Counterbore Size Chart for Socket Head Screws — Metric & Inch

Socket-head screw counterbore lookup

Use the table to search for a metric or inch socket-head cap screw and compare its head dimensions, through-hole requirement, and counterbore recommendation. Search for a size such as M6, M8, 1/4-20, or 3/8-16. Choose the clearance class where the source provides more than one option.

The table must identify the source and status of each dimensional field. A head diameter or head height taken from a fastener standard is a sourced value. A counterbore dimension derived by adding a stated allowance is a calculated recommendation. Those values answer different questions and must not be merged into one unlabeled standard column.

What a counterbore does

A counterbore is a cylindrical recess around a clearance hole. It lets a socket-head cap screw sit below the surrounding surface or at a controlled depth while leaving the hex socket accessible. The through-hole carries the screw shank. The counterbore carries the head clearance.

Counterbore geometry is a design choice. A flush screw head, a recessed head, a washer under the head, a coating, a burr, or a later finishing operation can change the required diameter and depth. Start with the complete fastener and assembly requirement, not only the thread size.

Counterbore diameter versus screw-head diameter

The minimum counterbore diameter must be larger than the applicable maximum or reference head diameter. The difference is the radial clearance around the head. If the design uses a calculated recommendation, the relationship should be visible:

recommended counterbore diameter = head diameter reference + 2 × radial allowance

The head diameter is sourced from the applicable socket-head screw standard. The radial allowance is an engineering assumption or project requirement. It accounts for tool runout, burrs, plating, coating, tolerance, assembly access, or other conditions only when the chosen allowance is appropriate to the job. The formula does not create a universal standard dimension.

Counterbore depth versus head height

Counterbore depth controls how far the screw head sits below the surface. For a flush result, the recess usually needs to accommodate the applicable head height plus a small axial allowance. For a recessed result, the depth is greater than the head height by the specified amount. For a proud result, the depth is less than the head height.

When the value is calculated, show the assumption:

recommended counterbore depth = head height reference + axial allowance

Use a zero allowance only when the drawing and process support a flush head with no additional clearance. A coating, washer, seal, chamfer, deburring step, or surface finish can require a different result. Do not hide those conditions behind a single “standard depth” label.

Clearance hole through the part

The hole below the counterbore is a clearance hole, not a tapped hole. Select the clearance class for the screw and application, then verify that the screw can pass through the part without binding. The applicable metric and inch clearance guidance should remain separately traceable from the socket-head screw dimensions.

For a broader metric and inch hole lookup, use the clearance hole size chart. If the hole will be tapped instead of cleared, use the tap drill size chart and confirm the tap manufacturer’s recommendation.

Flush, recessed, and accessible heads

Flush placement is only one possible design intent. A recessed head can protect the fastener or preserve a surrounding envelope. A shallow counterbore can leave the head proud for inspection or later access. The counterbore also must leave enough room for the hex key or driver to engage the socket without interference from the surrounding surface.

Check the fastener’s head height, socket size, wrench access, nearby walls, and assembly sequence. A counterbore that clears the head but blocks the tool is not a usable design.

Counterbore versus countersink

A counterbore has a cylindrical wall and a flat shoulder. A countersink has an angled conical wall and is normally used for a flat-head screw. Do not substitute a countersink angle or flat-head dimension for a socket-head counterbore. Use the future countersink size chart for angle-based flat-head geometry when it is available.

SOLIDWORKS connection

SOLIDWORKS Hole Wizard can help model a counterbored hole, but the feature settings are a CAD workflow, not the engineering authority. Establish the screw standard, clearance class, head dimensions, counterbore assumptions, and drawing requirements first. Then use Hole Wizard in SOLIDWORKS to represent the selected geometry.

Source and calculation note

Socket-head screw dimensions should be traced to ISO 4762:2004 for metric screws and ASME B18.3-2026 for inch socket screws. Clearance-hole values should be traced to ISO 273:1979 or ASME B18.2.8-1999 (S2021), as applicable. The table must identify whether each counterbore recommendation is standards-sourced or calculated from a head dimension plus an explicit radial or axial allowance. Verify the current standard and the manufacturing process for production-critical work.

Frequently asked questions

Is counterbore diameter standardized for every M8 or 1/4-inch screw?

No. The screw standard controls the applicable head dimensions, but the counterbore allowance depends on the design and manufacturing process. The chart must label a calculated recommendation separately from the sourced head diameter.

How deep should a socket-head screw counterbore be?

Set the depth from the applicable head height and the design intent. A flush or recessed head may use head height plus an explicit axial allowance. Confirm the allowance, finish, coating, washer, and tool access before releasing the drawing.

Can I use a counterbore for a flat-head screw?

Usually no. A socket-head cap screw uses a cylindrical counterbore. A flat-head screw normally uses an angled countersink. Match the recess geometry to the fastener head.

Does Hole Wizard decide the correct counterbore size?

Hole Wizard models the selected feature, but it does not replace the governing fastener and clearance standards or your project-specific allowance decisions.