square millimeter

The square millimetre is how the electrical trades name wire. Metric conductors are sold by cross-section in the IEC standard series, and an electrician who says "two-and-a-half" means 2.5 mm² cable, the usual choice for socket circuits, with 1.5 mm² for lighting and 6 or 10 mm² for cookers and showers. American practice names the same wires by gauge number instead, which is why equipment manuals for international markets carry a mm²-to-AWG table.

The subtlety inside those cables is that current rating does not scale with area. A 2.5 mm² conductor is typically rated near 24 A where a 1.5 mm² carries about 17.5 A under like conditions: two thirds more copper buys only about a third more current, because a conductor generates heat throughout its volume but sheds it only through its surface. In structural work the unit pairs with the newton to give N/mm², numerically identical to the megapascal as the caution says, so a 100 mm² tie bar carrying 35.5 kN is at 355 MPa with no conversion step at all. That identity is the reason the site's stress and section calculations can stay in millimetres from the drawing to the answer.