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

Get Standard Clearance Hole, Counterbore, and Countersink Dimensions for Cap Screws

Free counterbore and clearance hole calculator covering ASME B18.2 and ISO 273 standards. Select your screw type (Socket Head Cap Screw, Hex Cap Screw, or Flat Head), choose inch or metric sizes, and pick a fit class (Close, Normal, or Loose) to get complete dimensions for clearance holes, counterbores, and countersinks.

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Includes a cross-section visualization showing the screw seated in the counterbored hole. Covers #2 through 1 inch for inch screws and M3 through M24 for metric. The calculator outputs drill size, counterbore diameter and depth, and countersink angle and diameter.

Pro Tip: Always add at least 0.010 inches to the minimum counterbore depth from the standard. This accounts for face mill marks on the seating surface, slight variations in screw head height, and burrs from the counterbore operation. For CNC work, program the counterbore 0.015 to 0.020 deeper than the head height and you will never have a screw sitting proud.

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Counterbore & Clearance Hole Calculator
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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.

Frequently Asked Questions

Close fit allows minimum clearance between bolt body and hole wall, for precision assemblies. Normal fit provides enough clearance for easy assembly with standard tolerances. Loose fit adds extra clearance for field assembly and slotted holes.
Minimum depth equals the screw head height. In practice, add 0.010 to 0.020 inches to ensure the head sits below the surface.
The 82-degree included angle is the American standard per ASME B18.3. Metric flat heads use 90 degrees per ISO 10642. An 82-degree screw in a 90-degree countersink will sit proud.
For through holes, standard twist drills work fine. For blind clearance holes where a bolt must seat against a flat surface, use a flat-bottom counterbore tool.
Disclaimer: Dimensions per ASME B18.2, B18.3, and ISO 273. Always verify critical dimensions against the current revision of applicable standards.

Learn More

Machinist

Counterbore, Clearance Hole & Countersink Dimensions — The Complete Guide

How to determine clearance hole, counterbore, and countersink dimensions for socket head and hex cap screws using ASME and ISO standards.

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