How this instrument works
Two-stroke engines — common in chainsaws, string trimmers, leaf blowers, some outboard motors and older dirt bikes — burn oil directly in the combustion chamber along with gasoline, since they lack a separate oil sump like a four-stroke engine. Getting the gas-to-oil ratio right matters: too little oil starves the engine of lubrication and can cause serious damage, while too much causes smoking, fouled spark plugs and carbon buildup.
A mix ratio like '50:1' means 50 parts gasoline to 1 part oil by volume — so for any amount of gasoline, the oil needed is simply that amount divided by the ratio number. Different equipment calls for different ratios: many modern small engines, including most current STIHL equipment, specify 50:1, some manufacturers or older engines call for a richer 40:1 or 32:1 mix, and it always pays to check your specific equipment's owner's manual rather than assume.
This instrument works in whatever volume unit you enter — gallons, litres, or any consistent unit — since the ratio itself is unit-agnostic; entering 5 (gallons) at a 50:1 ratio and entering 5 (litres) at a 50:1 ratio both correctly return 0.1 of that same unit as the oil needed. Just keep the unit for fuel and the resulting oil figure consistent in your head.
- Enter Gasoline amount in whatever volume unit you're measuring — gallons or litres, matching what your fuel container or pump reads.
- Enter Mix ratio as just the fuel-side number — for a 50:1 mix, enter 50; for a 40:1 mix, enter 40.
- Read Oil needed directly beneath the inputs — it's returned in the same unit you entered for gasoline.
- Check your specific engine's owner's manual for its required ratio before mixing, since ratios vary by manufacturer and engine age.
- Mix only what you'll use within about 30 days, since ethanol-blended gas-oil mixtures can degrade and separate over time.
Worked example — 5 gallons of gas at a 50:1 ratio
A gas can holds 5 gallons of fresh gasoline, and the chainsaw's manual specifies a standard 50:1 mix. Entering 5 for gasoline amount and 50 for mix ratio gives oil needed = 5 / 50 = 0.1 gallons of two-stroke oil.
0.1 gallons works out to 12.8 fluid ounces (since a gallon holds 128 fluid ounces) of two-stroke oil to add to the 5 gallons of gas — the same 50:1 ratio STIHL specifies for its full gasoline-powered lineup, and a common default across many other 2-stroke brands as well.
Questions
How do I know what mix ratio my equipment needs?
Check the owner's manual for your specific chainsaw, trimmer, blower or outboard motor — the correct ratio is printed there and sometimes also stamped on the equipment itself near the fuel cap. Many current small 2-stroke engines specify 50:1, but plenty of equipment, especially older models, calls for a richer 40:1 or 32:1 mix; always defer to your equipment's actual specification rather than assuming 50:1 universally applies.
What happens if I get the ratio wrong?
Too little oil (a 'leaner' mix than specified, or accidentally running straight gas) starves the engine's internals of lubrication, risking piston seizure and serious, sometimes irreversible damage. Too much oil (a 'richer' mix than needed) causes excess smoke, spark-plug fouling, and carbon buildup that can hurt performance and starting reliability over time — neither direction is harmless, so matching the manufacturer's specified ratio matters.
Can I use this for litres instead of gallons?
Yes — the ratio math is unit-agnostic, so entering your gasoline amount in litres returns the oil needed in litres, using the exact same formula. Just be consistent: if you enter gasoline in gallons, read the oil result as gallons; if you enter litres, read the result as litres, and convert to a more convenient sub-unit like fluid ounces or millilitres afterward if that's easier for measuring.
Does the mix ratio change with different types of 2-stroke oil?
The specified ratio is set by the engine manufacturer for their specific equipment, not by the oil brand — but the oil itself does need to meet the right quality classification (commonly JASO FB, FC or FD, or the equivalent ISO-L-EGB/EGC/EGD standards) for that engine. Using a lower-quality oil than specified, even at the correct ratio, can still leave an engine under-protected, so match both the ratio and the oil-grade requirement in your equipment's manual.
Why do some engines use a richer ratio like 32:1 or 40:1 instead of 50:1?
Older 2-stroke engine designs, and some engines built for higher sustained loads like certain dirt bikes and older chainsaws, were engineered around a richer mix — more oil relative to gasoline — to ensure adequate lubrication under those specific operating conditions. Modern engine and oil-formulation improvements have let many current manufacturers move to leaner 50:1 mixes while still protecting the engine, but older or specialty equipment may still call for the richer ratio it was originally designed around.