Countersunk Hole Size for Flat Head Screws (ISO)

Countersunk-Flat-Head-ISO
Unsure on what size countersunk hole to use for your ISO flat head fasteners?

Then look no further!

This short post will give you the exact dimensions you need for your screws and bolts.

Jump to Chart

A countersunk hole is used for flat head fasteners which have a flat top face and then a taper down from the top face.

They are used when the top of a screw or bolt needs to be flush with the surface of the material it is inserting.

For example, a flat head screw with a countersunk hole sits flush with or just below the surface, whereas a screw that is not countersunk will sit above the surface:
 

Countersunk-Hole-vs-Clearance-Hole

An example of a flat head screw and the corresponding countersunk hole is shown in this 3D model:

Countersunk Hole
Flat Head (Countersunk) Screw

Use the image and chart below to determine what dimensions to use, all dimensions are in millimetres apart from the countersink angle.

For example, an ISO M10 flat head screw with a normal fit countersunk hole will require a pilot hole diameter of 11 mm, a countersunk diameter of 22.73 mm, and a countersunk angle of 90°.

Countersunk-Hole-Dimensions-Diameter

Countersunk Hole Size Chart for Flat Head Fasteners (ISO)

All dimensions are in millimetres apart from the countersink angle.

Fastener Size (Thread)

Pilot Hole Diameter (Close Fit H12)

Pilot Hole Diameter (Normal Fit H13)

Pilot Hole Diameter (Loose Fit H14)

Countersink Diameter

Countersink Angle (°)

M33.23.43.66.9490
M3.53.73.94.28.9690
M44.34.54.89.1890
M55.35.55.811.4790
M66.46.6713.7190
M88.491018.2590
M1010.5111222.7390
M121313.514.527.2190
M161717.518.533.9990
M2021222440.7190

If you want to learn more about countersunk holes, see our post here.

Looking for a Different Standard of Countersunk Hole?

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