You've seen it on engineering drawings. A strange little symbol that looks like an upside-down V, followed by numbers like "⌀10 × 90°". You may have typed it on your keyboard at some point, or maybe you just copied it from the internet. But do you actually know what it means?

That symbol is called the countersink symbol, and it looks like this: ⌵. In this guide, I'll explain what it means, how to read it, how to use it correctly, and what happens when you get it wrong. No engineering degree required – just plain English.

1. What Is the Countersink Symbol For?

A countersink is basically a conical recess carved around the top of a hole. Its job is simple: let a flat-head screw or bolt sit flush with or slightly below the surface of the material. No more screw heads sticking out, no more snagged cables, no more scratched hands.

Think about your phone case, your laptop lid, an airplane panel, or a car dashboard – all those screws are flush, not sticking out. That's because someone specified a countersink on the drawing. And the symbol that tells the machinist "put a cone-shaped recess here" is ⌵.

2. What Does the Countersink Symbol Look Like? How to Type It

The countersink symbol is Unicode character U+2335, and it looks like this: ⌵.

How to type it:

• Windows: Hold Alt and type 9013 on the numeric keypad, then release Alt.

• Windows (alternate): Type 2335, then press Alt+X.

• Mac: Open the Character Viewer and search for "countersink".

• Linux: Hold Ctrl+Shift, type U 2335, then release.

If you're in a hurry, just copy it from somewhere – no shame in that.

3. How to Call Out a Countersink on an Engineering Drawing

Here's the standard format:

⌵ ⌀10 × 90°

That translates to: make a countersink, major diameter 10 mm, included angle 90 degrees.

A complete countersink callout actually involves several dimensions:

Example: First you call out a through hole ⌀5. Then below that, you write "⌵ ⌀10 × 90°". Translation: drill a 5 mm hole for the screw shaft, then carve a 10 mm diameter, 90° cone at the opening to seat a flat-head screw.

4. Countersink vs. Counterbore – Don't Mix Them Up

Beginners often confuse the countersink symbol with the counterbore symbol. They look kind of similar, but they mean completely different things:

Here's an easy way to remember: countersink is a cone, counterbore is a cylinder.

Mixing them up is expensive. You'll have parts arrive with the wrong hole type – and every single one goes to scrap.

5. Countersink Angles – 90° vs. 82° (and Others)

The angle of your countersink depends entirely on what screw you're using:

• Metric fasteners → use 90° (ISO standard for flat-head screws)

• Imperial (inch-based) fasteners → use 82° (ASME standard)

Picking the wrong angle creates a mismatch. A metric 90° screw in an 82° countersink touches only at the outer rim, not the full cone surface. Clamping force isn't distributed evenly, the screw won't seat properly, and it's likely to loosen over time.

Other angles you might encounter:

• 100° – Common in aerospace for thin skins (shallow cone leaves more material)

• 60° and 120° – Specialty applications (countersinks for optical mounts or metrology)

Rule of thumb: metric screw = 90°, imperial screw = 82°. When in doubt, check your fastener spec sheet.

6. How to Calculate Countersink Depth

Here's the depth formula:

h = (D − d) / [2 × tan(θ/2)]

Where h = depth, D = countersink diameter, d = pilot hole diameter, θ = included angle.

Example: M6 flat-head metric screw. Head diameter ≈ 12 mm, pilot hole = 6.6 mm, angle = 90°.
At 90°: h = (12 − 6.6) / [2 × tan(45°)] = 5.4 / 2.0 = 2.70 mm.

What does that mean? Your material must be at least 2.7 mm thick to fully accept that countersink. If the material is thinner, the cone will cut all the way through, leaving a hole rather than a recess – and your screw head will stick out.

7. Four Common Mistakes on Countersink Drawings

1. Forgetting to specify the angle
Writing "⌵ ⌀10" without the angle is missing the most important detail. Flat 90°? 82°? 100°? The machinist can't guess. Always include the angle.

2. Getting the diameter wrong
The countersink diameter should match the maximum head diameter of your screw, plus a small tolerance (0.05–0.1 mm). Too small, and the screw won't fit. Too large, and the screw head will rock back and forth.

3. Putting a countersink too close to the edge
A countersink removes material right at the rim of your part. If it's too close to the edge, there may not be enough material left to hold the screw. The sidewall cracks when you tighten it.

4. Using the wrong symbol
This mistake happens over and over again. You put ⌵ on the drawing but meant a counterbore. The shop makes ⌴ instead. Your socket-head screw arrives and doesn't seat properly. Hundreds of parts get scrapped. Check your symbol twice.

8. Countersinks on PCBs – What's Different?

If you work with printed circuit boards, countersinks work the same way: a conical recess that lets a flat-head screw sit flush with the board surface.

What you need to specify on PCB drawings: pilot hole diameter, countersink diameter (or angle), depth (or angle), whether the hole is plated (PTH) or not (NPTH), and which side of the board gets the countersink.

The big difference: PCB material (FR4 fiberglass) is nowhere near as tough as metal. Cut too deep or at the wrong angle, and you can damage inner copper layers or crack the board around the hole opening. A good rule is to keep countersink depth to less than one‑third of total board thickness. If you need deeper, consider a counterbore with a washer instead.

9. Quick Summary

• The countersink symbol ⌵ means put a cone‑shaped recess at the hole opening to seat a flat‑head screw flush with the surface.

• Standard callout format: ⌵ diameter × angle (e.g., "⌵ ⌀10 × 90°").

• Metric screws use 90°; imperial (inch) screws use 82°. Don't mix them.

• Countersink (⌵) is a cone; counterbore (⌴) is a flat‑bottomed cylinder. They are not interchangeable.

• Leave enough material thickness, and always include the angle on your drawing.

That tiny little symbol can save your assembly from a major headache. Next time you see ⌵ on a drawing, you'll know exactly what it means – and more importantly, how to get it right. If you have a project that needs countersinks – whether on machined metal parts, PCBs, or rigid‑flex assemblies – feel free to send your drawings our way. We'll help you make sure that little ⌵ does exactly what it's supposed to do.