Three characters look like a circle with a slash, and all three appear in drawings, specifications and product listings as a diameter mark. Only one is actually the diameter sign. This guide explains which is which, when the substitutes are fine, and how to type each on every platform.
Quick answer
| Symbol | What it is | Windows | Mac | Word |
|---|---|---|---|---|
| ⌀ | Diameter sign (U+2300) | Win + . → search “diameter” | Character Viewer | 2300 then Alt + X |
| Ø | Capital O with stroke (U+00D8) | Alt+0216 | Option + Shift + O | 00D8 then Alt + X |
| ø | Small o with stroke (U+00F8) | Alt+0248 | Option + O | 00F8 then Alt + X |
| ∅ | Empty set (U+2205) | Win + . → search “empty set” | Character Viewer | 2205 then Alt + X |
Copy any of them from the tiles at the top of this page.
Which one is correct
- ⌀ Diameter sign. The character Unicode defines for diameter. It is slightly smaller than a capital letter and its slash extends past the circle, matching the mark used in ISO drawing standards. Use it in typeset documents, product data and anywhere the text may be searched or read aloud, because its name is “diameter sign”.
- Ø Latin capital O with stroke. A letter from Danish, Norwegian and Faroese. It is on every keyboard layout that supports those languages, has an Alt code and a Mac shortcut, and has been the de facto diameter mark in engineering for decades. Almost everyone will read it as diameter. Its drawback is that it is a letter: screen readers say “O” and searching for a diameter value will miss it.
- ∅ Empty set. A math symbol for a set with no elements. It is often the only slashed circle a math font provides, and it is not a diameter sign. Avoid it for measurements.
- ø lowercase o with stroke. Used in the same loose way as Ø, especially in older documents. Prefer the capital or the true sign.
Practical rule: in CAD and on drawings use whatever the software inserts (usually Ø); in Word, web pages and datasheets use ⌀ when the font supports it, otherwise Ø.
Windows
Alt codes for Ø and ø
- Turn on Num Lock.
- Hold Alt and type 0216 on the numeric keypad for Ø, or 0248 for ø.
- Release Alt.
The leading zero matters: Alt+216 without it gives a box-drawing character.
The true diameter sign
⌀ is outside the code page, so there is no classic Alt code. Press Win + ., open the Symbols tab and search “diameter”, or in Word type 2300 and press Alt + X.
Mac
- Option + Shift + O types Ø and Option + O types ø on the US layout.
- For ⌀ open the Character Viewer with Control + Command + Space and search “diameter”.
- Add a text replacement (System Settings → Keyboard → Text Replacements) such as
;dia→ ⌀ if you use it daily.
Word, Excel and PowerPoint
In Word, the fastest route for any of the four is the code followed by Alt + X (see the quick-answer table). Insert → Symbol → More Symbols with the font set to Segoe UI Symbol finds ⌀ under Miscellaneous Technical. In Excel, =UNICHAR(8960) returns ⌀ and =UNICHAR(216) returns Ø, which lets a custom label be built from a value: =UNICHAR(8960)&A2&" mm". PowerPoint accepts the Alt codes and paste but not Alt+X.
Google Docs and Sheets
Use Insert → Special characters and search “diameter” for ⌀ or “o with stroke” for Ø. Alt codes work in Docs on Windows and Option shortcuts on Mac. In Sheets use =CHAR(8960) or =CHAR(216).
AutoCAD and other CAD programs
- AutoCAD: type
%%cin any text, mtext or dimension override; it becomes the diameter symbol. The in-place text editor’s Symbol menu also lists Diameter. Dimension styles can add it automatically to diameter dimensions. - SolidWorks: in a note, use the Symbol library (the Ø button) or type
<MOD-DIAM>. - Inventor and Fusion 360: the text editor’s symbol dropdown includes Ø; Fusion also accepts pasting ⌀.
- Revit: type the character into the text (Alt+0216) or add it to a dimension prefix.
Most CAD programs insert Ø rather than ⌀, and that is the expected convention on drawings.
HTML and code
| Symbol | Named entity | Numeric | CSS | JavaScript |
|---|---|---|---|---|
| ⌀ | none | ⌀ |
"\2300" |
"⌀" |
| Ø | Ø |
Ø |
"\00D8" |
"Ø" |
| ø | ø |
ø |
"\00F8" |
"ø" |
| ∅ | ∅ |
∅ |
"\2205" |
"∅" |
Related marks
Diameter often appears alongside other drawing symbols: the degree sign ° for angles, the plus-minus sign ± for tolerances, and the radius R (which is a plain letter). See the degree symbol page and the plus-minus page for their codes.
Troubleshooting
- ⌀ shows as a box. The font lacks the glyph. Segoe UI Symbol, Arial Unicode MS, DejaVu Sans and Cambria Math include it; Calibri does not. Switch the font or fall back to Ø.
- The symbol prints too large or misaligned. Ø is a capital letter and sits on the baseline at full height. Use ⌀, which is designed as a symbol, or reduce the font size of the Ø by one or two points.
- Search cannot find my diameter values. Documents mix ⌀, Ø and ∅. Pick one for the whole document and use Find and Replace to normalise the rest.
Frequently asked questions
What is the Alt code for the diameter symbol? +
The true diameter sign ⌀ has no classic Alt code; type 2300 then Alt + X in Word. Most people use Ø instead, which is Alt+0216 (capital) or Alt+0248 (lowercase ø).
How do I type the diameter symbol on a Mac? +
Press Option + Shift + O for Ø or Option + O for ø. For the true ⌀ use the Character Viewer (Control + Command + Space) and search 'diameter'.
Is Ø the same as the diameter symbol? +
No. Ø is a Danish and Norwegian letter, ∅ is the mathematical empty set, and ⌀ is the diameter sign. They look alike and Ø is accepted in most engineering contexts, but for typeset documents ⌀ is correct.
How do I type the diameter symbol in AutoCAD? +
In a text or dimension field type %%c, which AutoCAD converts to Ø. The Symbol menu in the text editor also offers Diameter directly.