Every 2-stroke brush cutter engine needs oil mixed into the petrol because there’s no separate oil sump or pump — the oil in the fuel is the only lubrication the crankshaft bearings, piston and cylinder wall get. Get the ratio wrong and you’re either starving the engine of lubrication or drowning it in oil it doesn’t need. Below are the numbers for every common ratio, so you can measure straight from the chart without doing arithmetic at the workbench.
Find your ratio first
The ratio is not a matter of preference — it’s set by the engine manufacturer and printed somewhere on the machine or in the manual. Look in this order:
- The fuel cap. Most machines have the ratio moulded or printed directly on or around the cap (e.g. “MIX 50:1” or a fuel/oil pictogram).
- The engine shroud, usually near the recoil starter housing or air filter cover.
- The owner’s manual, under “fuel” or “specifications.”
- The oil bottle, if it’s a manufacturer-branded oil sold specifically for that machine — though this only confirms the oil is compatible, not that the ratio matches your engine.
If none of these are legible (faded decals are common on machines a few years old), go with 50:1 using a good semi-synthetic or full-synthetic 2-stroke oil — it’s the standard for the vast majority of modern air-cooled handheld engines from Stihl, Husqvarna, Makita, Echo and similar brands. Older machines and some professional-grade engines specify 40:1 or 33:1; a handful of older or budget engines call for 25:1.
Safety note. Never mix fuel and pour it near an open flame, running engine, or inside a shed with a pilot light. Mix outdoors or in a well-ventilated garage, away from ignition sources, and let a hot engine cool for a few minutes before refuelling.
The full ratio chart
Oil quantities below are for pure fuel volume (petrol only) before oil is added — this is standard practice and how oil bottle markings and mix jugs are calibrated.
| Ratio | Oil per 1 L | Oil per 2 L | Oil per 5 L | Oil per 10 L | Oil per 1 US gal |
|---|---|---|---|---|---|
| 25:1 | 40 ml | 80 ml | 200 ml | 400 ml | 5.1 US fl oz |
| 33:1 | 30 ml | 60 ml | 152 ml | 303 ml | 3.9 US fl oz |
| 40:1 | 25 ml | 50 ml | 125 ml | 250 ml | 3.2 US fl oz |
| 50:1 | 20 ml | 40 ml | 100 ml | 200 ml | 2.6 US fl oz |
A few notes on reading this table:
- 33:1 is rounded from the true value (30.3 ml/L); most pre-marked mixing jugs round to the nearest visible line, which is fine in practice.
- US fl oz figures assume a US gallon (3.785 L), not an imperial gallon (4.546 L) — if you’re working in imperial gallons, multiply the “oil per litre” figure by 4.546 instead.
- If your mixing jug only has litre markings and you need an odd quantity (say, 3 L), just add the “per 1 L” figure three times, or use the 5 L column and estimate down — precision to the nearest ml doesn’t matter, precision to the nearest ratio band does.
Quick mental shortcuts (no chart needed)
If you’ve left the chart at home, these are the numbers worth memorising for the two most common ratios:
- 50:1 — 20 ml of oil per litre of fuel. For a typical 1 L or 2-stroke fuel can size sold at fuel stations, that’s one “small” measuring cap fill on most bottled 2-stroke oils (check the bottle — most pre-measure to 100 ml lines, which handles 5 L in one go).
- 40:1 — 25 ml of oil per litre of fuel, or one-quarter more oil than 50:1.
A handy rule: 1 litre of fuel weighs roughly 750 g, but you don’t need to weigh anything — always measure oil by volume (ml) with a proper 2-stroke mixing jug, not by eye or by “glugs” from the bottle.
Litres to millilitres, worked examples
For the tank sizes most brush cutters actually take (typically 0.5–1 L per fill, mixed in a larger batch of 5 L or 10 L for convenience):
| Fuel volume | 25:1 oil | 40:1 oil | 50:1 oil |
|---|---|---|---|
| 0.5 L | 20 ml | 12.5 ml | 10 ml |
| 0.75 L | 30 ml | 19 ml | 15 ml |
| 1.5 L | 60 ml | 37.5 ml | 30 ml |
| 3 L | 120 ml | 75 ml | 60 ml |
Most people mix in 5 L batches (a full fuel can) since it’s the most common can size and lasts several fill-ups without the fuel sitting around long enough to degrade.
What happens if you get it wrong
The two failure modes are not equally forgiving, so it’s worth understanding which direction is the dangerous one.
| Mistake | What actually happens | How serious |
|---|---|---|
| Too little oil (mix too lean) | Reduced lubrication on the crank bearings, piston and cylinder wall; heat builds up faster; risk of piston-to-cylinder seizure, especially under sustained load | Serious — can destroy the engine in one tank if severe enough |
| Too much oil (mix too rich) | Excess oil burns as visible smoke, fouls the spark plug faster, leaves carbon deposits on the piston crown and exhaust port/muffler screen over time | Minor to moderate — annoying and can cause hard starting or lost power if left unchecked for a long time, but rarely catastrophic |
| No oil at all (pure petrol) | Complete lubrication failure; the engine will seize, often within minutes of running under load | Severe — treat this as an emergency stop the moment you realise |
| Wrong oil type (4-stroke oil, or non-JASO-rated oil) | Doesn’t burn as cleanly, leaves heavier ash/carbon deposits, may not provide adequate anti-scuff protection at 2-stroke operating temperatures | Moderate — use only oil labelled for air-cooled 2-stroke engines (JASO FB/FC or ISO-L-EGB/EGC) |
If you’ve run a tank on straight petrol or a mix far too lean and the engine has already shown signs of losing power, seizing, or excessive heat, stop the machine immediately and let it cool before investigating further — running it again risks scoring the cylinder wall permanently. For engines that already sound rough, tight, or have died suddenly under load, that’s a separate diagnosis from ratio maths; see our guide on troubleshooting a brush cutter that loses power under load or dies at full throttle.
If you’ve mixed a batch too rich by mistake (for example, doubled the oil by miscounting caps), it’s not an emergency — just drain the tank, remix correctly, and expect a bit more smoke than usual until the excess oil works through the system.
Mixing correctly, step by step
- Pour the oil into the empty mixing jug first, using the ml markings on the jug or a separate oil measuring cap.
- Add the fuel up to the target litre mark, then close the jug and shake vigorously for 15-20 seconds — oil and petrol don’t stay blended on their own, especially once cool.
- Label the jug with the date mixed if you keep fuel around for more than a few days.
- Shake again before each refuel, even from a jug mixed recently — oil can settle slightly if the jug has sat still.
- Never top up an old, part-used tank with a different ratio on top of what’s already in it; if you’re not sure what ratio is currently in the tank, run it dry (in a safe, ventilated area) before adding a freshly mixed batch.
Quick checklist
- Confirm the ratio from the fuel cap, shroud, or manual before mixing anything — don’t assume 50:1.
- Measure oil by volume in a proper 2-stroke mixing jug, never by eye.
- Use oil rated for air-cooled 2-stroke engines (JASO FB/FC or better); don’t substitute 4-stroke oil.
- Shake the mix thoroughly, and again before each refuel.
- Use mixed fuel within 2-4 weeks; don’t let it sit over winter in the tank.
- If in doubt between two ratios, err slightly rich rather than lean — smoke is a nuisance, seizure is not.