⌀ vs Ø vs ø vs ∅
⌀ (diameter sign) is the dedicated technical symbol for a diameter, used in engineering drawings and specs: ⌀10 means a round feature 10 units across. It is a circle crossed by a diagonal stroke, and it belongs to the technical symbols, not to any alphabet.
Ø and ø are letters of the Danish, Norwegian and Faroese alphabets, the capital and small "o with stroke". Because Ø is in almost every font and easy to type, it is the most common stand-in for ⌀, and most readers will understand either one.
∅ is the empty set from math, a set with no elements. It looks similar but has nothing to do with size, so avoid it in drawings. In LaTeX, \emptyset and \varnothing both produce the empty set, in slightly different shapes.
How to write a diameter
Put the sign before the number: ⌀12 mm or Ø12 mm. Engineering drawings use R for a radius (R6) and S⌀ for the diameter of a sphere. Add a tolerance with the plus minus symbol: ⌀10 ± 0.1. For sizes in inches, combine it with the inches symbol: Ø1/2″.
In some product listings, especially from East Asia, you will also see the Greek letter φ used for diameter. It means the same thing, but ⌀ or Ø is clearer for an international audience.
How to type the diameter symbol
On a Mac, press Option + Shift + O for Ø and Option + O for ø. On Windows, hold Alt and type 0216 for Ø or 0248 for ø on the number pad. For the true ⌀, type 2300 in Microsoft Word and press Alt + X, or copy it from this page.
In AutoCAD text, %%c inserts the diameter symbol. In LaTeX, the wasysym package provides \diameter for ⌀. In HTML, use Ø for Ø or ⌀ for ⌀. Related round shapes are on the circle symbols page.