Choose string trimmer line size from the approved diameters for your exact tool and cutting head. Then choose among those permitted options for the task. Battery power, engine size, or a line’s ability to fit through an eyelet does not independently establish compatibility.
Quick answer: Find the tool model and head model, check the manual or head instruction card, and buy a permitted diameter and line type. Do not install thicker line, wire, cable, or an unapproved attachment to compensate for a difficult cutting job.
Use this compatibility decision table
Identify both the power unit and cutting head because an approved replacement head can have different instructions. Disconnect power as the manual directs before inspecting or reloading it.
| What you know | What to check | Decision |
|---|---|---|
| Tool model only | Exact head and approved attachment list | Do not choose line until the head is identified |
| Head and tool models | Permitted line diameters and loading method | Use the shared approved specification |
| Several diameters permitted | Task, feed behavior, and manual guidance | Choose within the permitted range |
| Line jams or motor struggles | Loading, spool fill, guard, and line specification | Stop and correct the cause; do not bypass the guard |
Diameter and shape solve different questions
Diameter describes the line’s thickness. Shape describes its cross-section. Round, twisted, or edged profiles can differ in how they feed or cut, but an advertised heavy-duty profile is not permission to exceed the head’s specification. Check whether the head restricts line type as well as diameter.
Units also matter. Match the labeled measurement with the manual rather than choosing a nearby-looking number. Where packaging shows both inches and millimeters, confirm that it corresponds to an approved size instead of rounding beyond the permitted range.
Avoid universal battery-trimmer rules
Do not assume all cordless trimmers require thin line or that every gas trimmer accepts thick line. Tools and heads differ. Changing from the specified line can affect feeding and cutting behavior, so start with the approved configuration when troubleshooting.
If the line breaks frequently, inspect how it was loaded and the condition of the head and eyelets using the maintenance instructions. Repeated impact with masonry or fencing is a different issue from an incompatible diameter. Do not keep feeding line into an unresolved jam.
Reload and check safely
Follow the winding direction, permitted line length, and spool arrangement for the head. Use the specified guard and cutting attachment. Restore all covers before use and keep people and animals outside the safety area stated in the tool manual.
For replacement directions, see STIHL’s line replacement guidance. It directs users to the head instructions for size and loading information. Another brand’s manual remains the authority for that brand.
Questions before buying
Can I use thicker line if it fits? No. Physical fit alone does not establish approval for the head and tool combination.
Should I soak line before loading it? Follow the line manufacturer’s storage and preparation directions. Do not impose a universal soaking routine on every line material.
What should I record? Keep the tool model, head model, approved line specification, and loading instructions with your spare spool. Browse lawn equipment guides and use our winter storage checklist when putting the trimmer away.
