I have a paragraph of text:
Lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum
Depending on what font you're using you can set max-width
on the paragraph with a calculated value. It will not be exact, but I've found that in most cases that does not matter.
p {
max-width: calc(30em * 0.5);
}
The number you multiply with depends on what font it is, and how much a character takes up in a em square. More characters = less accurate.