[header] How to change the string on a lawn trimmer [title] Prepare the line. [step] The length and width of line you need varies based on your trimmer. If you purchase the wrong width of line, the trimmer will not work correctly, so don't waste your money by simply guessing at the hardware store. 
If you are not sure what size line your trimmer uses, check online-the manufacturer's website often has instructions, and if not the customer service department should be able to help you. The length to which the line should be cut also varies, from around 10' to 25'.