How to convert em to CSS millimetres
mm = em × (element font size in px) ÷ (3.77952755905512)
Convert em into CSS reference pixels, then divide by the pixel size of mm. The reference table uses 16 px for both element and root font sizes. Examples state any different context. For em in a font-size declaration, use the parent’s computed font size; for other properties, use the element’s computed font size.
Worked examples
1 em ≈ 4.233333333 mm
1 × 16 ÷ 3.77952755905512 ≈ 4.233333333
Font basis for this example: element 16 px; root 16 px.
1.5 em ≈ 7.9375 mm
1.5 × 20 ÷ 3.77952755905512 ≈ 7.9375
Font basis for this example: element 20 px; root 16 px.
Frequently asked questions
Which font or display assumption applies?
For font-size: 1.5em on a child with a 20px parent resolves to 30px. For other properties, em uses that element’s computed font size. rem uses the root font size; it is not necessarily 16px.