Stihl Weed Eater Gas Mix: FS Trimmer and Blower Ratios
Published October 4, 2026 · 8 min read
The gas mix for a Stihl weed eater is 50:1: 2.56 oz of 2-stroke oil per US gallon of gas, or 20 ml per litre. Stihl does not vary it by model, size or engine type. The FS 38 in a homeowner’s garage, the FS 131 on a landscaping crew’s trailer, the KM 131 KombiEngine and the BR 800 backpack blower all take the same mix from the same can. The only thing that changes between them is how fast they drink it, and whether the engine is a true two-stroke or Stihl’s 4-MIX, which looks like a four-stroke and still needs the oil in the gas.
Stihl trimmer and blower mix by model
| Model | Type | Engine | Mix |
|---|---|---|---|
| FS 38, FS 40, FS 50, FS 56, FS 70 | Homeowner trimmers | Two-stroke | 50:1 |
| FS 91, FS 111, FS 131 | Professional trimmers | 4-MIX | 50:1 |
| KM 56 | KombiEngine (attachment powerhead) | Two-stroke | 50:1 |
| KM 91, KM 111, KM 131 | KombiEngines | 4-MIX | 50:1 |
| BG 50, BG 56, BG 86 | Handheld blowers | Two-stroke | 50:1 |
| BR 350, BR 450 | Backpack blowers | Two-stroke | 50:1 |
| BR 600, BR 800 | Backpack blowers | 4-MIX | 50:1 |
If your model is not in the table, the answer is still 50:1. Stihl’s own mixing guide says it in one line: all Stihl gasoline-powered equipment uses a 50:1 gas and oil mixture. The engine type column matters for a different reason, covered below.
The KombiEngine point trips people up the same way Echo’s PAS system does. On a KM 56 or KM 131 the ratio belongs to the powerhead, so the edger, pole pruner, hedge trimmer and brushcutter attachments all run on whatever is in the powerhead’s tank. There is no separate mix for the attachment.
4-MIX: a four-stroke that still needs premix
The 4-MIX engine is the reason “gas mix for Stihl weedeater” is a real question rather than a solved one. The FS 91, FS 111 and FS 131 have valves and a four-stroke cycle, and they sound and pull like a four-stroke. Owners who know that four-strokes take straight gas, as their lawn mower does, reasonably assume the same here.
They do not. A 4-MIX engine has no crankcase oil and no dipstick. The fuel mix passes through the crankcase and a channel into the valve train, and the 2-stroke oil in that mix is the only lubrication the bearings, piston and valves get. Run one on straight gas and it fails the same way a two-stroke does: scored piston, seized engine, a repair bill that usually exceeds the trimmer’s value.
The quick test for any blower or trimmer: if there is an oil fill cap and a dipstick on the engine, it is a true four-stroke and takes unmixed gas. If there is only a fuel tank, it takes premix, whatever the marketing says about four-stroke power. Every Stihl gas trimmer and blower falls in the second group. The leaf blower gas oil mix guide covers the true four-strokes from other brands (Makita MM4, Honda-engined handhelds) that go the other way.
How much oil: batch sizes and tank fills
Stihl sizes its oil bottles around 50:1. The 2.6 oz bottle treats one gallon and the 6.4 oz bottle treats 2.5 gallons. If you are using another brand of air-cooled 2-stroke oil, or mixing in litres, these are the amounts:
| Fuel | Oil at 50:1 |
|---|---|
| 1 US gallon | 2.56 oz |
| 2 US gallons | 5.12 oz |
| 2.5 US gallons | 6.4 oz |
| 5 US gallons | 12.8 oz |
| 1 litre | 20 ml |
| 5 litres | 100 ml |
Matching the batch to the machine is the part most people skip. A professional FS 91 R carries a 710 cc (24 oz) tank, so a gallon of mix fills it about 5.3 times. A BG 86 handheld blower holds 440 cc (14.9 oz), about 8.6 fills per gallon. A BR 600 backpack holds 1.4 litres (47.3 oz) and empties a gallon can in under three fills. A homeowner who trims for twenty minutes a week can take two months to get through one gallon in a trimmer. A crew running a BR 600 all morning goes through it before lunch.
That difference should set your can size. For a single homeowner trimmer, mix a gallon at a time, or less. For a crew or a backpack blower, 2.5 or 5 gallons makes sense because it will be used inside the fresh window.
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Fuel: octane, ethanol and how long it keeps
Stihl recommends mid-grade unleaded of 89 octane or higher. Lower octane runs hotter and raises the risk of piston seizure, which matters more on a trimmer than people expect, because a trimmer spends most of its life at full throttle with a small, hot cylinder. Keep ethanol at 10 percent or less. E15 is not approved for small off-road engines in the US, and it degrades fuel lines and carburettor diaphragms faster than E10.
On storage, Stihl’s advice is to mix only enough for a few days of work and never store mixed fuel longer than three months. Three months is the outer limit, not the target. With ordinary ethanol-blended pump gas, about a month is the realistic shelf life before starting gets harder, and a stabiliser added at mixing time stretches that. Shake the can before every refuel, since oil slowly settles out of a can that has sat for a while.
Stihl also sells MotoMix, a premixed 50:1 ethanol-free fuel made with its HP Ultra oil. It costs several times what you would pay to mix your own, but it removes the two failure modes that kill trimmer carburettors: bad measuring and stale ethanol fuel. For a trimmer that is used a few times a month and stored over winter, it is a reasonable trade. For a backpack blower that burns a gallon a day, mixing your own makes more sense.
Mixing it right
The procedure is short, and the order matters:
- Use a clean, approved fuel can that holds only 2-stroke mix. Label it with the ratio and the date.
- Pour the measured oil in first, then add the gas. The incoming gas stirs the oil in better than shaking alone.
- Cap the can and shake it hard before the first fill and again before every later one.
- Clean around the fuel cap before opening it on the trimmer, so grit does not fall into the tank.
Measure, do not eyeball. One 2.6 oz bottle in a 2-gallon can gives roughly 100:1, half the oil the engine needs, and that single mistake causes more scored pistons in handheld tools than any brand of oil. The mixing guide covers the procedure in full, and measuring bottles and ratio cups make odd-sized batches repeatable.
Which oil, and the old 25:1 manuals
Stihl recommends its own HP, HP Super and HP Ultra oils, all mixed at 50:1. Any quality 2-stroke oil designed for air-cooled engines, rated JASO FD or ISO-L-EGD, is a safe substitute at the same ratio. What you must not use is marine TC-W3 outboard oil, which is formulated for water-cooled engines that run far cooler, or ordinary motor oil. The Stihl 2-stroke oil guide compares the three Stihl grades and the alternatives.
If you have an older manual, mostly a European edition, that lists 50:1 for Stihl oil and 25:1 for other brands, that rule dates from when generic oils carried only the older API TC rating. Today’s JASO FD oils exceed that, and Stihl’s current guidance is 50:1 across the board. Check the warranty terms on a machine that is still covered, though: like Echo, Stihl ties some extended warranty offers to its own oil.
When the mix is wrong
- Smoke and a wet, black plug right after a fresh can: too much oil, usually a full bottle in less than a gallon. Pour it off and remix.
- Power fading under load, then a seizure: too little oil. Stop at the first sign; a scored piston is visible through the exhaust port.
- Hard starting after a few weeks idle: stale fuel, not the ratio. Drain and refill with fresh mix.
- A 4-MIX that seized after a fill-up at the pump: straight gas. There is no rescue except a rebuild.
Echo owners have the same 50:1 answer with different model numbers in the Echo weed eater gas oil ratio guide. For the trimmer as a category, including the budget brands that still use 40:1, see the string trimmer page. The leaf blower page and the 50:1 mix chart cover the rest of the Stihl shed, and a Stihl chainsaw takes the same can, as the chainsaw page explains.
Common questions
[01] What is the gas mix for a Stihl weed eater?
50:1, meaning 50 parts gasoline to 1 part 2-stroke oil. That is 2.56 fl oz of oil per US gallon, or 20 ml per litre. Stihl uses 50:1 for every gasoline-powered tool it sells, including the FS trimmers, the KM KombiEngines and the BG and BR blowers.
[02] Do Stihl 4-MIX trimmers need mixed gas?
Yes. The FS 91, FS 111, FS 131 and the KM 91, KM 111 and KM 131 use Stihl's 4-MIX engine, which is a four-stroke design with valves but no oil sump. The 2-stroke oil in the fuel is the only thing lubricating the crank, piston and valve train, so they must run on 50:1 premix. Straight gas will destroy one.
[03] How much Stihl oil do I put in 1 gallon of gas?
2.56 fl oz, which is why Stihl sells its oil in 2.6 oz bottles: one bottle into one US gallon of gas makes a 50:1 mix. For 2.5 gallons use 6.4 oz, and for 5 gallons use 12.8 oz.
[04] What octane gas should I use in a Stihl trimmer?
Stihl recommends mid-grade unleaded of at least 89 octane. Lower octane raises engine temperature and the risk of piston seizure. Keep ethanol at 10 percent or less, and never use E15 or E85 in small off-road engines.
[05] How long does mixed gas last in a Stihl weed eater?
Stihl recommends mixing only enough for a few days of work and never storing mixed fuel for more than three months. With ordinary ethanol-blended pump gas, plan on about a month before it starts to go stale. Ethanol-free canned premix such as Stihl MotoMix lasts much longer.
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