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.
An example of a flat head screw and the corresponding countersunk hole is shown in this 3D model:
Use the image and chart below to determine what dimensions to use, all dimensions are in inches apart from the countersink angle.
For example, an ANSI Inch 9/16″ 100 degree machine screw with a normal fit countersunk hole will require a pilot hole diameter of 5/8″, a countersunk diameter of 1 10/69″, and a countersunk angle of 100°.
Countersunk Hole Size Chart for 100 Degree Machine Fasteners (ANSI Inch)
All dimensions are in inches apart from the countersink angle.
Fastener Size (Thread) | Pilot Hole Diameter (Close Fit) | Pilot Hole Diameter (Normal Fit) | Pilot Hole Diameter (Loose Fit) | Countersink Diameter | Countersink Angle |
| #0 | - | 6/79 | 3/32 | 5/42 | 100 |
| #1 | 3/37 | 4/45 | 8/77 | 13/89 | 100 |
| #2 | 3/32 | 7/69 | 7/62 | 16/93 | 100 |
| #3 | 5/47 | 8/69 | 9/70 | 1/5 | 100 |
| #4 | 3/25 | 9/70 | 14/97 | 9/40 | 100 |
| #6 | 2/13 | 10/59 | 5/27 | 12/43 | 100 |
| #8 | 9/50 | 10/51 | 13/61 | 1/3 | 100 |
| #10 | 15/73 | 21/95 | 5/21 | 5/13 | 100 |
| 1/4 | 17/64 | 9/32 | 19/64 | 36/71 | 100 |
| 5/16 | 21/64 | 11/32 | 23/64 | 40/63 | 100 |
Fastener Size (Thread) | Pilot Hole Diameter (Close Fit) | Pilot Hole Diameter (Normal Fit) | Pilot Hole Diameter (Loose Fit) | Countersink Diameter | Countersink Angle |
| 3/8 | 25/64 | 13/32 | 27/64 | 16/21 | 100 |
| 7/16 | 29/64 | 15/32 | 31/64 | 81/91 | 100 |
| 1/2 | 17/32 | 9/16 | 39/64 | 1 1/59 | 100 |
| 9/16 | 19/32 | 5/8 | 43/64 | 1 10/69 | 100 |
| 5/8 | 21/32 | 11/16 | 47/64 | 1 25/92 | 100 |
If you want to learn more about countersunk holes, see our post here.