The Basic Steps to Swap Out Your Trimmer Line

Replacing weed wacker string takes about five minutes once you know the process. Most trimmers use one of two designs: a fixed spool that you wind by hand, or an automatic head that advances line as you tap it on the ground. The steps differ slightly, but both start the same way—remove the old string, measure the new line, and feed it through the spool or head.

You do not need special tools. A pair of scissors or a utility knife to cut the line is all you need. The hardest part is usually getting the spool off, which sometimes requires a small flathead screwdriver to pop a retaining clip, but many newer models twist off by hand.

Key Takeaways

  • Most trimmers have either a fixed spool (manual wind) or an automatic tap head; check your manual or the head itself to know which you own.
  • Remove the spool or head by twisting counterclockwise or prying out a retaining clip with a flathead screwdriver.
  • Cut a length of replacement line (usually 15 to 20 feet for a standard head) and feed it into the spool slot or automatic head opening.
  • Wind the line tightly and evenly on a fixed spool, or straightforward insert it into an automatic head until both ends are even.
  • Reassemble the head, twist it back onto the trimmer, and test by tapping the head on the ground if it is an automatic model.

Identify Your Trimmer Head Type

Open your trimmer manual or look at the head itself. A fixed spool has visible grooves or slots where line sits, and you wind it by hand. An automatic head looks like a smooth dome or cylinder with a small opening at the bottom; you tap it on the ground and it feeds line automatically as it wears.

If you cannot find the manual, the head design will tell you. Automatic heads are usually round and featureless on top. Fixed spools have a visible winding pattern or a small arrow showing which direction to wind. Some trimmers have a bump-feed head, which is a hybrid—you tap it to advance line, but you wind it by hand when you first load it.

Remove the Old String and the Spool or Head

Unplug the trimmer or remove the battery. Locate the retaining clip or nut holding the spool or head in place. On most models, this is a small plastic clip on the side of the head, or a nut in the center. If it is a clip, use a flathead screwdriver to gently pry it out. If it is a nut, turn it counterclockwise by hand or with a wrench—it should not require much force.

Once the clip or nut is free, twist the entire spool or head counterclockwise and it will unscrew from the trimmer shaft. Pull out any remaining old string. Wipe the inside of the head or spool with a dry cloth to remove grass and debris. This step prevents the new line from jamming.

Cut and Prepare Your Replacement Line

Check the diameter of your old string or consult your manual—common sizes are 0.065 inch, 0.080 inch, and 0.095 inch. Buy replacement line that matches. Cut a length between 15 and 20 feet; if you are unsure, cut 15 feet to start. You can always add more if one side runs short.

Lay the line on a flat surface and straighten it as much as possible. A kinked or coiled line is harder to feed and may jam inside the head. If the line is very coiled from the spool, soak it in warm water for a few minutes to relax it, then dry it completely before feeding it in.

Load String Into a Fixed Spool

Look for the slot or groove on the spool where the line sits. Most fixed spools have a small notch or hook at one end to anchor the line. Insert one end of your cut line into that notch, or straightforward hold it in place with your thumb. Wind the line tightly and evenly around the spool in the direction shown by the arrow on the head—usually counterclockwise when looking down at the spool from above.

Keep tension on the line as you wind so it sits flat and does not bunch up. Wind until you have about 2 to 3 inches of line left on each side of the spool. Do not overfill; if line is stacked on top of itself, it will jam when you reassemble the head. Once both sides are loaded, hold the line in place and move to reassembly.

Load String Into an Automatic or Bump-Feed Head

Automatic heads have a small opening, usually at the bottom or side, where you insert the line. Find that opening—it is often a slot or a hole about the size of a pencil. Insert one end of your cut line into the opening and push it in until about 2 to 3 inches stick out the other side. The line should sit in a channel or groove inside the head.

For bump-feed heads, you may need to wind the line on a small spool inside the head before closing it. Consult your manual for the exact winding pattern, as it varies by brand. Once the line is seated, make sure both ends are even—if one side is much longer than the other, the head will be unbalanced and may vibrate.

Reassemble and Test

Align the spool or head with the trimmer shaft and twist it clockwise until it is snug. Do not overtighten; you should be able to turn it by hand without a tool. Replace the retaining clip by sliding it back into its groove, or screw the nut back on and tighten it by hand. Again, snug is enough—over-tightening can crack the plastic.

Plug in the trimmer or reinsert the battery. If you have an automatic or bump-feed head, hold the trimmer away from your body and tap the head gently on the ground a few times. The line should advance slightly each time. If it does not, the line may not be seated correctly in the opening; remove the head and try again. Once the line advances, you are ready to trim.

Common Mistakes to Avoid

Do not wind the line in the wrong direction. Check the arrow on the spool before you start—winding backward will cause the line to unravel as soon as you use the trimmer. If you are unsure, wind a few turns, then test by gently pulling on the line; it should not slip.

Do not use line that is too thick or too thin for your trimmer. A line that is too thick will jam the head or stall the motor. A line that is too thin will break when ready. The diameter is printed on the package and should match your manual or the old string you removed.

Do not overfill the spool. If line is piled on top of itself, it will tangle when you reassemble the head and may not feed at all. Load until you have about 2 to 3 inches of space left, then stop.

Do not forget to unplug or remove the battery before you start. Even though the trimmer is not running, accidentally bumping the trigger while the head is exposed can cause injury.

Frequently Asked Questions

How do I know what size string my trimmer uses?

Check your manual or look at the old string you removed—the diameter is usually printed on the spool or the package. Common sizes are 0.065, 0.080, and 0.095 inch. If you cannot find the old string, measure it with a ruler or caliper, or bring a piece to a hardware store and compare it to the spools on the shelf.

Can I use a different brand of string than what came with my trimmer?

Yes, as long as the diameter matches. String is string—the brand does not matter. Buy whatever is available at your local hardware store in the correct size. Avoid extremely cheap string, which may be brittle and break quickly, but you do not need to buy the trimmer manufacturer's brand.

What if the line keeps breaking or shredding?

The line may be too thin for your trimmer, or you may be hitting rocks or hard objects. Try a thicker diameter line (for example, go from 0.065 to 0.080 inch) and avoid trimming near gravel or concrete. If the head itself is damaged or the spool is cracked, the line will also fray; inspect the head for cracks and replace it if needed.

How often do I need to replace the string?

It depends on how often you trim and what you are cutting. Light residential use may need a new string every few months. Heavy use or trimming thick weeds may require a new string every few weeks. You will know it is time when the trimmer stops cutting cleanly or the line becomes very short.

Why is my automatic head not advancing line when I tap it?

The line may not be seated correctly in the opening, or it may be wound too loosely. Remove the head, unwind the line, and reload it, making sure it sits fully in the slot or channel. If the head is cracked or the internal mechanism is broken, you may need to replace the head itself rather than just the string.