How Do You Refill a String Trimmer? Step-by-Step Guide for Every Head Type

Direct Answer: How to Refill a String Trimmer

Turn off the trimmer and fully disconnect power — unplug corded models, remove the battery pack on cordless units, or pull the spark plug boot on gas trimmers. Press the spool-retaining tabs, remove the cap and spool from the head. Cut 15–25 feet of replacement monofilament line at the diameter specified in your owner’s manual (commonly 0.065 inch for light residential, 0.080 inch for medium-duty, or 0.095 inch for heavy-duty trimming). Anchor the end of the line in the spool’s retainer hole or notch, wind it firmly and evenly in the direction of the arrow molded onto the spool, and leave approximately 6 inches of feed tail per strand protruding through the eyelets on the trimmer head. Snap the spool and cap back into place until the tabs click.

That 50-word summary covers the core procedure, but the full details below will walk you through every nuance — from identifying your head type to troubleshooting the most frustrating post-refill problems.

What “Refilling” Actually Means (and Which Method Applies to You)

Before you unwind a single foot of trimmer line, it is critical to understand that “refilling a string trimmer” can refer to two fundamentally different tasks, and the procedure for each is not interchangeable.

Re-winding bulk line onto an existing spool is the traditional method. You buy a spool of bulk monofilament line, cut it to the recommended length, thread it into a retainer hole on your trimmer’s spool, and manually wind it by hand. This is the most common refill path for bump feed and some automatic feed heads.

Dropping in a pre-wound spool cassette is the quicker alternative available on many modern consumer trimmers. The manufacturer sells a sealed cassette pre-loaded with the correct line length and diameter. You simply remove the old cassette and snap the new one in — no winding required. These cassettes are typically brand- and model-specific, meaning a Ryobi cassette will not fit a Husqvarna head, and they generally cost more per foot of line than bulk monofilament. However, they eliminate winding errors and tangling, which makes them popular with first-time users.

Which method you need depends on your trimmer head type. Here is a quick identification guide:

Bump Feed Head

The most prevalent head type on residential and prosumer trimmers. It features a spring-loaded spool inside a cap. When the head is tapped (“bumped”) against the ground during operation, a mechanism releases a short length of line from the spool. You refill it by winding bulk line onto the spool. Look for a round cap on the bottom of the head with visible tabs or arrows indicating bump direction.

Fixed-Line Head

Common on commercial-grade trimmers. There is no internal spool. Instead, short pre-cut segments of line (typically 10–12 inches each) are inserted directly into holes or channels on the head’s outer housing. When the line wears down, you stop the trimmer, pull out the stubs, and push in fresh segments. You refill it by inserting pre-cut line pieces — no winding involved.

Automatic Feed Head

Uses a small internal motor or centrifugal clutch mechanism to advance line automatically as it wears. Many automatic feed heads accept either bulk line winding or a proprietary pre-wound cassette. Some brands market these as “EasyLoad” or “SpeedLoad” systems. Check your owner’s manual to determine whether your model uses a refillable spool or a drop-in cassette.

Pre-Wound Spool Cassette

A sealed, disposable unit sold by the trimmer manufacturer. You replace the entire cassette when the line runs out. Found on many entry-level consumer trimmers from brands like Ryobi, Greenworks, and Worx. The refill task is simply removing the old cassette and inserting the new one. No winding, no cutting — just match the part number to your trimmer model.

Bottom line: If you have a bump feed or refillable automatic feed head, the step-by-step winding instructions in the sections below apply to you. If you have a fixed-line head, jump to the fixed-line section. If you use a cassette system, jump to the pre-wound cassette section.

Choosing the Right Trimmer Line Diameter and Type

Using the wrong line diameter is the single most common setup error that leads to poor cutting performance, premature line breakage, and even damage to your trimmer’s spool or head housing. The correct diameter is specified by the trimmer manufacturer and is usually printed on the spool itself, on a sticker on the trimmer head, and in the owner’s manual.

Line Profile: Round, Twisted, and Multi-Sided

• Round line is the standard profile. It feeds reliably, resists welding (fusing) inside the spool, and is the safest choice for general-purpose trimming around fences, trees, and delicate plantings.
• Twisted or serrated line has a slightly aggressive surface that can cut through thicker weeds more efficiently. It produces a louder hum and may wear faster against hard surfaces.
• Multi-sided line (star, square, pentagon) features sharper cutting edges on each face. It delivers a cleaner cut on tough vegetation but is more prone to snapping on contact with hard objects such as concrete or chain-link fencing.
• Heavy-duty or reinforced line is formulated with added polymers or aluminum-core composites for extreme conditions. Use it only in heads rated for the corresponding diameter.

Key rule: Never exceed the maximum line diameter rated for your trimmer. Running 0.095 inch line in a head designed for 0.065 inch line can jam the spool, overload the motor, and void your warranty.

How Much Line Per Strand?

For most bump feed spools, cut 15 to 25 feet of line per strand. Your spool’s sidewall or your owner’s manual will specify the exact length. For dual-line spools, cut two equal lengths. For fixed-line heads, each segment is typically 10 to 12 inches (the length that protrudes from the head during use).

Safety First: How to Power Down Your Trimmer Before Reloading

Skipping the safety shutdown is the one step you cannot afford to skip. A spinning trimmer head can cause severe lacerations in a fraction of a second. Follow the procedure for your trimmer’s power source:

Gas-Powered Trimmers

1. Turn the ignition switch to the OFF position.
2. Pull the spark plug boot off the spark plug terminal. This prevents accidental starting if the starter cord is pulled. The boot is the rubber cap connected to the thick wire running from the engine to the spark plug on the side or top of the engine cylinder.
3. Allow the engine to cool for at least 2–3 minutes before handling the trimmer head. The muffler and engine housing can retain enough heat to cause burns.

Corded Electric Trimmers

1. Turn the trimmer’s power switch to OFF.
2. Unplug the extension cord from the wall outlet. Do not simply unplug the trimmer from the extension cord — disconnect at the source to eliminate any possibility of reconnection by another person or accidental contact.

Battery-Powered (Cordless) Trimmers

1. Turn the trimmer’s power switch to OFF.
2. Remove the battery pack from the trimmer body. This is the equivalent of unplugging a corded model and ensures zero power to the motor.

In all cases, lay the trimmer on a flat, stable surface with the head facing up before beginning the refill procedure.

How to Refill a Bump Feed Spool (Single-Line and Dual-Line)

Bump feed heads are the most common type in the consumer market and the head type most frequently asked about in search. The procedure below covers both single-line and dual-line spools. Steps that differ between the two are clearly labeled.

Step 1: Remove the Spool Cap

Locate the bump cap on the bottom of the trimmer head (the part that faces the ground during operation). Depending on your model:

• Tab-release style: Press the two side tabs inward simultaneously while pulling the cap straight down and off the head.
• Twist-release style: Grip the cap and turn it counterclockwise (usually about a quarter turn) until it releases from the housing.
• Center-bolt style: Some commercial heads require you to hold the spool with a wrench or the built-in spanner slot while unscrewing a central bolt.

Set the cap aside. Lift the spool straight out of the head housing. Inspect the inner spool for old line remnants and remove them.

Step 2: Locate the Retainer Hole or Notch

Look at the spool’s center hub. You will find a small hole, a slot, or a slit — this is the anchor eyelet or line retainer hole. Some spools have two retainer holes positioned on opposite sides of the hub for dual-line loading.

Step 3: Measure, Cut, and Anchor the Line

Cut your monofilament line to the length specified on the spool sidewall or in the owner’s manual (typically 15–25 feet).

• Single-line spool: Insert one end of the line approximately ½ inch into the retainer hole. The end should be locked in place by the hole’s internal friction or a small hook.
• Dual-line spool: Cut two equal lengths. Insert each end into the two separate retainer holes, one on each side of the spool hub. The two strands will be wound in opposite directions onto their respective halves of the spool.

Step 4: Wind in the Correct Direction

Look closely at the spool’s sidewall. You will see an arrow molded or printed into the plastic — this indicates the required winding direction. Winding against the arrow will cause the line to unspool incorrectly during operation, resulting in jams and feed failures.

• Single-line spool: Wind the line evenly and firmly in the direction of the arrow. Each wrap should lay tight against the previous one without overlapping. Continue until approximately 6 inches of line remains unwound.
• Dual-line spool: Wind each strand on its respective side of the divider in the direction of the arrow. The two strands move in the same rotational direction but sit on separate halves of the spool. Wind each strand until 6 inches of tail remains.

Pro tip: Wind with consistent tension. Loose winding causes the line to tangle, overlap, and “weld” (fuse) together from friction-generated heat during operation. If the line feels floppy on the spool, unwind it and start again with firmer hand tension.

Step 5: Leave Feed Tails and Thread Through Eyelets

The 6 inches of line that remain unwound are the feed tails. These tails are what the bump mechanism releases when you tap the head on the ground.

Thread each tail through the corresponding eyelet (line exit hole) on the trimmer head housing. There will be one eyelet for a single-line head or two eyelets positioned on opposite sides of the housing for a dual-line head. Pull the tails through until the spool sits snugly inside the housing.

Step 6: Reassemble the Head

1. Place the spool back into the head housing, ensuring the feed tails remain threaded through the eyelets.
2. Align the cap with the housing. Press or twist it back into position until the tabs click or the cap locks firmly in place.
3. Gently tug each feed tail to confirm it moves freely through the eyelet without snagging.

Your bump feed trimmer is now refilled and ready to use. Reconnect power (plug in, insert battery, or reattach spark plug boot) and test on a patch of grass.

How to Refill a Fixed-Line Trimmer Head

Fixed-line heads eliminate spool winding entirely. Instead of a continuous line wrapped around an internal spool, they use short pre-cut segments that insert directly into channels or holes in the head housing.

1. Power down and lay the trimmer flat following the safety steps above.
2. Remove the old line segments. Most fixed-line heads have two line channels (occasionally three or four). Pull or twist out the worn stubs from each channel.
3. Cut fresh line segments to the length specified by the head manufacturer — typically 10 to 12 inches per segment. Use the same diameter line specified for your head.
4. Insert each segment into its channel. Push the line into the hole until it clicks or seats firmly against the internal retainer. Some designs require you to thread the line through an outer eyelet first, then push it into the hub channel. The line should protrude evenly from both sides of the head.
5. Verify the protruding length. Each side should stick out far enough to trim effectively (about 5–6 inches from the head housing) but not so far that it exceeds the trimmer’s cutting swath guard.

No winding direction or spool tension is involved. The primary risk with fixed-line heads is using the wrong diameter (which causes the segment to fall out or jam) or cutting segments too long (which overloads the head and reduces RPM).

How to Replace a Pre-Wound Spool Cassette (Automatic and EasyLoad Heads)

Pre-wound cassettes are the simplest refill option but the most brand-dependent. Each cassette is engineered to fit a specific head model, and using the wrong part number can result in an improper fit, feeding issues, or head damage.

1. Identify your trimmer model number (usually on a sticker on the shaft or motor housing).
2. Purchase the correct replacement cassette. Match the part number exactly. Cassettes from the same brand but a different trimmer model line are generally not interchangeable.
3. Remove the old cassette. Press the release tabs or unscrew the cap (per your head’s design) and lift out the spent cassette.
4. Insert the new cassette. Align it with the spool housing, push it down until it seats, and replace the cap or click the tabs back into position. Thread the pre-extended feed tails through the eyelets if they are not already routed.
5. Reconnect power and test. Tap the head lightly on the ground to confirm line feeds correctly.

Cost consideration: Pre-wound cassettes typically cost $8–$15 each and contain 15–20 feet of line. Bulk monofilament of the same diameter costs $5–$12 for a 100- to 200-foot spool, making bulk winding roughly 3–5 times cheaper per foot. If you trim frequently, learning to wind your own spool pays for itself quickly.

Troubleshooting: Line Breaks Immediately, Won’t Feed, or Winds Unevenly

Even when you follow every step, things can go wrong. Here are the three most common post-refill failure modes, their root causes, and their fixes.

Problem: Trimmer Line Breaks Immediately After Refilling

Possible causes:

• Wrong line diameter. Using a diameter smaller than the head is rated for allows the line to whip at excessive speed, generating centrifugal stress beyond the line’s tensile rating. Using a diameter larger than rated overloads the feed mechanism and can cause the line to snap at the exit eyelet from excessive friction.
• Old or sun-degraded line. Monofilament line dries out and becomes brittle over time, especially if stored in direct sunlight or extreme heat. If your bulk line has been sitting in the garage for more than a year, it may have lost its flexibility. Discard it and buy a fresh spool.
• Multi-sided or specialty line in a tight eyelet. Star- and square-profile lines can catch on the edges of narrow eyelets, creating a stress point that leads to snapping. If your head has small eyelets, stick with round profile line.
• Contact with hard objects. Concrete edging, fence posts, rocks, and tree bark will sever any line quickly. This is normal wear, not a refill error. Angle the trimmer head so the line strikes vegetation, not hardscape.

Problem: Line Won’t Feed When You Bump the Head

Possible causes:

• Feed tails are too short. If the tails protruding from the eyelets are less than 4 inches long, the bump mechanism may not have enough line to release. The fix: remove the cap, pull additional line through the eyelets by hand until 5–6 inches protrudes per strand, and reassemble.
• Line is fused (welded) inside the spool. When wound too loosely, overlapping layers of line generate friction heat during high-RPM operation, causing the nylon to soften and bond to adjacent wraps. The fix: disassemble the head, pull the line free, and rewind with firm, even tension. Applying a light coat of silicone lubricant or soaking the line in water for 24 hours before winding can reduce fusing.
• Bump spring is broken or compressed. The internal spring that pushes the spool outward when the head is tapped against the ground can lose tension over time or break. The fix: replace the spring (usually available as an inexpensive OEM part) or replace the entire head assembly.
• Cap is not fully seated. If the cap does not click firmly into place, the spool cannot travel downward when bumped, preventing line release. Press the cap firmly until you hear and feel it lock.

Problem: Line Winds Unevenly, Tangles, or Comes Out in Clumps

Possible causes:

• Winding in the wrong direction. If the line unspools loosely during operation instead of feeding through the eyelets, you likely wound against the directional arrow. The fix: rewind in the correct direction.
• Inconsistent winding tension. Wrapping some sections tight and others loose creates uneven layers that shift during operation, causing tangles and jams. The fix: maintain steady, moderate hand tension from start to finish. Avoid rushing the winding process.
• Overloading the spool. Filling the spool beyond its rated capacity (leaving no clearance for the line to slide under the cap’s retaining lip) prevents smooth feed. The fix: trim the line to the length specified in the manual rather than guessing.
• Mismatched dual-line lengths. If the two strands on a dual-line spool are not equal in length, one will run out before the other, creating an imbalance that leads to vibration and uneven cutting. The fix: always cut two equal lengths before winding.

Frequently Asked Questions

How often should I refill my string trimmer line?

There is no fixed schedule. You refill the line whenever the trimmer’s cutting performance drops noticeably — typically when the line has worn down to the point where it no longer extends to the full cutting swath, or when the bump feed mechanism advances line but the trimmer still struggles to cut cleanly. For average residential use, a refill every 1 to 3 trimming sessions is typical. Heavy commercial use may require a refill or line replacement multiple times per day.

Can I use any brand of trimmer line in my string trimmer?

Yes, as long as the line diameter matches the specification for your trimmer head. Bulk trimmer line from any reputable manufacturer (not just the trimmer brand) will work in a bump feed or fixed-line head. However, if your trimmer uses a pre-wound cassette system, you must use the manufacturer’s specific cassette — a generic spool of line will not fit into a cassette-based head.

Why does my trimmer line keep breaking inside the spool after I refill it?

The most likely cause is loose or uneven winding, which allows overlapping layers of nylon to rub against each other at high RPM. Friction generates enough heat to soften the monofilament and fuse the wraps together. When the bump mechanism tries to release line, the fused sections snap. The fix is to rewind with consistent, firm tension, keeping each wrap snug against the previous one. Some manufacturers also recommend briefly soaking new monofilament in water for 24 hours before winding, which adds moisture and flexibility to the nylon.

What happens if I use a line diameter that is too large for my trimmer?

Using a line that exceeds your head’s rated maximum diameter can cause the line to jam in the eyelets, prevent the bump feed mechanism from advancing line, overload the motor (increasing amperage draw on electric models or bogging RPM on gas models), and in severe cases crack the spool housing or head cap. Always match the line diameter to the specification printed on the spool, the head housing, or in the owner’s manual. When in doubt, go with the diameter the trimmer shipped with originally.