The model of weed trimmer used determines the length of the trimmer line; for example, 10 feet of line is suitable for a traditional bump head trimmer, while 30 feet of line is optimal for a Craftsman electric model. Some trimmers require pre-cut replacement lines. Consult the owner's manual to determine what length is appropriate for the trimmer.
Pre-cut trimmer line is available through online stores such as Sears.com. It comes in a variety of lengths, including 8, 10, 12 and 18-inches. For example, Maxpower Precision Parts pre-cut line comes in packages of six 16.5-inch strips. Consult the manufacturer's requirements to determine the best pre-cut line to install on the trimmer. Users should not alter the length of pre-cut line specially made for a particular trimmer brand.
Spool-head model trimmers require users to wind longer lengths of line around the trimmer head. Craftsman tools by Sears only use nylon line and should not be equipped with substitute materials, such as titanium or plastic. Sears also sells pre-loaded replacement trimmer heads, which are already fitted with properly sized line and are suitable for different brands.
The diameter of trimmer line is also an important consideration, because different sizes are appropriate for different purposes. Thinner, round lines are suitable for general use around the home and yard and for smaller projects. Serrated lines with a thicker diameter can cut through dense, large weeds and are better suited for commercial landscaping because they do not wear down as quickly. Square-shaped lines can cut through medium-sized weeds and may be useful for home and commercial trimmers.