Gas Oil Ratio Calculator works out the 2-stroke oil a can of gas needs at 50:1, 40:1, 32:1, or any ratio, then checks a finished mix and shows the gas or oil to correct a poor mix.
How the Gas Oil Ratio Calculator Mixes 2-Stroke Fuel
The Gas Oil Ratio Calculator works out how much 2-stroke oil to add to gas, or how much gas to add to oil, for any mix ratio. Owners of chainsaws, trimmers, blowers, outboards and dirt bikes use it to mix a can without guessing.
It has four modes. Two build a fresh mix, one checks a can you already mixed, and one fixes a can mixed at the wrong ratio.
Gas can be entered in US gallons, liters or UK gallons. Oil comes out in US fluid ounces or milliliters, with tablespoons and cups for measuring at home.
The Gas Oil Ratio Calculator Formula
A ratio like 50:1 means 50 parts gas to 1 part oil, by volume. So the oil you need is the gas divided by the ratio.
$$\text{Oil} = \frac{\text{Gas}}{\text{Ratio}}$$
Working backward from oil, multiply instead.
$$\text{Gas} = \text{Oil} \times \text{Ratio}$$
The Gas Oil Ratio Calculator converts everything to milliliters first. A US gallon is exactly 128 US fluid ounces, or 3,785.41 mL, and one fluid ounce is 29.57 mL.
With the defaults, 2 gallons at 50:1 needs 5.12 fl oz of oil. That is 151.42 mL, or about 10 tablespoons.
A common mistake is entering the size of the can instead of the gas actually in it. If a 5-gallon can holds 3 gallons, enter 3, or the mix will come out oil-rich.
Oil per Gallon at Common Ratios
The second result card shows the dose at your ratio for other fuel amounts. The table below comes straight from the formula, using 128 fl oz per US gallon and 1,000 mL per liter.
| Ratio | Oil per US gallon | Oil per liter | Oil share of the mix |
|---|---|---|---|
| 100:1 | 1.28 fl oz | 10.0 mL | 0.99% |
| 50:1 | 2.56 fl oz | 20.0 mL | 1.96% |
| 40:1 | 3.20 fl oz | 25.0 mL | 2.44% |
| 32:1 | 4.00 fl oz | 31.3 mL | 3.03% |
| 25:1 | 5.12 fl oz | 40.0 mL | 3.85% |
| 20:1 | 6.40 fl oz | 50.0 mL | 4.76% |
| 16:1 | 8.00 fl oz | 62.5 mL | 5.88% |
Notice that 50:1 is 1.96% oil, not 2%. The ratio compares oil to gas, so the oil share of the finished mix is 1 divided by the ratio plus one. The fourth card shows that share along with the total volume of mixed fuel.
The third card shows the oil for the same gas at nearby common ratios. At the defaults, 40:1 would take 6.40 fl oz and 32:1 would take 8.00 fl oz.
Why the 2.6 oz Bottle Isn’t Exactly 50:1
Oil makers sell pre-measured 2.6 oz bottles for one gallon at 50:1. The exact figure is 2.56 fl oz, so a full 2.6 oz bottle in one gallon actually makes about 49.2:1.
That small extra oil is harmless and errs on the safe side. It only matters when you split bottles across odd amounts of gas, which is where the calculator helps.
Checking a Can You Already Mixed
Pick Check My Mix Ratio in the Gas Oil Ratio Calculator, then enter the gas in the can and the oil you poured in. The hero result shows the ratio you actually made.
Say you added 5 fl oz of oil to 2 gallons. That works out to 51.2:1, about 2% leaner than 50:1.
The alert box turns green when the mix is within 10% of your target. It turns red when the mix is short on oil and yellow when it is oil-rich. The first card then shows how much gas or oil to add to reach the target.
Fixing a Can Mixed at the Wrong Ratio
Fix a Wrong Mix is the mode most calculators leave out. Enter the total mixed fuel in the can and the ratio it was mixed at, and the Gas Oil Ratio Calculator splits the can back into gas and oil.
$$\text{Oil in can} = \frac{\text{Mixed fuel}}{\text{Current ratio} + 1}$$
The rest of the can is gas. If the mix has too much oil, the tool adds gas until the oil matches the target. If it has too little, it adds oil instead.
$$\text{Gas to add} = \text{Oil in can} \times \text{Target ratio} – \text{Gas in can}$$
$$\text{Oil to add} = \frac{\text{Gas in can}}{\text{Target ratio}} – \text{Oil in can}$$
Thinning a mix takes more gas than people expect. One gallon of 32:1 needs about 0.55 gallons of fresh gas to reach 50:1, leaving 1.55 gallons in total. The alert reminds you to check that your can has room.
Going the other way is easier. One gallon of 100:1 needs only about 1.27 fl oz of oil to reach 50:1.
Which Ratio Your Engine Needs
The engine label or owner’s manual sets the ratio, not the Gas Oil Ratio Calculator. Most modern chainsaws, trimmers, blowers and outboards call for 50:1.
ECHO states that its older engines listed 32:1 or 16:1 in their manuals, but can now run 50:1 because of advances in oil. Other brands may differ, so follow the manual for older equipment from any other maker.
A 100:1 mix is only for oils made and rated for it. A standard 2-stroke oil at 100:1 can leave the engine short of lubrication.
The oil itself matters too. Air-cooled handheld equipment is typically specified for oil meeting the JASO FD or ISO-L-EGD ratings, printed on the bottle.
Limits and Edge Cases
Every amount and ratio in the Gas Oil Ratio Calculator must be greater than zero. The tool flags any empty or zero field in red and clears the results.
The alert switches to a warning when the target ratio falls below 8:1 or above 200:1. That is well outside the 16:1 to 100:1 range 2-stroke engines use, so a value like that usually means a typo.
When you pick a ratio outside the preset list, the tool switches the Engine Ratio menu to Custom Ratio. The comparison card then shows two nearby standard ratios.
Mixing Mistakes the Tool Can’t Catch
In Fix a Wrong Mix, entering only the gas instead of the total mixed fuel throws off the split. Enter everything that is in the can.
Mixing in a UK 5-gallon can but entering US gallons adds about 17% too little oil. Set the gas unit to Gal (UK) for imperial cans.
Oil only mixes well when it is shaken through the gas. Pour the oil in first, add the gas, then shake the can gently before each use.
Two-Stroke Mix Questions
How much oil do I mix with 1 gallon of gas at 50:1?
Exactly 2.56 fl oz, which rounds to the 2.6 oz on most oil bottles. In metric, that is 20 mL of oil per liter of gas. For a 2.5-gallon can, use 6.4 fl oz, and for 5 gallons use 12.8 fl oz. If your can holds a partial fill, enter only the gas actually in it.
What does 50:1 mean in a gas oil ratio calculator?
It means 50 parts gas for every 1 part oil, measured by volume. The unit does not matter as long as both sides use the same one. So 50 mL of gas takes 1 mL of oil, and 50 gallons takes 1 gallon. The oil makes up 1.96% of the finished mix, since the ratio compares oil to gas and not to the total.
Can I run 40:1 in an engine that calls for 50:1?
You can, but it adds 25% more oil than the engine was set up for. At 40:1, one gallon takes 3.2 fl oz instead of 2.56. Extra oil can bring more smoke, spark plug fouling and carbon buildup over time. Too little oil is the bigger risk, since it can overheat and seize the engine, so never go leaner than the label.
How long does mixed gas last?
Premix made with E10 gas is best used within about 30 days. Ethanol pulls moisture from the air, which can separate from the gas and settle in the can. A fuel stabilizer or ethanol-free fuel extends that, and sealed canned premix lasts much longer. Mix only what you expect to use in a month.
Can I use regular motor oil in a 2-stroke mix?
No. Four-stroke motor oil is made to stay in a crankcase, not to burn with the fuel. In a 2-stroke engine it can foul the plug, clog the exhaust port and damage the rings. Use oil labeled for 2-stroke or 2-cycle engines, and check that it carries the rating your manual asks for.