A trimmer line is the nylon cutting cord in a string trimmer that slices grass and light weeds by spinning fast at the head.
What A Trimmer Line Does
A string trimmer cuts with flexible line, not a blade. Spin that cord fast enough and it stiffens. The arc shears grass while the guard defines the path. Because the line wears down as it hits fences, stone, and soil, fresh line feeds out to keep the cut width steady.
Most lines are nylon or tougher copolymers. Round line is the classic choice. Newer designs twist or square the profile to bite into thicker stems and keep noise down. The goal stays the same: quick, neat trimming where a mower cannot reach.
Sizes: Choosing The Right Diameter
Pick a size that matches your tool and the work. Thin line gives a neat edge on turf and lasts longer on light jobs. Thicker line handles weeds and rough edges with fewer breaks but draws more power. Always check the head for allowed diameters and stay within that range.
| Diameter (in.) | Typical Use | Works Best With |
|---|---|---|
| 0.065 – 0.080 | Light grass, edging beds, tight corners | Corded or battery trimmers |
| 0.095 | Mixed grass, moderate weeds, weekly yard work | Strong battery or small gas units |
| 0.105 – 0.130+ | Tough weeds, tall grass, rough edges | Mid-to-pro gas trimmers |
Brands list diameters and compatible heads. If your unit supports more than one size, try the mid option. For loading steps, see the official STIHL guide. Many heads also stamp the winding direction on the spool cap, which saves time and prevents jams. Keep that arrow in mind every time you reload a fresh pair of strands.
Why diameter matters: the line tip speed stays high. Thin line spins up quicker on small motors and leaves a refined edge on soft grass. Thick line keeps its edge on stalks and gravel but needs a stronger powerhead. Match size to the densest patch you plan to cut, not the easiest corner.
Main Parts And How They Work
Head And Spool
The head holds the spool and shields your hands. Line feeds through eyelets on each side. Most heads take two equal strands for balance. A small blade on the guard clips excess line to the set radius so the cut width stays consistent.
Feed Systems
You will see three styles. Bump feed releases more line when you tap the head on the ground. Auto feed advances line as it wears. Fixed heads use short pre-cut lengths that you slip into slots. Bump heads are fast for most yards. Fixed heads shine with heavy line where spooling is fussy.
Cutting Path And Technique
Keep the head level and sweep right to left if it spins clockwise. Use the tip, not the full arc, and take shallow passes on tall growth. Edge with the guard between you and the cut. Keep feet clear of debris and watch for hidden wire or stones.
Types And Shapes Of Trimmer Line
Round
Durable and easy to wind. It glides over chain link and concrete with less fray. If you mostly trim turf and edge beds, this shape is simple and reliable.
Square Or Star
Multiple edges bite into thicker stems. It scours along curbs well and leaves crisp edges. In rocky spots it can nick faster than round, so keep spare line ready.
Twisted
Spiral profiles cut clean and run with less whistle at speed. Many midrange and pro spools use a twist to balance noise, life, and cut quality.
Serrated
Toothy ridges grab fibrous weeds. It’s a good pick for neglected areas, though it can wear quicker on hard borders.
Materials And Durability
Basic line is nylon. Copolymer blends add toughness. High-grade lines may layer materials or use reinforced cores to resist welding inside the head. Keep spare spools sealed and out of sun. If line turns brittle after storage, pre-soak the spool in clean water for a day to restore flex, a tip shared by major makers on their help pages.
Welding happens when heat fuses wraps together. It shows up as a feed that sticks even with a good bump. Tight, even winding lowers friction. So does pausing the trigger during long sweeps to let air cool the head. If your yard chews through line, try a tougher blend or a twisted profile that runs quieter and sheds heat better. A tiny dab of dry silicone on the spool lip can help too.
Loading, Feeding, And Trimming Tips
Load Neatly
Cut two equal lengths, then wind in the direction shown on the spool. Keep wraps tight and even. Crossed wraps grind and melt under heat, which causes jams.
Set The Length
After loading, bump once to seat the line. Spin the head and let the guard blade trim to length. If the arc feels narrow, add a short bump to feed a bit more.
Trim With The Tip
Use just the outer inch or two. That part moves fastest and cuts cleanly. Ramming the head into stems slows the motor and chews through line.
Edge In Stages
On thick borders, make two light passes. The first pass knocks growth down. The second pass cleans the line and leaves a straight edge.
Mind Obstacles
Ease up near bark, fences, and stucco. A light touch saves line and prevents scuffs. For lots of fence work, round line lasts longer.
Wear eye and hearing protection, long pants, and gloves when you trim. A longer guard helps keep chips off your legs. State extension programs also urge basic PPE for string trimmers; the UC ANR guide is clear on that point.
Care And Storage For Fewer Breaks
Keep Line Moist
Nylon is hygroscopic, so it loses moisture in hot, dry sheds. Soaking the spool in water for 12–24 hours before use brings back flexibility and reduces snap-offs during bumps. Store spare rolls in a sealed bag with a splash of water to hold that moisture between jobs.
Clean The Head
Grass dust packs inside the housing and raises friction. Open the head and tap out debris when you reload. Stuck dust can weld wraps together under heat.
Match The Size To The Job
Line that is too thin breaks fast on rough edges. Line that is too thick can bog a small motor. When in doubt, choose the mid size your head supports and test on a short pass.
Replace On A Schedule
Heavy trimming chews through spools faster than a weekly touch-up. If you trim often, keep a spare spool ready. Swapping a pre-wound insert saves time mid-task.
Line Shape Cheat Sheet
| Shape | What It Does | Best For |
|---|---|---|
| Round | Long life, smooth feed | Regular turf and edging |
| Square/Star | Sharp edges for bite | Stems, curb lines, crisp edges |
| Twisted | Lower noise, clean cut | Mixed yards and long sessions |
| Serrated | Grabs fibrous weeds | Overgrown corners and ditches |
Choosing A Size That Fits Your Trimmer
Every head lists a range. If the cap or manual says 0.065–0.080 in., stay within it. A larger size may not feed or could stress the drive. Many battery units run best on 0.080 in. or 0.095 in. short cuts. Gas models handle 0.095 in. all day and step up to 0.105 in. for rough spots. When you swap heads, check the thread and guard pairing so the blade trims the line to the right arc.
Real-World Use Cases
Small Lawn With Lots Of Beds
Round 0.080 in. keeps noise down and edges clean without chewing mulch borders. Auto feed heads shine here since the wear is gentle and steady.
Corner Lot With Curbing
Square 0.095 in. scours along concrete and keeps edges sharp. Bump feed gives control over length as you switch between sidewalk and fence lines.
Acreage With Rough Fencelines
Twisted 0.105 in. takes on wiry stems and stays quiet over long walks. Fixed heads with pre-cut sticks prevent jams and make field swaps quick.
Troubleshooting Quick Fixes
Line Breaks Right After A Bump
The line may be brittle or too thin for the spot. Pre-soak the spool, switch to a thicker size within spec, and trim with the tip rather than the head.
Feed Sticks Inside The Head
Rewind with tight, even wraps. Check for dust build-up and clean the hub. If the cap is cracked or the spring is weak, replace those parts.
Cut Path Feels Narrow
Let the guard blade clip the line once, then add half a bump. If the motor strains at full width, drop one size within the allowed range.
Buying Tips That Save Time
- Check the cap or manual for the permitted diameters and the winding direction.
- Keep two sizes on hand: a lighter roll for turf and a heavier roll for weeds.
- Choose high-contrast colors so breaks are easy to spot in tall grass.
- Use pre-cut sticks with fixed heads for thick line; they reload fast too.
- Carry a spare spool and a rag in your pocket to wipe dust during reloads.
Ready For Cleaner Edges
Now you know what a trimmer line is, how it works, and how to pick one that fits your yard and your tool. Match the diameter to the job, pick a shape for the cut you want, wind neatly, and keep the spool moist and clean. Do that and your string trimmer will cut crisp borders with less fuss, season after season.