What Size Strimmer Line Do I Need? Diameter, Shape and Spool Fit Explained

The size of strimmer line you need is set by your machine, not by the job in front of you. Small mains-powered and battery trimmers usually take 1.2mm to 1.6mm line, mid-range cordless models run 1.6mm to 2.0mm, and petrol strimmers or brushcutters typically use 2.4mm to 3.0mm. Fitting anything thicker than the manufacturer states is the single most common way people wreck a spool head.

Why line diameter is fixed by the machine

A strimmer head is a balancing act. The motor has to spin the line fast enough to cut, and thicker line means more mass at the end of the arm, more air resistance and more current drawn. Go up a size and the head slows down, the motor runs hotter, and the cut gets ragged rather than cleaner. On battery machines you will also notice runtime dropping noticeably.

There is a mechanical limit too. The eyelets in the head are sized for a particular diameter. Force oversized line through and it either will not feed at all, or it wears the eyelet oval, at which point the line starts snapping at the exit point every few minutes. That damage is permanent on most plastic heads.

Typical sizes by machine type

• 1.2mm to 1.5mm - small corded electric trimmers, usually 250W to 450W, and entry-level 18V cordless. Fine for lawn edges and soft growth.
• 1.6mm to 2.0mm - the most common range. Mid-power corded models, 18V and 36V cordless with a decent head. Handles thicker grass and light weeds.
• 2.0mm to 2.4mm - larger cordless machines and small petrol strimmers. Copes with nettles, docks and rough ground.
• 2.4mm to 3.0mm - petrol strimmers and brushcutters with a bike handle or harness. Built for brambles and neglected areas.
• 3.0mm and above - professional brushcutters only. Rare on domestic machines.

If your machine sits on the boundary between two sizes, the manual will normally list a permitted range rather than one figure. Staying at the lower end of that range gives you more runtime and a quieter cut. Going to the top end gives you more durability against fences, walls and stones.

Line shape changes the cut as much as diameter

Once you know your diameter, the profile is the next decision, and it makes a bigger difference than most people expect.

Round line

The default and the cheapest. It is the most durable against hard surfaces because there are no corners to chip away, but it tears grass rather than slicing it. Good for general work and for anyone strimming near brick and fence posts.

Twisted line

A round line with a spiral twist. Quieter than round, slightly better cut, and it resists welding to itself on the spool in hot weather. A sensible all-round upgrade if your line keeps fusing together.

Square, star and serrated line

These have cutting edges, so they slice cleanly and are noticeably better on thick weeds and woody stems. The trade-off is that the edges break down faster against walls and kerbs, so you get through more of it. Best kept for open rough ground rather than tight edging work.

How to find the right size without the manual

Manuals go missing. There are three reliable ways to work it out:

1. Check the head itself. Most spool covers are moulded with the line diameter and often the spool part number. Look on the underside and around the eyelets.
2. Measure the old line. If there is any left, measure it with a vernier caliper or take it to a garden machinery shop. Do not measure a worn section, as line thins with use.
3. Search the model number. It is on the rating plate near the motor or handle. The replacement spool listing for that model will state the diameter.

Buying a pre-wound spool made for your model removes the guesswork entirely, though loose line on a reel works out far cheaper if you strim regularly.

How much line to load

Length matters less than diameter, but overfilling causes its own problems. A spool packed too full will not feed, because the coils bind against each other. Wind it neatly in tight rows, keep it below the rim of the spool, and leave the tension even. Loose, criss-crossed winding is the usual reason a bump feed stops working a week after it was refilled.

Cutting length is set by the trimmer guard, not by you. The small blade on the guard trims the line to the right length on the first spin. If your line is being cut back to almost nothing, that guard blade is doing its job and the head is simply feeding too much out.

Sizing mistakes worth avoiding

• Going thicker to stop breakages. Line usually breaks because of technique or because it has gone brittle, not because it is too thin. Oversizing masks the problem and strains the motor.
• Mixing diameters on a twin-line head. Both sides must match, or the head runs out of balance and vibrates badly.
• Using old, dry line. Nylon loses moisture in storage and turns brittle. Soaking a spool in water overnight genuinely helps if it has been sitting in a shed for a year.
• Assuming all 2mm line is equal. Cheap line is often out of round and inconsistent, which makes feeding unreliable regardless of the stated size.