Trimmer Line Size Chart: Which Diameter and Shape Your Trimmer Takes

Find the size your trimmer takes

Three places to look. In order.

1. The manual. Look under Specifications or Line Replacement. The EGO ST1500 manual lists .095 twist line as recommended and .080 as compatible, with part numbers for both.
2. The head card. STIHL says not all of its line fits every STIHL head, and points you to the head's instruction card for diameter and length.
3. The old line. Lost the manual? Bring a piece of the old line to the store. Most packages list the diameter. Manufacturers post digital manuals too. STIHL and EGO both do.

Found the number? Write it on the trimmer shaft with a paint marker. Done forever.

Going thicker: skip it unless the manual lists it

Thicker line is tougher. It also drags harder through the air and the weeds. The motor pays for that.

EGO spells it out. Its manual says to use no more than 2.4 mm (.095) nylon line, because heavier line "increases the load on the motor and reduces its operating speed," which leads to overheating and damage. The same manual's warranty excludes damage from line thicker than .095. That's a hard ceiling.

Gas trimmers have more headroom on some heads. STIHL lists the FS 56 with the AutoCut C head on .095 and room for .105. Swap to the DuroCut 20-2 head and .120 is standard, with .080 to .105 optional. Note the catch. The head decides. Swap to a bigger size without the matching head and you're guessing.

Line shapes: what each one does

Shape is where the aisle gets loud. Here's what the manufacturers actually claim. Nothing more.

Round

The default. Oregon calls round the most popular shape and says it's "more durable and longer lasting than square trimmer line." STIHL calls its Premium Round Line a multi-purpose line that fits all STIHL and competitive heads. Cheap. Winds easy. Works.

Twisted

A spiral profile. Oregon says its twist line "produces fewer decibels and less drag than other shapes." STIHL's Quiet Line uses spiral grooves to cut wind noise, and STIHL aims it at electric and battery trimmers in noise-restricted areas. EGO specs twist line for its 15-inch trimmer. Less drag matters most on a battery.

Square, pentagon, and star

Edges. STIHL says the edges on its Commercial Square Line "cleanly slice through grass and heavy weeds," and pitches it for daily professional work. STIHL's pentagon line combines square line's hard edges with round line's abrasion durability and easy re-spooling. Oregon's All Purpose star line comes in .065 to .105. Oregon says the star gives "6 sharp-cutting sides" for low to medium tall grass.

The tradeoff Oregon states: round lasts longer than square. Edged line bites. Round line goes along fences and curbs, where EGO's manual says string wears fast.

Serrated and other specialty shapes

Battery vs gas: line choice

Battery trimmers are the touchy ones. Every bit of drag comes out of the pack.

• Battery trimmers: stay at or under the stated max, full stop. EGO specs twist line for its trimmers. STIHL points battery owners to its grooved Quiet Line. The EGO manual also says to cut with the tip only. Burying more line in the grass overloads the motor.
• Gas trimmers: more options, if the head supports them. Square or pentagon line earns its keep on heavy weeds. Round line is the cheap everyday choice.

Wind and replace the line

Heads differ. The steps below follow the EGO ST1500 manual. Check your own head card for length and direction.

1. Pull the battery. On gas, shut it down. STIHL says fully powered off and cleaned before any maintenance.
2. Line up the LOAD mark on the spool with the eyelets, per the EGO manual.
3. Cut one piece of line 16 feet long. That's EGO's length for the ST1500.
4. Feed it through the mounting hole until equal lengths hang out both sides.
5. Wind it on following the arrow on the spool.
6. Stop with about 5.5 inches showing on each side, the length EGO specifies.
7. Push the spool retainer down and pull both ends to confirm the line feeds.

Bump heads feed on a tap. EGO says to tap on bare ground or hard soil. Tapping in tall grass can overheat the motor.

And one warning straight from EGO's manual: never load wire, metal-reinforced line, or rope. Those pieces break off and fly. Nylon only.

Shop math: reloads per spool

Givens: a 125-foot donut of .095 line and an EGO ST1500 head that takes 16 feet per reload.

1. Reload length: 16 feet, per the EGO manual.
2. 125 divided by 16 is about 7.8.
3. That's 7 full reloads, with about 13 feet left over.

Your head's length goes in step 1. Most of the time the arithmetic is the whole buying decision.

Storage: line dries out

Old line snaps. Husqvarna's support answer says line ages and "dries out," which makes it brittle. Their fix: buy fresh line, or soak your line in water for several days to rehydrate it.

Husqvarna lists fresh line first. So buy the spool size you'll use in a season or two. Got a spool that keeps snapping? Soaking costs nothing. Try it on that one.

Keep line off tree trunks

Trimmer line cuts bark. That's the job it's built for, and bark doesn't know the difference. Purdue Extension warns that young, smooth-barked trees like maples and birches can have bark less than 1/16 inch thick. Cut through that and the tree loses the pathways that move food and water. Circle the trunk and the tree dies.

Purdue's fix is cheap. A 2- to 3-inch layer of mulch over the root zone gives equipment a visual line to stay out of. EGO's manual says the same about the machine side: avoid trees and shrubs, because the string easily damages bark, siding, and fence posts.

Brown strip along the walk a week after trimming? Scalp from a low head, or disease, or grubs. They look alike from standing height. Snap a photo for a free AI diagnosis and get the cause first. Then shop. Mulch rings and bed borders help too. The lawn edging ideas post has options.

Picks by use

Named products, matched to the job. Check your manual's diameter before any of them.

No verdict on price here. Prices move weekly. Buy the size your head takes, in the shape your job needs, at whatever the shelf says that day.

What other guides get wrong

Most size charts sort line by yard size or by who you are. Homeowner gets .080. Pro gets .095. Acreage decides. That skips the part that matters. The trimmer head sets the range, and the manual prints it.

A pro-grade line in a small battery trimmer is a bigger load on a smaller motor. That mistake never shows up on a chart sorted by lawn size. Look up the number. Then shop.

Second miss: shape gets sold as a cutting upgrade, full stop. Oregon's own product copy says round outlasts square. Edged line trades life for bite. Pick it for heavy weeds. Leave it for everything else.

Where trimming fits the rest of the lawn

Trimmer line is a wear part. The lawn it edges is the real job. Measure it once with the lawn size calculator and every bag and spreader setting gets easier. Not sure what grass you're trimming? Get a free photo ID. From there, Premium's 12-month care plan sets a mowing-height target for your grass and ZIP code, and smart reminders time the season's jobs. Edge to that same height.

The trimmer line punch list

1. Find the diameter in your manual or on the head card. Write it on the shaft.
2. Never go above the stated max. EGO's manual caps its 15-inch trimmer at .095.
3. Go thicker only when the manual lists the size for your head.
4. Pick shape second: round for life, twisted for less noise and drag, square for heavy weeds.
5. Cut the reload length your head card lists. EGO's ST1500 takes 16 feet.
6. Nylon only. No wire.
7. Buy line you'll use in a season or two. Soak brittle line if you like; Husqvarna says it helps.
8. Mulch ring the trees. Keep line off the bark.

Right size, right shape, one reload at a time. Leave the rest on the shelf.