Equipment used in the demonstration: Huawei Enjoy 20, EMI9.1,mobile phone QQ8.5, and mobile phone sogou input method 10.36.

First, because there is no separate symbol for square millimeters, it is generally composed of square symbols combined with millimeters ㎜? ; Or a lowercase m followed by a square meter symbol in front of it forms m㎡, but because the latter is not beautiful, the input method of the former is demonstrated here, and it is also demonstrated here in QQ. Click the QQ message bar to pop up the input method panel, and then click "Symbol".

Second, after opening the symbol, find the math label and click Enter.

Third, find the symbol ㎜ in the mathematical symbols and click Enter.

Fourth, then enter the corner mark, find the superscript number 2, and click Enter.