Counterbore & Clearance Hole Calculator - ASME B18.2 & ISO 273

How It Works

1. Select Screw Type

   Choose Socket Head Cap Screw (SHCS), Hex Cap Screw, or Flat Head (Countersunk). Each type has different head geometry that determines the counterbore or countersink dimensions.

2. Choose Unit System and Size

   Select Inch or Metric, then choose the screw size. Inch sizes range from #2 through 1 inch; metric from M3 through M24.

3. Select Fit Class

   Choose Close, Normal, or Loose fit. Close fit for precision assemblies with tight positional tolerance, Normal for general purpose, Loose for field assembly and alignment slots.

4. Review Dimensions

   See clearance hole diameter, counterbore diameter, counterbore depth, and countersink angle. All dimensions in both decimal inches and millimeters.

5. Verify with Cross-Section View

   Check the visual diagram showing the screw head seated in the counterbore. Helps catch errors before cutting metal.

Built For

• CNC programmers setting up counterbore cycles for socket head cap screw pockets
• Mechanical designers dimensioning clearance and counterbore holes on engineering drawings
• Tool and die makers fitting cap screws into mold plates and fixture components
• Manual machinists drilling and counterboring holes for assembly hardware
• Maintenance mechanics selecting the right drill and counterbore for field repairs
• Manufacturing engineers specifying clearance fits for assembly process documentation

Features & Capabilities

Three Screw Types

Complete dimension tables for SHCS (ASME B18.3), Hex Cap Screws (ASME B18.2.1), and Flat Head (Countersunk) screws with correct head height, head diameter, and drive clearance.

Three Fit Classes

Clearance hole diameters per ASME B18.2 and ISO 273 for Close, Normal, and Loose fits.

Cross-Section Visualization

CSS-rendered diagram showing screw head, clearance hole, counterbore pocket, and seating surface. Updates dynamically with selection changes.

Inch and Metric Coverage

Full range of standard sizes in both systems. Inch: #2 through 1". Metric: M3 through M24.

Countersink Data

For flat head screws, calculates countersink diameter and angle (82° inch, 90° metric).

PDF Export

Export clearance and counterbore dimensions as a branded PDF for setup sheets or job travelers.

Assumptions

• Clearance hole diameters per ASME B18.2.8 for Close, Normal, and Loose fits
• Socket head cap screw dimensions per ASME B18.3 (head diameter, head height, socket size)
• Hex cap screw dimensions per ASME B18.2.1 (head across flats, head height)
• Flat head (countersunk) screw included angle: 82 degrees for inch (ASME B18.3), 90 degrees for metric (ISO 10642)
• Counterbore depth equals screw head height plus recommended clearance of 0.010 to 0.020 inches
• Metric clearance holes per ISO 273 for Close, Medium (Normal), and Coarse (Loose) fits

Limitations

• Does not cover specialty fastener types: shoulder screws, set screws, weld studs, or self-tapping screws
• Counterbore depth does not account for washer thickness or gasket compression under the screw head
• Thread engagement depth for the mating tapped hole is not calculated; see tap drill calculator for that
• Does not verify sufficient material thickness beneath the counterbore for structural integrity
• Cross-section visualization is schematic only and not drawn to dimensional scale
• Some non-standard screw manufacturers may have head dimensions outside ASME/ISO tables

References

1. ASME B18.3 - Socket Head Cap Screws (head dimensions and tolerances)
2. ASME B18.2.1 - Square and Hex Bolts and Screws, Inch Series (hex cap screw dimensions)
3. ASME B18.2.8 - Clearance Holes for Bolts, Screws, and Studs (clearance hole sizing)
4. ISO 273 - Fasteners - Clearance Holes for Bolts and Screws (metric clearance fits)
5. ISO 10642 - Hexagon Socket Countersunk Head Screws (90-degree metric flat heads)
6. Machinery's Handbook, 31st Edition - Counterbore and Countersink Dimensions

Frequently Asked Questions