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What size strimmer line do I need for this machine

Updated: October 2026

You know the machine in the shed and not the millimetres on the coil. What size strimmer line do I need is the question the head answers, and a grass chart only helps when that head allows a range. The listings in this comparison print anything from 1.5 mm to 3.5 mm, and two of them print two figures at once. The [trimmer line comparison](/hub/lawn-and-garden-equipment/trimmer-line) is the table. By the end you should be able to match a head to a row, and to leave 3.5 mm in the Bosch-pattern exception where it belongs. Our links can earn a commission.

You know the machine in the shed and not the millimetres on the coil. What size strimmer line do I need is the question the head answers, and a grass chart only helps when that head allows a range.

Our top picks

DRERIO DRERIO 3.5 mm x 25

Best overall

DRERIO DRERIO 3.5 mm x 25
6.2/10about £8.99
View offer
Dapetz Dapetz 1.5 mm x 20 m

Preis-Tipp

Dapetz Dapetz 1.5 mm x 20 m
8.6/10about £2.99
View offer
Oregon Oregon 2.4 mm x 15 m round

Rank 3

Oregon Oregon 2.4 mm x 15 m round
9.0/10about £5.99
View offer
On this page
  1. 1.Read the head before the grass
  2. 2.What the bands mean when the head allows a range
  3. 3.The diameters on these twelve listings
  4. 4.3.5 mm is a Bosch-pattern strand

Read the head before the grass

The diameter is written on the head, on the spool, or in the manual. Yourspares says to check that recommended diameter first: line which is too thick mis-feeds and strains the machine, while line which is too thin snaps. A small lawn with a head marked 2.4 mm is not a 1.2 mm job. STIHL UK’s AutoCut 2-2 takes 1.6 mm and 2.0 mm, and the AutoCut 27-2 takes 2.4 mm and 2.7 mm. Those pages show the head winning. They are not a chart for a Flymo or a Ryobi cassette. If you cannot find a number, measure the line that last fed, or read the manual, before you trust a title that says “universal”.

What the bands mean when the head allows a range

Yourspares’ bands are a rule of thumb, not a laboratory scale. Light grass is about 1.2 mm to 1.6 mm. General home use is about 1.6 mm to 2.0 mm. Thick grass is about 2.0 mm to 2.4 mm. A corded or 18 V strimmer is usually in the 1.5 mm to 2.0 mm band, and a 2.4 mm reel is the wrong buy unless the manual lists it. If the head states one millimetre, you do not get to choose a band. Too thin, it snaps. Winding is on the how-to page, not a reason to change the diameter.
DRERIO DRERIO 3.5 mm x 25

Testsieger

DRERIO DRERIO 3.5 mm x 25
6.2/10about £8.99
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The diameters on these twelve listings

Dapetz, this variant, is 1.5 mm by 20 m round at £2.99. Zeqeey is titled 1.6 mm by 100 m round at £10.99, and a bullet says 1.65 mm. Viabrico square at 2.0 mm is £11.99. Oregon 69-362-OR is round 2.4 mm by 15 m at £5.99. The other Viabrico square reel is 2.4 mm by 100 m at £10.99. Navoys is titled 2.4 mm by 15 m at £3.75 and a bullet says 1.65 mm, so do not treat it as 2.4 mm. KINGTAIXIN at £2.99 prints no diameter.

3.5 mm is a Bosch-pattern strand

DRERIO at £8.99 is 25 strands of 37 cm by 3.5 mm, and Eyoloty at £12.99 is 24 of the same stated size. Both are sold against Bosch part F016800431 for the AFS 23-37. Bosch DIY UK’s own card is £28.04. The aftermarket packs are not that card. One UK review on the Eyoloty listing says the twist is 3.5 mm at the widest point and narrower along the strand, and that the lines lasted about a quarter to a half as long as the original. That is one review, not a specification. These strands do not wind into a domestic bump spool, and 3.5 mm is not the next size up from 2.4 mm.

Common questions

What size strimmer line do I need?
The size printed on the head, the spool or in the manual. If that number and a grass chart disagree, buy the head’s number. Many corded and 18 V machines at home sit around 1.5 mm to 2.0 mm. A 3.5 mm strand is for an AFS 23-37 pattern head, not for a general bump spool.
Where is the diameter written?
On the head housing, on the old spool, or in the model’s manual. Listings sometimes print it in the title and again, differently, in a bullet. If you cannot find it on the machine, measure the line that last fed, rather than guessing from the colour of a new coil.
Can I choose a thicker line because the grass is long?
Only inside the range the head allows. Yourspares’ thicker bands are for thicker growth on a machine that can spin that cord. Cut the long grass in stages with the correct diameter. A neglected lawn does not change the eyelet.
Why does one listing show two millimetre figures?
Because the title and a bullet were not written as one description. Zeqeey prints 1.6 mm and 1.65 mm. Navoys prints 2.4 mm and 1.65 mm. You cannot know which figure will arrive from the text alone. Leave that row until a single size matches the head, rather than hoping the cheaper price is the thicker line.
Is 3.5 mm a stronger version of 2.4 mm?
No. In this set, 3.5 mm is a 37 cm pre-cut strand for a Bosch AFS 23-37 pattern head. It is not a heavier refill for a bump spool that was using 2.4 mm round line. The maker’s own UK card for that strand is £28.04. The cheaper packs here are aftermarket.
What happens if the line is too thick for the eyelet?
It may not enter, the bump feed jams, and a light motor works harder. UK reviews on the thicker square reels describe difficult loading and a cheap strimmer running hot. Too thin, the line snaps. Match the head, then wind it with the guard left on.

Related pages

Trimmer line buying guide

Trimmer line

Best strimmer line for the diameter you already know

How to wind strimmer line on a standard bump spool

Round or square strimmer line for a domestic eyelet

Sources

Sources checked for this comparison; last retrieved on 17 October 2026.

  1. HSE, chain-flail bulletin for brush cutters (retrieved 17 October 2026)
  2. HSE INDG480, hand-arm vibration (retrieved 17 October 2026)
  3. GOV.UK, outdoor noise regulations 2001 (retrieved 17 October 2026)
  4. ISO 11806-1:2022, brushcutters and grass trimmers (retrieved 17 October 2026)
  5. STIHL GB safety manual for FS machines (retrieved 17 October 2026)
  6. STIHL UK, AutoCut 2-2 mowing head (retrieved 17 October 2026)
  7. Yourspares, how to choose strimmer line (retrieved 17 October 2026)
  8. Consumer Rights Act 2015, section 22 (retrieved 17 October 2026)
  9. Bosch DIY, extra-strong line 37 cm x 3.5 mm (retrieved 17 October 2026)