Counterbore Hole Dimensions Chart Calculator for recommended clearance and hole dimensions for socket head cap screws from #0 through 2 inches size. There are two classes of fit category; close- and normal-fit category. The recommended drill bit size for each class of clearance hole is shown in calculation results.

Nominal Screw Size
Fit Class1,2

Note 1: Close Fit: The close fit is normally limited to holes for those lengths of screws which are threaded to the head in assemblies where only one screw is to be used or where two or more screws are to be used and the mating holes are to be produced either at assembly or by matched and coordinated tooling.

Note 2: Normal Fit: The normal fit is intended for screws of relatively long length or for assemblies involving two or more screws where the mating holes are to be produced by conventional tolerancing methods.


Clearance Hole Counter Bore
Parameter Value
Nominal Screw Size --- ---
Max. Body Diameter [Dmax] --- in
Fit Class --- ---
Nominal Inch Drill Size [A] --- ---
Decimal Size [A] --- in
Counterbore Diameter [B] --- in
Countersink Diameter [C] 1 ---
Max. Head Height [Hmax] ---
Max. Fillet [Fmax ---

Note 1: It is considered good practice to countersink or break the edges of holes which are smaller than (D Max + 2F Max) in parts having a hardness which approaches, equals or exceeds the screw hardness. If such holes are not countersunk, the heads of screws may not seat properly or the sharp edges on holes may deform the fillets on screws thereby making them susceptible to fatigue in applications involving dynamic loading. The countersink or corner relief, however, should not be larger than is necessary to insure that the fillet on the screw is cleared.


Clearance hole: A hole on a part where threaded section of a bolt can pass.

Counterbore: A hole used when the head of the socket head cap screw is needed to be flushed with or below the level of the work piece's surface.

Countersink: A hole with conical shape. In this case, it's used to remove the burr left from a drilling operation thereby removing any sharp edges.

Drill bit: A cutting tool used in drilling machines to drill a hole.



  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.