How, let's say, to force "em" to take into account the line-height?

Question

css

You need to set the height of the textarea to 20 lines. It would be desirable through css. Through attribute "rows" works. But setting it via css as "height: 20em" did not give the expected results, because there is also such a property as "line-height", which affects the height of the line and which takes into account the "rows" attribute, but does not take into account "em". Any "pt", "%", "vw", "vh", etc. - also, the campaign, they do not take into account (or looked badly).
Any solutions besides manually multiplying 20 by line-height? I am using bootsrap.

Answer 1

line-height cannot change the height of a textarea. Multiply.
And remember that 20 em = 20 lines only if line-height = font-size.