Diameter symbol
The diameter symbol ⌀ is the circle-with-a-line you meet on every engineering drawing, pipe fitting and lens filter — one character that replaces a whole phrase. Drawings also write it with the letter Ø and older notes spell it out as the dia symbol (“DIA 25”). This page copies the true sign and separates it from its two famous lookalikes.
Copy the sign and its lookalikes
Drawing callouts ready to copy
⌀ vs Ø vs ∅ — three different characters
| Character | What it actually is | Right use |
|---|---|---|
| ⌀ U+2300 | The diameter sign itself, from technical-drawing standards | Drawings, specs, parts lists — the correct choice for diameter |
| Ø U+00D8 | A letter — capital O with stroke in Danish and Norwegian | Borrowed for diameter on many drawings and in CAD fonts; reads fine, but it is alphabet, not symbol |
| ∅ U+2205 | The empty set of mathematics (“contains nothing”) | Set theory only — using it for a pipe size quietly changes the subject |
Why three twins? Printers and programmers each solved the same drawing twice: engineers needed a diameter mark, Scandinavian alphabets already had a slashed O to lend, and mathematicians drew their own empty set. On paper they pass for each other; in a file they are different characters with different names — and a search for one will not find the others.
How to type it
| Platform | Method |
|---|---|
| Windows / Word | Type 2300 then Alt + X for ⌀ |
| Mac | Option + O gives Ø (the letter, not ⌀) — for the true sign, copy it here or use Character Viewer |
| HTML | No named entity — numeric code 8960 |
| CAD / drawings | Most CAD text editors insert ⌀ from their symbol menu (often %%c in AutoCAD-style codes) |
Explore more
The measurement family gathers on the units and SI symbols page, the angle cousin is the degree symbol, and the full science set lives on the math symbols hub.
Frequently asked questions
What does ⌀12 mean?
A diameter of 12 — in whatever unit the drawing uses (usually millimetres on metric drawings). The sign precedes the number and replaces the word “diameter” entirely.
Can I use Ø instead of ⌀?
On paper, people do it constantly and everyone understands. In digital files, prefer ⌀: it is the actual diameter sign, while Ø is a Scandinavian letter that screen readers will pronounce as one.
Is ∅ a diameter symbol?
No — ∅ is the empty set from set theory. It looks like a slashed circle, but in maths it means “the set with nothing in it”, and in a spec it is simply the wrong character.
What does DIA mean on old drawings?
The written-out shorthand for diameter, used before symbol fonts were common — which is why the search “dia symbol” lands here: DIA 25 and ⌀25 are the same callout.
How is diameter different from radius?
Diameter crosses the whole circle through its centre; radius runs from centre to edge — exactly half. Drawings mark radius with R (R25) and diameter with ⌀ (⌀50 describes the same circle).