The basic steps for replacing trimmer line

Replacing string trimmer line takes about five to ten minutes once you know where the spool is and how your trimmer's head works. Most trimmers have a spool — a plastic bobbin that holds the line — inside a round head at the end of the shaft. You remove the head (usually by twisting or pressing a button), take out the old spool, wind new line onto it in the direction marked on the spool, and snap it back in. The exact steps depend on whether your trimmer has a fixed spool you wind by hand or an automatic spool that feeds line as you use it.

Before you start, buy the correct line diameter for your trimmer. Check your trimmer's manual or the head itself — the size is usually stamped on the plastic. Common sizes are 0.065 inch, 0.080 inch, and 0.095 inch. Line that is too thin will break constantly; line that is too thick will not fit through the feed holes or will strain the motor. You can buy line in bulk spools or pre-wound cartridges, depending on your trimmer model.

Key Takeaways

  • Check your trimmer's manual or the head itself to find the correct line diameter before you buy replacement line.
  • Most trimmer heads come off by twisting counterclockwise or pressing a release button on the side.
  • Wind new line onto the spool in the direction marked by an arrow on the plastic, leaving 4 to 6 inches free on each side to feed through the holes.
  • If your trimmer has an automatic spool, you may only need to insert a pre-wound cartridge and snap the head back on.
  • Test the line feed by tapping the trimmer head gently on the ground while the engine is off to see if line extends from the holes.

How to remove the trimmer head

The trimmer head sits at the end of the shaft and is held on by either a twist-lock or a button release. Look at the head from the side and bottom. If you see a small button or tab, press it while turning the head counterclockwise — the head will unscrew and come free. If there is no button, grip the head firmly and turn it counterclockwise by hand. Some heads require you to hold the shaft steady with one hand while you turn the head with the other.

Once the head is loose, you may see a metal or plastic collar underneath it. Do not lose this collar — it holds the spool in place. Set the head, collar, and any springs or washers on a clean surface in the order you removed them so you can reassemble them the same way. Take a photo with your phone if you are unsure about the order.

Winding line onto a manual spool

A manual spool is a plastic bobbin with two or three slots or notches around the edge. Look for an arrow printed on the spool that shows which direction to wind. This direction matters — winding backward will cause the line to tangle or not feed. Cut a length of new line about 15 to 20 feet long (you can estimate by laying it on the ground). Insert one end into a slot on the spool and hold it there with your thumb.

Wind the line tightly and evenly around the spool in the direction the arrow points. Keep tension on the line as you wind so it does not bunch up or overlap. Wind until you have about 4 to 6 inches of line left free — this free end will feed through the holes in the trimmer head when you reassemble it. Tuck the free end into the slot or notch to hold it in place. Repeat this process with a second line if your spool has two slots (most do).

If your spool has three slots, wind line into two of them and leave the third empty. Some older trimmers use a single long line that winds back and forth across the spool instead of two separate lines — check your manual to see which type you have.

Installing a pre-wound cartridge

Some trimmers use pre-wound cartridges that snap into the head instead of requiring you to wind line by hand. These cartridges look like small plastic spools with line already wrapped around them. To use one, remove the old spool from the head, insert the new cartridge into the same slot or opening, and push or twist it until it clicks into place. The line ends should already be positioned to feed through the holes.

Pre-wound cartridges cost more per use than bulk line but save time and eliminate the frustration of winding. They are worth the extra cost if you trim frequently or if hand-winding has given you trouble in the past. Check your trimmer's manual to confirm that your model accepts cartridges — not all do.

Reassembling the trimmer head

Once your spool is wound or your cartridge is installed, thread the free ends of the line through the small holes or eyelets in the trimmer head. Most heads have two holes, one on each side, but some have four. The line should pass through cleanly without kinking. If the line does not fit through the hole, the diameter is wrong — go back and check that you bought the correct size.

Place the spool back into the head in the same position it was in before. If there was a collar, spring, or washer underneath the spool, put those back in the same order. Screw the head back onto the shaft by turning it clockwise. Tighten it firmly but do not force it — you should feel it stop naturally when it is seated. If your head has a button release, press the button and turn clockwise until the head clicks into place.

Testing the line feed

Before you use the trimmer, test that the line feeds correctly. Make sure the engine is off. Hold the trimmer upright and tap the head gently on the ground a few times. You should see the line extend slightly from the holes — usually about a quarter inch. If the line does not move, the spool may be wound backward, or the line may be stuck in the slot. Remove the head again and check the winding direction against the arrow on the spool.

If the line extends too far (more than half an inch), trim it back with scissors so it does not drag on the ground while you work. Once the line feeds smoothly and extends the right amount, you are ready to use the trimmer. The line will wear down as you trim, and the automatic feed (if your trimmer has one) will release more line as needed.

When to call a professional

If the trimmer head will not come off after you have tried turning it counterclockwise, stop and do not force it. The threads may be stripped or the head may be stuck with old line wrapped around the shaft. Take the trimmer to a repair shop rather than risk breaking the shaft or the head. Similarly, if you have reassembled the head and the line will not feed no matter how many times you tap it on the ground, the spool mechanism may be damaged and will need professional service.

If your trimmer is very old or the head is cracked or broken, replacement heads are available for most models and cost between fifteen and forty dollars. A repair shop can install one for you, or you can do it yourself if the shaft threads are still in good condition.

Frequently Asked Questions

What size line does my trimmer use?

Check your trimmer's manual or look for a size stamp on the plastic head itself. Common sizes are 0.065, 0.080, and 0.095 inch. If you cannot find the size, take the trimmer to a hardware store or repair shop and they can measure the old line or the feed holes to tell you what you need.

Can I use a different line diameter than what the manual says?

No. Line that is too thin breaks easily; line that is too thick will not fit through the feed holes or will overstrain the motor. Always match the diameter your trimmer is designed for.

How much line should I wind onto the spool?

Wind until you have 4 to 6 inches of free line left on each side. Too much line will not fit in the head; too little will run out quickly. The exact amount depends on your spool size, but the 4 to 6 inch rule works for most residential trimmers.

Why does my line keep breaking?

Line breaks when it hits rocks, thick weeds, or hard ground. It also breaks if the diameter is too thin for your trimmer or if the spool is wound too loosely. Check that you are using the correct size and that you wound it tightly and evenly. If breakage continues, the feed holes may be cracked or rough — a repair shop can smooth them or replace the head.

Do I need to wind both slots on the spool?

Most two-slot spools work best with both slots wound. If you wind only one slot, the trimmer will be unbalanced and may vibrate. Check your manual to see if your model allows single-slot operation.