Several methods are common today. Translations may be invoiced per line, per word or based on the number of signs. Common PC software offers the option statistics for counting the lines, words and signs of a text.
The most precise invoicing method is based on the number of signs in a text. The number of lines may vary depending on size and type of character. 1800 signs correspond to one A4 page of 30 lines and 60 signs each.