Where 2.56 comes from
A US gallon is 128 fluid ounces. A 50:1 ratio means 50 parts gasoline to 1 part oil, so the oil is 128 divided by 50, which is 2.56 fluid ounces. There is no rounding to be clever about. At 40:1 it is 3.2 ounces, at 32:1 exactly 4 ounces, at 25:1 5.12 ounces, and at 100:1 1.28 ounces. In metric the arithmetic is friendlier: a litre is 1000 millilitres, so 50:1 is 20 mL per litre, 40:1 is 25 mL and 32:1 is 31.25 mL. Those two systems agree with each other, because 2.56 fl oz is 75.7 mL and a gallon is 3.785 litres.
| Ratio | Per US gallon | Per 2.5 US gal | Per litre | Per 5 litres |
|---|---|---|---|---|
| 100:1 | 1.28 fl oz | 3.2 fl oz | 10 mL | 50 mL |
| 50:1 | 2.56 fl oz | 6.4 fl oz | 20 mL | 100 mL |
| 40:1 | 3.2 fl oz | 8.0 fl oz | 25 mL | 125 mL |
| 32:1 | 4.0 fl oz | 10.0 fl oz | 31.3 mL | 156 mL |
| 25:1 | 5.12 fl oz | 12.8 fl oz | 40 mL | 200 mL |
| 16:1 | 8.0 fl oz | 20.0 fl oz | 62.5 mL | 313 mL |
The one thing this table cannot tell you is which row is yours. The ratio is a property of the engine and the oil together, and it is printed in the manual and usually on the fuel cap or a decal near it. Older equipment and modern equipment do not agree, and two machines in the same shed can want different things. Nothing on this page is a recommendation for your machine.
The partial can problem
Most people do not mix a whole gallon. They mix whatever the can holds, or whatever is left of the gas jug, and the ratio quietly drifts because the oil bottle is sized for a round number of gallons. A 2.6 fl oz bottle is a 1 gallon dose at 50:1. If you are filling a can with 0.8 gallons of gasoline, you need 2.05 ounces, which is about four fifths of that bottle, and pouring four fifths of a bottle by eye is where the error lives.
Two honest ways out. Either measure the gasoline to match a whole bottle, which is why so many people mix in exact gallon increments, or use a graduated mixing bottle with millilitre markings and pour to a line. A kitchen measuring cup is not accurate at three ounces and should not be reused for food afterwards. Getting the gasoline volume right matters as much as the oil, because the ratio is a ratio and both halves count.
Fixing a batch you already got wrong
The fix mode does the only two things that are physically available. If the mix has too much oil for your target, the oil in the can does not go anywhere, so you add plain gasoline until that quantity of oil is spread across enough fuel. A gallon mixed at 32:1 holds 3.88 ounces of oil, and 3.88 ounces is a 50:1 dose for 1.55 gallons, so you add about half a gallon of straight gasoline. If the mix has too little oil, you add oil, shake it hard and let it stand.
The asymmetry between those two cases is not academic. Too much oil is an inconvenience: fouled plugs, a smoky exhaust, carbon in the port. Too little oil is the failure mode that scores a cylinder, and it happens on the hot pass through a hardwood log rather than politely at idle. If you cannot account for what went into a can, the cheap answer is to put that fuel in something that runs on straight gasoline and start over, not to guess.
Gasoline is the hazard, not the oil
Mixing means handling open gasoline, and gasoline vapour is heavier than air, travels along the ground and ignites at a distance from the container. Mix outdoors, away from water heaters, pilot lights, running engines and anything that sparks. Never fuel an engine that has just been running, because the muffler is far above the ignition temperature of the vapour coming out of the tank. Use containers made for fuel, keep them out of living space, and keep them closed.
Once the can is mixed, the next question is how long it stays usable, which is a different problem from the ratio and is covered in the fuel storage and stabiliser planner. If the machine you are mixing for is a saw, the chainsaw bar and chain calculator covers the other half of the consumables, and the service interval planner tracks what wears out on a run-hour basis.
Questions people ask
How much oil per gallon at 50 to 1?
2.56 US fluid ounces, which is 75.7 millilitres. The arithmetic is 128 fluid ounces in a gallon divided by 50. For two gallons it is 5.12 ounces, for 2.5 gallons 6.4 ounces, and for five gallons 12.8 ounces. Single-dose oil bottles are usually sized so one bottle treats a round number of gallons at a common ratio, so a 2.6 ounce bottle is a one gallon dose at 50:1 and a 6.4 ounce bottle is a 2.5 gallon dose. Match the gasoline quantity to a whole bottle where you can, because pouring a fraction of a bottle accurately is harder than it sounds.
What happens if I use 40:1 in an engine that calls for 50:1?
You have put more oil in than the design assumes, and the effects are all on the nuisance end: more visible smoke, quicker plug fouling, more carbon building in the exhaust port and on the piston crown, and over time a muffler screen that clogs and costs the engine power. It is not the failure case. The failure case is the other direction. What this page will not do is tell you that any given substitution is fine for your machine, because the oil specification and the ratio are set together by the manufacturer and the manual is the only authority on it. If you have mixed a batch richer in oil than you wanted, dilute it with plain gasoline using the fix mode rather than running it and hoping.
Can I run straight gasoline in a two-stroke by mistake and get away with it?
No, and the damage is quick. A two-stroke has no oil sump and no oil pump for the crankshaft bearings and cylinder wall. The only lubrication those surfaces get arrives dissolved in the fuel. Run a tank of straight gasoline and the engine will usually start, run well for a few minutes because it is making more power without the oil, then seize. If you realise the mistake before starting, drain the tank and the carburettor bowl or bulb, refill with correct mix and it is a non-event. If it has already run, stop and get it looked at rather than restarting it.
Does the ratio mean gasoline to oil, or total mix to oil?
Conventionally it is gasoline to oil: 50:1 is 50 parts gasoline plus 1 part oil, giving 51 parts of finished mix. That is the convention this calculator uses, and it is why the finished volume shown is slightly larger than the gasoline you entered. In practice the distinction is worth about two percent at 50:1, which is smaller than the error in most measuring, so it rarely changes what you pour. It matters more at rich ratios like 16:1, where the oil is six percent of the total, and it matters when the calculator has to work backwards from a can of finished mix in the fix and check modes.
Do I need to shake the can, or does it mix itself?
Shake it. Two-stroke oil and gasoline are miscible and will stay mixed once combined, but they do not combine on their own in a can standing still. Oil poured on top of a full can of gasoline sits there as a layer, and the first pour out of that can is mostly gasoline while the last is mostly oil. The order that works is oil into the empty can, roughly half the gasoline, cap and shake, then the rest of the gasoline and shake again. Give it a shake before each fill after that too, particularly if the can has been sitting.