Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
Every string trimmer head has a maximum line diameter, and it is a property of the head, not of trimmers in general. Toro gives .065 in (1.65 mm) as the only diameter for its 51480 electric trimmer. EGO gives 0.095 in (2.4 mm) as the only diameter for its ST1620T trimmer. A Husqvarna RapidReplace head takes a wider span, .065 in to .105 in. Buy a diameter above the number your own head names and the makers agree on the result: the motor works harder, and on a cordless machine it drains the battery faster.
Shape is a different question, and the honest answer is that nobody has published an independent test comparing round, twisted, square, pentagon and serrated line. What exists instead is each maker's own claim for its own product: STIHL says its square line slices cleanly through dense weeds, Oregon says the same for its square Gatorline, and ECHO calls its square-twisted Black Diamond line its fastest cutting line yet. This page names every claim and who makes it, rather than presenting a marketing sentence as a tested result. Round line remains the shape every source here names as fitting the widest range of heads.
Necessary equipment
| Item | Note |
|---|---|
| Documentation for your trimmer head | The documentation names the maximum diameter for the head. |
| Calipers or a small ruler optional | Measure the diameter of any line already in the head. |
| Bright light or a flashlight optional | The print on a trimmer head can be small. |
| Work gloves | Wear gloves before you handle a square, pentagon, or serrated line end. |
| Pen and paper optional | Record the diameter and the shape before you shop. |
| New trimmer line material | Buy line with the diameter and the shape your documentation allows. |
Find the maximum diameter for your head
Estimated 6 min to 12 min
Source: 14in Electric Trimmer, Model No. 51480 — Operator's Manual (Form No. 3379-685 Rev B) · tier 1 · accessed 2026-07-29
Source: Operator's Manual — 56-Volt Lithium-Ion Cordless 16in String Trimmer, Model ST1620T / ST1620T-FC · tier 1 · accessed 2026-07-29
Source: RapidReplace trimmer head for high speed feed · tier 1 · accessed 2026-07-29
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Find the make and the model number of the trimmer head.1mReport a problem with step 1, Find the maximum diameter for your head
- The number is often on the head, on the cap, or on the card.
- Keep the number for every later step.
- Read the maximum line diameter from the label on the head.2mReport a problem with step 2, Find the maximum diameter for your head
- Many heads print the diameter as a number with IN or MM after it.
- Use a bright light if the print is small.
- If the head has no label, read the documentation for your trimmer.3mReport a problem with step 3, Find the maximum diameter for your head
- STIHL states that not all diameters suit every STIHL trimmer.
- Toro names .065 IN as the only diameter for the 51480 trimmer.
- EGO names 0.095 IN as the only diameter for the ST1620T trimmer.
- Husqvarna names .065 IN to .105 IN for the RapidReplace head.
- Write the maximum diameter on paper.<1mReport a problem with step 4, Find the maximum diameter for your head
- A range gives a low number and a high number.
- A range is not one fixed size.
Compare your current line with the maximum diameter
Estimated 2 min to 4 min
WEAR GLOVES BEFORE YOU HANDLE A SQUARE, PENTAGON, OR SERRATED LINE END.
STIHL states that a square edge cuts cleanly. The same edge can cut skin.
- If the line already extends from the head, measure it with calipers.2mReport a problem with step 1, Compare your current line with the maximum diameter
- Measure across the round part of a shaped line, not a corner.
- A worn end reads thinner than new line.
- Compare your measurement with the number from your documentation.<1mReport a problem with step 2, Compare your current line with the maximum diameter
- A large difference suggests a wrong line in the past.
Compare the shapes before you choose one
Estimated 5 min to 7 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Decide if the job needs a shape other than round.<1mReport a problem with step 1, Compare the shapes before you choose one
- STIHL states that round line fits the widest range of heads.
- Husqvarna names TrimForce as the round line for general use.
- Read the maker's own claim for a shape before you buy it.2mReport a problem with step 2, Compare the shapes before you choose one
- STIHL claims its square line cuts cleanly through heavy weeds.
- Oregon claims its square Gatorline cuts cleanly through vegetation.
- ECHO calls its square-twisted Black Diamond line its fastest line yet.
- Treat each claim as the maker's own claim for its product.
- Make sure that the shape comes in a diameter your head accepts.1mReport a problem with step 3, Compare the shapes before you choose one
- ECHO names .095 IN and .105 IN as the sizes for Black Diamond.
- A shape is no use if it comes in the wrong diameter.
- Read the documentation again for a statement about shape.1mReport a problem with step 4, Compare the shapes before you choose one
- A statement about shape is rarer than one about diameter.
- The document for your own head is the final authority.
Buy line that matches your numbers
Estimated 3 min to 3 min
DO NOT BUY A LINE DIAMETER ABOVE THE MAXIMUM IN YOUR DOCUMENTATION.
Toro warns that a larger diameter can overheat the motor. An overheated motor can fail.
- Compare the package diameter with the number from your documentation.<1mReport a problem with step 1, Buy line that matches your numbers
- Toro warns that a larger diameter can overheat the motor.
- EGO gives the same warning for heavier line.
DO NOT BUY METAL-REINFORCED LINE, WIRE, OR CORD FOR THE HEAD.
EGO warns that this material can break at speed. A broken piece can strike and injure a person.
- Make sure that the package names nylon or another polymer, not metal.<1mReport a problem with step 2, Buy line that matches your numbers
- EGO warns that metal-reinforced line can break at speed.
- A broken piece can strike and injure a person.
- Buy line that names both the diameter and the shape.1mReport a problem with step 3, Buy line that matches your numbers
- The package usually prints both numbers on the front.
- Keep your written diameter and shape with the trimmer.<1mReport a problem with step 4, Buy line that matches your numbers
- You need both numbers again at the next purchase.
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- trimmer line
- Also called: weed eater string, weed wacker string, strimmer line, trimmer string, grass trimmer line, cutting line, nylon line, the string in a weed eater
- string trimmer
- Also called: weed eater, weed wacker, weedwhacker, weed whipper, strimmer, grass trimmer, line trimmer, edge trimmer
- trimmer head
- Also called: cutting head, bump head, spool head, string head, mowing head, the part that holds the line
- line diameter
- Also called: line gauge, line thickness, string gauge, string size, how thick the line is
- line shape
- Also called: line profile, line type, string shape, cross section of the line
When to use a professional
Stop if you cannot find the maximum diameter for your head. Do not guess a number. Speak to the manufacturer of your trimmer or to a dealer.
Stop if the label on the head and your written manual disagree. Do not choose between them. Speak to the manufacturer before you buy line.
Stop if no diameter on the market matches your head's rating. A gap between the rating and the market can happen. Do not buy the closest size. Speak to the manufacturer.
Stop if your documentation names an approved shape that you do not have. Do not use a different shape instead. Speak to the manufacturer or to a dealer.
Stop if your trimmer head has cracks or damage. Do not put any line into a damaged head. Speak to a service dealer.
Common questions
What size trimmer line do I need?
Whatever your head is built for, and that number lives on your equipment rather than on this page. Toro gives .065 in (1.65 mm) as the only diameter for its 51480 electric trimmer. EGO gives 0.095 in (2.4 mm) as the only diameter for its ST1620T trimmer. A Husqvarna RapidReplace head takes a span, .065 in to .105 in, rather than one fixed size. The number is normally printed on the head itself, on the cap, or on the instruction card; if it is not, your owner's manual has it.
What is the difference between round, twisted, square and serrated trimmer line?
Shape changes the edge the line presents to the grass, and every maker's description is a claim about its own product rather than an independently measured result. STIHL describes its round line as fitting the widest range of heads and its square line as slicing cleanly through dense weeds; its pentagon line is sold as combining a square edge with a round line's durability. Oregon makes a similar claim for its square Gatorline, and ECHO markets its square-twisted Black Diamond line as its fastest cutting line to date. None of the manuals or product pages read for this page compares one shape against another in a test. Round remains the shape every source here names as fitting the widest range of heads.
Does thicker or a sharper-edged line cut heavier weeds better?
A shape with edges, such as square or pentagon line, is marketed by its maker for exactly that job — STIHL positions its square line for professionals who face dense weeds daily. Thicker line is a different question from shape, and it has a real limit: Toro and EGO both warn that a diameter larger than the one they specify can overheat the motor and damage the trimmer. So a heavier job can justify a different shape within your head's rated diameter, but it does not justify a diameter above that rating.
My head says it takes .065 to .105 in line. What does that mean?
It means the head, in this case a Husqvarna RapidReplace, accepts a span of diameters rather than one fixed size, and any diameter inside that span sits within the maker's own rating. It does not mean every diameter performs the same: a thinner line inside the range wears faster, and a thicker one loads the motor more, within the approved limit. Read your own instruction card, because a different head that quotes a range may quote a different one.
Can I use wire or metal-reinforced line for extra strength?
No. EGO's own warning is direct: metal-reinforced line, wire, or rope can break off and become a dangerous projectile. A trimmer head throws whatever is inside it at speed, and nylon line is chosen because a broken piece of it is far less dangerous than a piece of wire. Buy line described as nylon or as the polymer your manufacturer names, never a metal-cored product.
Is expensive shaped line worth it over plain round line?
That depends on the job, and this page will not tell you it is or is not worth the money, because no source read for this page tested that question. What is verifiable is narrower: round line is the shape every maker here names as fitting the widest range of heads, and a shaped line only helps if your head's documentation allows the diameter it comes in. Treat every cutting-speed or durability claim for a shaped line as the maker's own marketing for its own product.
Related procedures
- How to find and record the model and serial number of every appliance in the house
- How to put new line in a string trimmer, and what size line to use
- How to grease the gearbox on a string trimmer or hedge trimmer
- How to change the blade on a lawn edger
- What drill bit and what speed should I use for wood, metal, plastic or concrete?
- What size blade fits my circular saw, and does the RPM rating on the blade matter?
- What NRR do I need for ear protection with power tools and a lawn mower?
- How should I store my cordless mower and trimmer batteries over the winter?
Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 114in Electric Trimmer, Model No. 51480 — Operator's Manual (Form No. 3379-685 Rev B)
- Tier 1Operator's Manual — 56-Volt Lithium-Ion Cordless 16in String Trimmer, Model ST1620T / ST1620T-FC
- Tier 1RapidReplace trimmer head for high speed feed
- Tier 1Know Your STIHL Trimmer Line
- Tier 1TrimForce Round Trimmer Line
- Tier 1Black Diamond Trimmer Line
- Tier 1Magnum Gatorline Square Trimmer Line, .105 in x 938 ft
Revision history
Current version 1 · updated 2026-08-03 · next review due 2027-08-03
- v1 · 2026-08-03First draft. Maximum line diameter is published per head from three already-registered manufacturer manuals — Toro 51480 (.065 in only), EGO ST1620T (0.095 in only), Husqvarna RapidReplace (.065 in to .105 in) — and is treated as a real, verified limit, not a preference, per the backlog note. Shape (round, square, pentagon, square-twisted) is deliberately kept out of the specifications block as verified fact: it is published as attributed manufacturer marketing from STIHL, Husqvarna, ECHO and Oregon Tool, with an explicit statement in the summary, a section note and one specification entry that no independent test comparing shapes was found. The four shape sources are new and are proposed in data/sources-additions-choose-string-trimmer-line-by-diameter-and-shape.json; they are not yet in data/sources.json, so check 3 will report them unresolved until scripts/sources_admit.py runs, which this drafting session is not permitted to do.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.