What Size String Trimmer Line Do I Need?

String trimmer line is sold by diameter, measured in thousandths of an inch, and the correct size for your machine depends on the motor’s power class and the head it feeds through. Sizing is not a matter of picking whatever is cheapest at the hardware store. Line that is too thick for a light trimmer drags on the motor and can overheat the drive, while line that is too thin for heavy growth breaks constantly and needs re-feeding every few minutes.

Common trimmer line diameters and what they are for

Trimmer line is typically sold in the following diameters, and each range corresponds to a class of machine:

• .065 to .080 inch: Light-duty electric and small battery trimmers used for lawn edges, light grass, and touch-up work around flower beds.
• .085 to .095 inch: Mid-power battery and gas trimmers handling mixed grass, light weeds, and moderate yard size.
• .105 to .110 inch: Higher-power residential and light-commercial gas trimmers cutting tougher weeds, tall grass, and overgrown patches.
• .130 inch and thicker: Commercial-grade trimmers and brush-capable machines working through woody stems, brambles, and dense growth where standard line would snap immediately.

These bands are not universal law, but they reflect how manufacturers rate their heads and how much torque a given motor can push through a line of that thickness without stalling.

Why matching line size to the motor matters

A trimmer head spins the line at high RPM to build enough centrifugal force for the line to cut. Thicker line has more mass, so it takes more torque to spin it at the same speed. A small electric motor built for .065 line can struggle to keep .095 line spinning at cutting speed, which shows up as bogging down in grass, shorter run time on battery models, and more heat in the motor housing over a long session.

Going too thin creates the opposite problem. If you load .065 line into a gas trimmer built for .105, the line will spin fine, but it lacks the mass and stiffness to survive contact with tougher stems, curb edges, or gravel. It snaps quickly, so you spend more time re-feeding line than actually trimming.

How to find your trimmer’s rated line size

The fastest way to confirm the correct diameter is to check the owner’s manual, which usually lists a specific size or a narrow range. Many trimmer heads also have the size molded or printed directly on the housing near the eyelets. If neither is available, look up the model number on the manufacturer’s support page, since most brands publish a parts and specifications sheet online. Avoid guessing based on what was already in the trimmer when you bought it used, since a previous owner may have installed the wrong size.

What happens if you go outside the recommended range

Using line slightly above the recommended size is sometimes tolerated by mid-power and commercial machines with reinforced gearheads, but it is a real risk on lighter consumer models. Overloading the motor with oversized line can shorten motor life, trip thermal protection on battery trimmers, or strip a plastic gearhead over time. For a full breakdown of when it is safe to size up, see our guide on putting thicker line in a string trimmer. If you are unsure how long a cut length should be once the line is loaded, our guide on how long trimmer line should be covers the feed and cutting radius side of the equation.

Line shape also factors into sizing decisions

Diameter is only half the picture. The cross-sectional shape of the line, round, square, or twisted, changes how much cutting edge that diameter delivers and how the line behaves against different types of growth. A .095 round line and a .095 square line will not perform identically even though they share the same diameter rating. Our comparison of round, square, and twisted trimmer line explains how shape changes cutting force and noise at a given size.

Choosing line for a specific yard

For most homeowners maintaining a standard lawn with occasional edging along driveways and beds, .080 to .095 inch line on a mid-power trimmer covers the majority of tasks without excess strain on the motor. Larger properties with thicker weeds, ditches, or fence lines benefit from stepping up to .105 or higher on a trimmer rated for it. If you are shopping for a new machine rather than just line, our trimmers and edgers buying guide breaks down which machines are built for heavier line and tougher growth from the factory.

When in doubt, start at the low end of your trimmer’s rated range. It is easier to size up gradually and confirm the head still feeds smoothly than to overload a motor with the thickest line available and shorten its service life.