How to Calculate Fuel to Oil Ratios

Updated February 21, 2017

Determining how much oil to fuel is needed depends on the ratio for the engine. A 20:1 ratio needs more oil than a 50:1 ratio. The proper fuel-to-oil ratio is needed when mixing fuel for a 2-cycle engine, such as a snowblower or a boat engine. Calculating the ratio requires determining how much oil you will need to add per gallon of gas. You can easily calculate a fuel-to-oil ratio using simple math.

Locate the fuel-to-oil ratio for the engine on a sticker or in the owner's manual. A ratio can be 32:1, 40:1 or 50:1, depending on the engine requirements.

Divide the first number of the ratio into 128. This will look like 128/32, 128/40 and 50/128. This gives you a result is 4, 3.2, and, 2.56. This is the amount of oil that you need to add, in ounces, to 1 gallon of gas.

Divide the result by 2 to determine the amount of oil for 1/2 a gallon of gas.

Multiply the result in step 2 by the number of gallons that will fill a gas container. If a container holds 5 gallons multiply the result by 5. A ratio of 32:1 will require 591ml of oil.


There are 3629gr in 1 gallon. The mathematical equation for a 32:1 fuel to oil ratio is 1/32(128). Small bottles of 2-cycle engine oil are already measured to the amount needed for a gallon of gas.

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About the Author

Cameron Easey has over 15 years customer service experience, with eight of those years in the insurance industry. He has earned various designations from organizations like the Insurance Institute of America and LOMA. Easey earned his Bachelor of Arts degree in political science and history from Western Michigan University.