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.