🔧 Countersink Diameter Calculator
Calculate the exact countersink diameter, depth, and pilot hole size for any screw and material
| Screw # | Shank Dia (in) | Head Dia (in) | Pilot – Softwood (in) | Pilot – Hardwood (in) | CS Dia 82° (in) | CS Depth (in) |
|---|---|---|---|---|---|---|
| #2 | 0.086 | 0.172 | 0.047 | 0.070 | 0.172 | 0.055 |
| #4 | 0.112 | 0.225 | 0.062 | 0.089 | 0.225 | 0.073 |
| #6 | 0.138 | 0.262 | 0.078 | 0.104 | 0.262 | 0.088 |
| #8 | 0.164 | 0.322 | 0.094 | 0.125 | 0.322 | 0.111 |
| #10 | 0.190 | 0.373 | 0.109 | 0.144 | 0.373 | 0.130 |
| #12 | 0.216 | 0.435 | 0.125 | 0.166 | 0.435 | 0.153 |
| #14 | 0.242 | 0.497 | 0.140 | 0.185 | 0.497 | 0.176 |
| Screw | Shank Ø (mm) | Head Ø (mm) | Pilot – Wood (mm) | Pilot – Metal (mm) | CS Dia 90° (mm) | CS Depth (mm) |
|---|---|---|---|---|---|---|
| M3 | 3.0 | 5.5 | 2.0 | 2.5 | 6.5 | 1.75 |
| M4 | 4.0 | 8.0 | 2.8 | 3.3 | 9.4 | 2.50 |
| M5 | 5.0 | 9.5 | 3.5 | 4.2 | 10.4 | 2.90 |
| M6 | 6.0 | 11.5 | 4.2 | 5.0 | 13.0 | 3.60 |
| M8 | 8.0 | 15.0 | 5.5 | 6.8 | 17.6 | 4.80 |
| M10 | 10.0 | 18.0 | 7.0 | 8.5 | 20.4 | 5.60 |
| M12 | 12.0 | 22.0 | 8.5 | 10.2 | 24.5 | 6.80 |
| Material | HSS Bit RPM (1/4 in) | HSS Bit RPM (1/2 in) | Carbide RPM (1/4 in) | SFM Range | Feed Rate (in/rev) |
|---|---|---|---|---|---|
| Softwood | 3000–5000 | 1500–2500 | 5000–8000 | 500–900 | 0.010–0.020 |
| Hardwood | 2000–3500 | 1000–1800 | 3500–6000 | 300–600 | 0.008–0.015 |
| Plywood | 2500–4500 | 1200–2200 | 4000–7000 | 400–700 | 0.009–0.018 |
| MDF | 2000–4000 | 1000–2000 | 3500–6000 | 350–550 | 0.008–0.016 |
| Aluminum | 2000–4000 | 1000–2000 | 3000–5000 | 200–400 | 0.004–0.010 |
| Mild Steel | 600–1200 | 300–600 | 900–1800 | 60–120 | 0.003–0.007 |
| Stainless 304 | 300–600 | 150–300 | 500–900 | 30–80 | 0.002–0.005 |
| Acrylic | 1500–3000 | 800–1500 | 2500–4500 | 300–500 | 0.005–0.012 |
| Bit Size | Flutes | Angle | Screw Range | Best For | Max RPM |
|---|---|---|---|---|---|
| 3/16 in (5mm) | 1–3 | 82° | #4–#6 | Softwood, MDF | 5000 |
| 1/4 in (6mm) | 1–6 | 82° | #6–#8 | Wood, Plastic | 4500 |
| 5/16 in (8mm) | 3–6 | 82° | #8–#10 | Hardwood, Ply | 4000 |
| 3/8 in (10mm) | 6 | 82° | #10–#12 | All Wood | 3500 |
| 1/2 in (12mm) | 6–8 | 90° | #12–#14 | Metal, Hard | 3000 |
| 3/4 in (19mm) | 6–8 | 82°/90° | Large screws | Heavy Wood | 2000 |
The diameter of countersink belongs to those themes that seems hard, but really it is only basic stuff. A countersink is made up of a cone shaped cut in the material, that allows the head of a screw or bolt to sit flat with the surface. When one designs it, three main dimensions matter: the diameter of the mounting hole, the diameter of the countersink top, and its angle.
It represents the biggest diameter on top where the head of the screw sits. Usually one sizes it according to the main sizes of the used fastener. The goal is that a flat head fastener sits flat with the surface.
How to Choose the Right Countersink Size and Angle
Sometimes the head sits even a bit below, what requires a biggre diameter.
Here angles play a big role. For metric screws the standard angle of a countersink is 90 degrees, while for imperial it is 82 degrees. In United States most sold countersink bits have 82 degrees, that works for flat head screws.
In planes one uses usually 100 degrees for special screws and bolts in aerospace industry. Wrong choosing the angle is commonly happening, for instance using 90 degrees for a standard screw, when 82 degrees works.
Here is a clear example. For an ANSI screw with half inch long flat head with average heads require a hole of 9/16 inch diameter, a countersink of 1 inch and 82 degree angle. In metric system M6 flat head screws require a hole of 6.6 mm, a countersink of 13.71 mm and 90 degree angle.
For an M5 screw with 9.2 mm head a countersink of 11.2 mm works. 11.0 mm would be too narrow for safe support, while 12.0 mm removes to much material.
When one marks countersink cuts in designs, it is good to mention the included angle and the diameter at the surface. The depth is less important than precise surface diameter compared to the head of the fastener that enters the hole. Most tools calculate the depth from a set diameter of countersink using basic math.
Talking about depth, its calculation uses simple math. Consider the countersink as a half right triangle. One side is the half of the top diameter, one angle the half of the countersink angle, and the other leg the depth.
For a three quarter inch diameter with 82 degrees the depth reaches around 0.4312 inches.
For measuring the diameter of a countersink one can use tools like a caliper. Press the cap in the countersink, until the flat part sits flat against the surface, and it shows the diameter. On the other hand, in tight spaces withdetails around the usual tools can be hard to use.
