the result with a suitable gauge. For stacked parts, consider accumulated positional error as well as diameter. Choosing the smallest fit that still assembles reliably helps preserve bearing area under the head or washer and limits unnecessary movement.

Clearance hole examples

Fastener and fitCalculated holeUse case
8 mm fastener, normal fit8.20 mm holeGeneral workshop assembly with a balanced allowance.
12 mm fastener, loose fit12.50 mm holeEasier field alignment or room for coating buildup.
6 mm fastener, close fit6.10 mm holeControlled alignment where excess movement is undesirable.

How to calculate a clearance hole

  1. Confirm the nominal metric diameter of the fastener that must pass through the part.
  2. Choose close, normal, or loose fit according to alignment and assembly needs.
  3. Calculate the target diameter by adding the displayed diametral clearance.
  4. Select an available drill and verify the finished hole against the governing drawing or standard.

Clearance hole FAQ

What is a clearance hole?

A clearance hole is larger than the fastener shank so the fastener passes through without creating threads. A nut or a threaded feature in another component provides the clamping action.

How is a clearance hole different from a tapping hole?

A tapping hole is intentionally smaller than the thread's major diameter so a tap can cut internal threads. A clearance hole is larger and is not meant to engage the threads.

Which fit should I select?

Use close fit for controlled, accurately aligned work and normal fit for most general assemblies. Loose fit is appropriate when easy field alignment, coating thickness, or wider positional variation matters.

Should I round to the nearest drill size?

Choose a standard drill that does not reduce the required clearance, which usually means rounding upward. Confirm that the resulting hole still meets edge-distance and washer requirements.

Does this replace an ISO clearance table?

No, this calculator applies simple fixed allowances for quick planning. Use ISO 273, an approved drawing, or the relevant engineering standard whenever compliance or a safety-critical fit is required.