Clearance Hole Calculator

What Is a Clearance Hole?

A clearance hole is a hole drilled or machined through a workpiece at a diameter larger than the fastener passing through it. Unlike a tapped hole — which threads directly onto the bolt — a clearance hole allows the fastener to pass freely, with clamping force supplied by a nut, threaded insert, or captured thread on the far side. Correct clearance hole sizing ensures reliable assembly, accurate part alignment, and protection against fastener galling or binding during installation.

Governing Standards

Two internationally recognized standards define metric clearance hole diameters:

• ISO 273:1979 — Fasteners: Clearance holes for bolts and screws — Establishes three fit classes (fine, medium, and coarse) for metric fasteners from M1 through M150. This standard is the primary reference for global manufacturing and engineering design.
• ASME B18.2.8 — Clearance Holes for Bolts, Screws, and Studs — Published by the American Society of Mechanical Engineers, this standard covers both inch and metric clearance holes and is widely specified across North American construction and industrial fabrication projects.

The Clearance Hole Formula

Clearance hole sizing is not derived from a single algebraic expression. Instead, it follows a standardized lookup relationship defined as:

D_hole = f(fastener size, fit class)

Each input combination maps to a tabulated diameter specified in ISO 273 or ASME B18.2.8. The two independent variables are:

• Fastener Size (Metric): The nominal thread diameter of the bolt or screw, expressed in millimeters — for example, M4, M8, M16, or M24.
• Fit Class: The tolerance category selecting how much clearance is added above the nominal bolt diameter. ISO 273 designates three classes: Close (fine), Normal (medium), and Loose (coarse).

Understanding the Three Fit Classes

According to Engineers Edge — Standard Metric Clearance Hole Sizes, each fit class serves a distinct engineering purpose:

• Close Fit: The minimum clearance above the bolt nominal diameter. Specified for precision assemblies where accurate part location is essential — including jigs, fixtures, coordinate measuring machine tooling, and structural connections with strict bolt-pattern tolerances.
• Normal Fit: The most commonly specified general-purpose clearance. Accommodates typical manufacturing positional tolerances and is the industry default for the vast majority of bolted joints in mechanical and construction applications.
• Loose Fit: The largest clearance, designed for assemblies subject to significant thermal expansion, rough field installation, large structural steel frameworks, or joints requiring deliberate positional latitude.

Standard Metric Clearance Hole Sizes (ISO 273)

The following tabulated values represent the most frequently used nominal fastener sizes under ISO 273:

• M3: Close = 3.2 mm, Normal = 3.4 mm, Loose = 3.6 mm
• M4: Close = 4.3 mm, Normal = 4.5 mm, Loose = 4.8 mm
• M5: Close = 5.3 mm, Normal = 5.5 mm, Loose = 5.8 mm
• M6: Close = 6.4 mm, Normal = 6.6 mm, Loose = 7.0 mm
• M8: Close = 8.4 mm, Normal = 9.0 mm, Loose = 10.0 mm
• M10: Close = 10.5 mm, Normal = 11.0 mm, Loose = 12.0 mm
• M12: Close = 13.0 mm, Normal = 13.5 mm, Loose = 14.5 mm
• M16: Close = 17.0 mm, Normal = 17.5 mm, Loose = 18.5 mm
• M20: Close = 21.0 mm, Normal = 22.0 mm, Loose = 24.0 mm

Worked Examples

Example 1: Steel Bracket — Normal Fit, M10

A structural engineer specifies M10 bolts for a steel bracket assembly in a standard indoor environment. Selecting Normal fit yields a clearance hole of 11.0 mm — 1.0 mm larger than the bolt nominal diameter — accommodating typical positional variation without sacrificing joint rigidity or alignment accuracy.

Example 2: Precision Machining Fixture — Close Fit, M6

A toolroom engineer designs a repeatable machining fixture requiring precise locating. Close fit for M6 bolts specifies a hole diameter of 6.4 mm, providing just 0.4 mm of clearance. This tight tolerance ensures the fixture returns to the same position across multiple setup cycles, maintaining dimensional repeatability in the manufactured parts.

Example 3: Outdoor Steel Frame — Loose Fit, M16

A construction engineer designs a large exposed structural steel frame subject to seasonal temperature variation. Specifying M16 bolts with Loose fit produces a clearance hole of 18.5 mm — 2.5 mm over nominal — accommodating thermal expansion movement and simplifying bolt insertion during on-site erection in variable conditions.

Selecting the Correct Fit Class

The correct fit class depends on three factors: required positional accuracy, expected operating temperature range, and assembly environment. Use Close fit for precision-critical applications. Default to Normal fit for everyday mechanical and construction joints. Specify Loose fit wherever thermal movement, field assembly difficulty, or large positional tolerances are anticipated. Always verify that the selected hole diameter also satisfies minimum edge-distance and pitch requirements specified by the applicable structural or mechanical design code.