What Is the Oil to Gas Ratio?
Every 2-stroke engine — the kind found in chainsaws, string trimmers, leaf blowers, mopeds, dirt bikes and many outboard motors — burns oil and gas together as a single fuel. Unlike a 4-stroke engine with a separate oil reservoir, a 2-stroke relies entirely on oil mixed directly into the gasoline to lubricate the piston, rings and crankshaft bearings.
The oil to gas ratio tells you exactly how much oil to blend into a given volume of gasoline. Get it right and the engine runs clean and lasts for years. Get it wrong in either direction — too little oil or too much — and you risk anything from fouled spark plugs to a seized piston.
The Oil to Gas Ratio Formula
Mix ratios are written as gas:oil, such as 50:1, meaning 50 parts gasoline for every 1 part oil. To find the oil volume for a given amount of gas:
[
\text{Oil Volume} = \frac{\text{Gas Volume}}{\text{Ratio}}
]
Both volumes must end up in the same unit before you divide — the calculator above converts gallons, liters and quarts to milliliters automatically so the math always lines up.
Worked Example: 1 Gallon at 50:1
Say your trimmer's manual calls for a 50:1 mix and you're filling it with 1 US gallon of gas. One gallon is 128 fluid ounces, so:
[
\text{Oil Volume} = \frac{128 \text{ fl oz}}{50} = 2.56 \text{ fl oz}
]
In metric terms, 1 gallon is 3,785.41 mL, so:
[
\text{Oil Volume} = \frac{3785.41 \text{ mL}}{50} = 75.7 \text{ mL}
]
That's roughly a 5-tablespoon pour of oil per gallon of gas — small enough that measuring accurately matters.
Common Mix Ratios and Oil per Gallon
Richer ratios (smaller second number relative to the first, like 16:1) use more oil; leaner ratios (like 100:1) use less. Here's how much oil each common ratio needs per US gallon of gas:
| Ratio | Oil per 1 gal gas | Typical use |
|---|---|---|
| 16:1 | 8.00 fl oz | Older 2-stroke motorcycles, some vintage outboards |
| 25:1 | 5.12 fl oz | Older chainsaws, some mopeds |
| 32:1 | 4.00 fl oz | Many chainsaws and dirt bikes |
| 40:1 | 3.20 fl oz | Common trimmers, blowers, outboards |
| 50:1 | 2.56 fl oz | Most modern handheld equipment |
| 100:1 | 1.28 fl oz | Select synthetic-oil rated engines |
Try 40:1 with 5 liters of gas in the calculator above and you'll get 125 mL of oil — the same ratio, just scaled to a different volume.
Why the Ratio Matters So Much
A 2-stroke engine has no dedicated oiling system, so the fuel mixture is the only thing lubricating the moving parts on every single stroke. Too lean on oil and metal-on-metal contact builds heat fast, often destroying an engine in minutes of hard use. Too rich and unburned oil fouls the spark plug, clogs the exhaust port, and leaves you with smoke and lost power — annoying, but far less catastrophic than running dry.
That asymmetry is why most manufacturers err toward slightly richer ratios than the theoretical minimum, and why guessing "close enough" is never a good idea when mixing fuel.
Quick Recap
- Oil Volume = Gas Volume ÷ Ratio, with both volumes in the same unit.
- Always use the ratio printed on the equipment or in its manual — don't assume 50:1 fits every engine.
- Mix only what you'll use within a month or two; old pre-mixed fuel separates and gums up carburetors.
- Use the calculator above to convert any gas volume and ratio into an exact oil amount.
If you're mixing fluids for your engine's cooling system too, the coolant ratio calculator works the same way for antifreeze and water.