Changing grass trimmer string takes about five minutes and follows a standard pattern: remove the spool, wind on fresh line, and thread it through the head.
The process is the same whether you’re handling a corded electric, battery-powered, or gas trimmer. The core steps stay consistent — shut off the power, open the head, reload the spool, and reassemble. A clean, properly wound spool is the difference between a trimmer that feeds line smoothly and one that jams every few minutes. Before you begin, park the trimmer on a flat, dry surface and inspect the head for caked-on grass or dirt that could block the line.
Safety First: Power Down The Trimmer
Disconnecting the trimmer from its power source is the one step you should never skip. For corded electric models, unplug the unit. For battery-powered trimmers, remove the battery pack entirely. With gas trimmers, shut off the engine and disconnect the spark plug wire.
This prevents the head from spinning if you accidentally bump the throttle or trigger. Working on a flat surface also stops the trimmer from shifting while you’re trying to pop off the spool cap.
Opening The Trimmer Head And Removing The Old Line
Trimmer heads open a few different ways depending on the brand and model. Most use either a tab-release system or a screw-on cap. Press the tabs on the sides of the head to release the cap, or unscrew it if your model uses threads. Some models feature a bump-feed head where you push the whole assembly down and twist, while others have a small knob on the bottom that you hold while turning the cap.
Once the head is open, pull out the spool. Remove any leftover line and inspect the spool for wear. If the spool or the cover is cracked or worn, buy a replacement rather than reusing damaged parts — a worn spool causes constant feeding problems. Clean out any debris that has collected inside the head.
Winding New Line Onto The Spool Correctly
Most trimmer spools take between 10 and 25 feet of line, but the right length depends on your spool’s capacity. Check the owner’s manual or the markings on the spool itself for the exact amount and the correct line diameter. Using line that’s too thick for your trimmer will cause jams and can strain the motor.
Find the anchor slot or small hole on the spool and insert one end of the line. Wind the line tightly and evenly in the direction shown by the arrow on the spool. Multiple manufacturers — including STIHL in their official tutorial — stress following the arrow rather than assuming a direction.
| Step | What To Do | Common Mistake |
|---|---|---|
| Wind the line | Follow the arrow on the spool, keep it tight and even | Crossing the line over itself |
| Fill the spool | Keep line within the spool’s capacity | Overfilling, which causes jams |
| Secure both ends | Tuck line into the notches on the spool edge | Failing to secure, line unwinds during assembly |
| Thread the head | Push both ends through the eyelets on the head | Threading only one end |
| Reassemble | Snap or screw the cap back on firmly | Misaligning the spool with the head housing |
Leave a gap at the top of the spool, about a quarter inch, so the line can feed freely. Tuck the loose ends into the notches on the spool’s rim, then place the spool back into the head, aligning any tabs with their slots. Feed both line ends through the eyelets on the head housing, pull them snug, and snap or screw the cap back into place.
Common Missteps And When To Replace The Trimmer
Three mistakes cause most feeding problems: overfilling the spool, winding the line in the wrong direction, and using the wrong line thickness. Overfilled spools bind and stop feeding entirely. Wrong-direction winding means the line unwinds instead of locking into the head. And an oversized line jams the eyelets, which is sometimes mistaken for a broken trimmer head.
If your trimmer’s head keeps jamming even after a careful restring, the spool may be worn or the head may simply be an older auto-feed design that benefits from prewound replacement spools, which Home Depot’s trimmer maintenance guide notes are available for many models.
If the trimmer itself is aging and repairs are adding up, a newer model can be the practical fix. For readers ready to upgrade, our roundup of the best 4-stroke grass trimmers tested here covers quiet, low-emission options that hold their tune longer. But if your current unit restrings cleanly and starts reliably, a new spool is all the maintenance it needs.
FAQs
How much string should I put on a trimmer spool?
The right amount depends on the spool’s capacity, which is printed on the spool or listed in the owner’s manual. Most residential spools take 10 to 25 feet. Leave roughly a quarter inch of space at the top of the spool so the line feeds without binding.
Can I use any brand of trimmer line?
You can use any brand that matches the diameter specified in your trimmer’s manual. The brand matters less than the thickness. Line that is too thick will jam the feed mechanism, while line that is too thin wears out quickly and cuts poorly.
What do I do if the string keeps jamming inside the head?
Open the head and check whether the line is wound in the direction of the arrow on the spool. Also confirm you haven’t overfilled the spool and that both line ends are threaded through the eyelets. If the spool is worn or cracked, replace it.
References & Sources
- STIHL USA. “Wind and Replace String Trimmer Line.” Official tutorial covering power disconnection, line diameter, and winding direction.
- Home Depot. “String Trimmer Maintenance.” Covers bump-feed heads and prewound auto-feed spool options.