Diameter symbol ⌀

• Engineering drawings: a dimension such as ⌀ .250 gives a hole or shaft diameter. On US drawings to ASME standards, inch dimensions under 1 have no leading zero (.250), while metric dimensions do (⌀ 0.5).
• Product specs: “Tabletop ⌀ 36 in,” “Planter ⌀ 12 in.”
• Geometric dimensioning and tolerancing (GD&T): ⌀ in front of a tolerance value means the tolerance zone is cylindrical.
• Photography: lens filter thread sizes are often printed with a diameter sign, as in ⌀58 mm.

The sign goes before the number. Whether you add a space depends on the drawing standard and company practice; follow the convention your drawings already use.

Four characters look alike, and the wrong one is common in specs:

• ⌀ (diameter sign) is the correct technical symbol.
• Ø and ø are Danish and Norwegian letters. They often stand in for ⌀ because they are easy to reach (Option+Shift+O on a Mac, Alt+0216 on Windows) and look close enough in many fonts.
• ∅ (empty set) is a math symbol, not a measurement sign.

Readers will understand Ø, but it has drawbacks: search, filters and data imports treat it as a letter, and screen readers announce it as one. If your spec sheet or product data will be searched or parsed, use ⌀ consistently, or write “dia.” as the plain-text fallback.

⌀ lives in Unicode’s Miscellaneous Technical block, which not every font covers. If it shows as an empty box, switch to a font with broader symbol support.

• Word: use the Alt+X code from the table above, then save it as an AutoCorrect entry (for example “dia1” → ⌀) under File > Options > Proofing.
• Mac: open the Character Viewer with Control+Command+Space and search “diameter.”
• CAD software: most programs have their own diameter code or symbol menu; check the text-editing help.
• Phones: copy it here once and save a text replacement.
• LaTeX: \diameter from the wasysym package.