Single Line vs Dual Line String Trimmer: How the Heads Differ

The single line vs dual line string trimmer question comes down to a count: a dual-line head shows two ends of line, one on each side, and a single-line head shows one. Makers use the words for the head or the spool, and the words say nothing about motor power. I check that count last, after line diameter, cutting swath and feed type, because those three decide how a trimmer behaves along my tree rows and my gravel edge. Below you get what each head is, what the maker manuals say, why spools do not swap between models and how to reload either one safely.

The Short Answer on Single Line vs Dual Line String Trimmer

Single line means one line end sticks out of the head, and dual line means two do. In the EGO, Craftsman, DeWalt, Milwaukee, Echo and Ryobi manuals, the user pushes one length of line through the eyelets until equal amounts show on each side, so the two ends come from a single piece. DeWalt’s product page calls the line in its attachment-capable trimmer “0.080 in. dual line”; my guide explains how an attachment-capable string trimmer differs from a dedicated model.

The line count describes the head. The spool, the line diameter and the maximum load for that exact head decide what you can buy and fit. When I pick a trimmer, I treat the head it ships with as fixed and I check that its spool is still sold.

What Dual Line Means on Real Trimmer Heads

Search for a dual line string trimmer and you meet more than one design under that label, so look at the head in front of you.

One Piece Through Two Eyelets

On the EGO, Craftsman, DeWalt, Milwaukee, Echo and Ryobi heads, you cut one length of line, push it through one eyelet until it comes out of the opposite one, and pull until equal lengths show. The EGO ST1610T manual calls for a 14 ft piece of 0.095 in line, Ryobi’s gas trimmer manual for 18 ft, and the DeWalt DCST972X1 kit’s string trimmer attachment manual and the Craftsman CMCST920 Quickwind manual set a maximum of 20 ft of 0.08 in line. The Craftsman CMCST900D1 is a different case: its auto-feed head takes only 0.065 in round nylon line. My guide answers how does a string trimmer work from the head down to the drive.

That equal-ends rule is the core of the layout, because the head spins with a tip on each side. I check both lengths before I start. The guard’s small blade then trims the ends to the length the maker set, which is why the guard stays on.

A True Two-Line Head From STIHL

STIHL describes its AutoCut 27-2 as a semi-automatic two-line head. It takes round line, 2.4 mm (0.095 in) as standard, feeds when you tap it on the ground and lets you refill by removing the reel. STIHL lists it for gas and battery brushcutters such as the FS 91 and the FSA 135, and says it may only be used with a special mowing guard or a STIHL universal guard. If you run a brush cutter head like that, my guide to string trimmers and brush cutters sorts out which tool you need.

What a Single Line Spool Looks Like

Single-line designs exist, and the clearest maker example is Black+Decker’s auto-feed spool. Black+Decker sells a replacement spool, the SF-080-BKP, under the name single line auto feed replacement spool: 20 ft of 0.080 in line, factory wound.

A single line trimmer head of this kind feeds on its own. A Black+Decker auto-feed manual describes one line end protruding about 5-3/8 in from the housing, and the AF-100 spool carries 30 ft of 0.065 in line. The LST136 kit ships with an AF-100 spool.

Auto feed changes how you treat the tool. The Black+Decker manual says not to bump the head on the ground, because bumping disrupts the feed mechanism, and the guard’s blade trims the line to length. On the LST136 the manual also limits the tool to 0.065 in round line and to growth under 12 in. That 12 in limit rules it out for the overgrown fence line on my place, but it suits the house lawn edge. My Black+Decker LST136 review takes a closer look at this 40V MAX trimmer-edger kit.

What the Line Count Changes at the Head

The line count changes how many tips pass a point on each turn of the head: one tip passes once per turn and two tips pass twice. Cut quality in the yard depends on the tip, the swath and the line length, which the next three sections cover.

Let the Tip Do the Work

EGO’s manual says to let the very tip of the line do the cutting, and warns that cutting with more than the tip reduces efficiency and may overload the motor. Husqvarna says the end of the cord does the work and never to press the cord into the area to be cut. Along the tree-row strip I hold the head level and let the tip brush the grass edge, which keeps the line off the thin bark at the base of the trunks.

Equal Ends and Vibration

The manuals ask for equal ends and a set line length, and a vibration study shows why length matters. Mallick varied nylon thread length, engine speed and handle material on a gas grass trimmer and measured the lowest hand-arm vibration, 2.76 m/s2, with a 100 mm thread, about 3000 rpm and an ABS handle (1). That was one machine, so read it as a reason to keep the line at the length your manual sets, not as a figure for your trimmer.

Husqvarna’s manual adds that a line wound unevenly on the drum makes the machine generate harmful vibration. Vibration reaches the hands whatever the line count: in a laboratory test of grounds equipment, a grass trimmer gave 3.5 to 5.8 m/s2 at a gloved hand against 1.1 to 2.0 m/s2 for a backpack blower, and the authors found gloves reduced hand vibration (2). I wear gloves on the long edge for that reason, and I stop to shake out my hands between rows.

Swath and Line Length

Swath is the circle the line draws, and the maker’s number belongs to a line at its proper length. Craftsman warns that extending line past the 13 in swath hurts performance, runtime and motor life, and EGO ties extra line to motor overheating. The guard’s cut-off blade holds the line at the right length. On the house lawn edge I let the guard blade trim the ends instead of guessing the length by eye. If you are weighing a 13 in swath against a wider one, my guide answers what cutting width do I need on a string trimmer yard by yard.

Why a Dual Line String Trimmer Replacement Spool Must Match Your Model

Spools and heads are built as sets. Black+Decker’s AF-100 page lists the trimmers it fits, from the GH400 and ST6600 to the LST136, and its SF-080-BKP page lists one, the GH3000. The LST136 manual allows only 0.065 in round line, so the 0.080 in spool is a wrong match even from the same maker.

Line Diameter Is Set by the Maker

STIHL adds that not all diameters are approved for all STIHL trimmers, so the manual for your model settles it. DeWalt and Craftsman say another maker’s line can reduce performance, damage the trimmer or cause personal injury. EGO and DeWalt both ban metal-reinforced line, wire, rope and cable, because pieces can break off as projectiles. I keep a spare spool of the maker’s own line in the shed and I do not mix brands. I answer what size trimmer line should I use in a guide that starts from the manual too.

How to Load Either Head Safely

These steps apply to every head. The three loading methods below come after them, and each names the models it covers. Reloading puts your hands next to the guard’s cut-off blade, so the order of the steps matters more than the line type. Every manual opens with the same step: stop the tool and take away its power before you touch the head.

1. Switch off the trimmer and wait until the head has stopped.
2. Remove the battery. On a gas trimmer, switch the engine off, let it cool where your manual says so, and disconnect the spark plug wire. On a corded trimmer, unplug it.
3. Put on gloves, eye protection that meets ANSI Z87.1 or the standard your manual names, heavy long pants and sturdy slip-resistant boots.
4. Cut the length of line your manual gives. Ryobi, for example, warns that line may jam if more than 18 ft is loaded on its gas trimmer.

Two-Eyelet Heads Wound by Hand

This method covers the manual-winding designs named earlier: the EGO, Craftsman CMCST920 Quickwind, DeWalt, Milwaukee, Echo and Ryobi heads. Do it after steps 1 to 4 above.

1. Line up the arrows, push the line through one eyelet until it comes out of the opposite one, and pull until equal lengths show on each side.
2. Wind the line in the direction of the arrows on the head. Echo and Ryobi say clockwise on their heads; follow your own manual.
3. Leave the exposed length your manual sets, about 5 to 6 in each side on the Echo, Ryobi and DeWalt heads, and check that the guard is fitted so its blade can finish the length.
4. Refit the battery only when the head is closed, and keep people and pets back as far as the manual says.

Factory-Wound Spools

An auto-feed head such as Black+Decker’s loads differently. After steps 1 to 4 above, the manual has you release the tabs on the spool cap, take off the cap and the empty spool, fit a new AF-100 spool and put the cap back, and only then refit the battery. For bulk line, the manual describes winding by hand, and it notes that hand-wound spools tangle more than factory spools.

Power-Wound Heads

The EGO ST1610T’s POWERLOAD head winds the line for you. With the battery removed, you thread a 14 ft piece through the eyelets until equal lengths show, refit the battery and hold the POWERLOAD button, release it when about 8.5 in is left on each side, then tap it briefly to set 7 in per side. According to EGO, a full spool can usually be loaded in 12 seconds, and repeated winding in rapid succession risks motor damage. I load a power-wound head at the bench the evening before a long edging day, so the field work does not stop for a reload. On a long edging day, my picks of string trimmers for edging are the ones with a real edge mode or an edger kit.

Eye protection is a rule once the head spins. US emergency departments treated an estimated 81,907 lawn trimmer injuries from 2000 to 2009, and the eye accounted for 42.5% of them, mostly from foreign objects (3). Published eye trauma cases also include a bystander (4). I walk the gravel yard for loose stones before I trim along its edge.

What to Check Before You Buy

The spec sheet gives you rows with documented consequences. Read them before the line count.

If you shop for a dual line cordless string trimmer, filter on those rows and read the head’s description in the manual. Power source matters for the body too: NIOSH sampling of 80 landscaping workers found significantly more noise and carbon monoxide exposure when they used fuel-powered string trimmers than battery versions (5). My guide to battery and gas string trimmers helps you choose which power source fits your yard.

Match the Head to the Job

• House lawn edges: the LST136 is limited to 0.065 in round line and growth under 12 in, which fits edging a mown lawn. If your lawn is small, my picks of string trimmers for small yards compare nine easy-to-handle models.
• Fence lines and overgrown corners: pick a head whose manual allows a heavier line, such as the EGO ST1610T’s 0.095 in twist line or the 0.095 in option on the DeWalt DCST972X1 kit.
• Tree rows: keep the line off young trunks, because the line strips bark.
• Gravel edges: keep the guard on and your eye protection in place, because the head throws stones.

On my place the fence line and the corners drive my choice more than anything printed on the spool. For that kind of ground, my list of string trimmers for thick weeds covers tougher models.

Mistakes I See With Line Heads

Most line trouble comes from loading and feeding. Three mistakes need more than the loading steps above.

• Bumping an auto-feed head. Black+Decker and Craftsman both say bumping disrupts the feed. Let the head feed itself. My guide to bump feed string trimmers and automatic feed heads explains how each behaves.
• Loading by guesswork. Equal ends, no more than the maximum load and only the diameter and maker’s line your manual lists. DeWalt and Craftsman warn that another maker’s line or a heavier one reduces performance, damages the tool or causes injury, and Craftsman and EGO tie line past the swath to lost performance and motor overheating.
• Skipping the shutdown order. A trigger lock or a switch in the off position does not replace it: switch off, wait for the head to stop, then remove the battery, unplug the cord or disconnect the spark plug wire, and only then touch the head.

The mistake I run into most is a tap on an auto-feed head out of habit from older tools.

Pick the Trimmer by the Job, Then the Head

Three things to remember: count the line ends only to name the head you hold, and check the spool part number, the line diameter and the maximum load in the manual. Keep the order on every reload: switch off, wait for the head to stop, then remove the battery, unplug the cord or disconnect the spark plug wire, and only then touch the head. Use only the battery and charger the maker specifies. When you are ready to choose a tool, my roundup of string trimmers for any yard compares nine picks across budgets and power types.

My tip is to write the spool part number on the inside of the shed door, so the right line is never a guess when the old spool runs out. Choose a machine whose approved line, swath and feed method suit your job, then confirm that its replacement spool is sold. The single line vs dual line string trimmer choice follows from those three, because the line count alone does not make one head cut better than the other. My guide on what size string trimmer do I need covers swath, power and weight for your yard.

Frequently Asked Questions

What is the easiest line trimmer to use?

The easiest line trimmer to use is the one you can reload without a fight, carry through the whole edge and feed by the rule in its own manual. An auto-feed head such as the Black+Decker LST136’s takes no bumping, while a bump head such as the EGO ST1610T’s needs a tap on bare ground. Read the feed rule for your model first, because the two types have opposite rules.

Can I use only 1 string or do have to use both strings?

A head built for two ends should carry both, because the EGO, Craftsman, DeWalt, Milwaukee, Echo and Ryobi manuals tell you to leave equal lengths on each side. A head that shows a single end, as the Black+Decker auto-feed manual describes, takes only that one. Load exactly what your manual shows.

What is the toughest trimmer line?

The toughest trimmer line is the one your manual approves, because a toughness claim on a package does not make a line fit your head or motor. Use the diameter and shape your manual lists, since a heavier line than allowed overloads the motor. Metal-reinforced line, wire and rope are never an option, since EGO and DeWalt warn they can break off as projectiles.

How to choose trimmer line size?

Choose trimmer line size from the diameter your trimmer’s manual allows, because the motor and head are built for that range. The Black+Decker LST136 allows 0.065 in round line only, while the EGO ST1610T lists 0.095 in and the DeWalt DCST972X1 kit lists 0.08 in with 0.095 in as an option. A heavier line than the manual lists is a motor-overload risk.

Are trimmer blades better than string?

Trimmer blades are better than string only for woody growth such as saplings and brambles, and only when the maker rates a brush or saw blade for that stem diameter on your exact tool. Grass blades are not for woody stems, line suits grass and light weeds, and a metal blade never goes on a head not rated for one.

References

1. Mallick Z. Optimization of the operating parameters of a grass trimming machine. Appl Ergon. 2010;41(2):260-265. doi:10.1016/j.apergo.2009.07.010
2. Oh J, Chen N, Boyd C. Laboratory evaluation of occupational exposure to hand-arm vibration (HAV) during grounds maintenance equipment operations. Ann Agric Environ Med. 2023;30(2):384-389.
3. Leinert J, Griffin R, Blackburn J, McGwin G. The epidemiology of lawn trimmer injuries in the United States: 2000-2009. J Safety Res. 2012;43(2):137-139. doi:10.1016/j.jsr.2012.03.001
4. Lubniewski AJ, Olk RJ, Grand MG. Ocular dangers in the garden. A new menace: nylon line lawn trimmers. Ophthalmology. 1988;95(7):906-910. doi:10.1016/s0161-6420(88)33076-9
5. Alexander BM, Graydon PS, Pena M, Feng HA, Beamer BR. Hazardous exposures and engineering controls in the landscaping services industry. Journal of Occupational and Environmental Hygiene. 2025;22(3):189-202.

Safety Note: Read your manual first. A string trimmer or brush cutter throws debris with force, so clear stones, glass and wire from the area before you cut. Wear impact-rated eye protection with side coverage that meets ANSI Z87.1, hearing protection, gloves, heavy long pants, long sleeves and sturdy slip-resistant boots (steel-toe where your manual says so). Keep people and pets at least as far away as your manual says (50 feet in some manuals, 100 feet in others), and stop if anyone comes closer. Keep the correct guard fitted, use only the cutting attachments, handles and harness the maker approves for your exact tool, and never replace trimmer line with wire. A blade can kick sideways when it hits a stump or a thick stem, so fit the handlebar and harness your manual requires for blade work, hold both handles firmly and keep secure footing. Before you clear a jam, clean the tool or change line, a blade or an attachment, switch it off, wait for all movement to stop, then remove the battery, unplug the cord or disconnect the spark plug wire. Refuel a gas model outdoors with the engine off and cool, away from flames or sparks, and move away from the fueling spot before starting, as far as your manual says (often 10 to 30 feet); never run it in a garage, shed or other closed space. For a battery model, use only the maker’s battery and charger, charge it within the temperature range in your manual and never use a damaged or swollen pack. For a corded model, use a GFCI outlet, keep the cord clear of the cutting line and avoid wet conditions.