The most common mixtures for small engines are 32:1, 40:1, 50:1, or 100:1. Currently, the most common mixtures are 50:1 and 40:1. If you don’t have the user manual, try searching online for your weed wacker model and recommended oil mixture, or contact the manufacturer. When you learn the correct oil mixture, write it down somewhere or on the side of the machine so you don’t forget.

1 US gal (3. 8 L) is also a good amount because in the US, mixing oil usually comes in small bottles measured to a 50:1 mixture for a gallon of gas. This way, you can just pour in the whole bottle, unless you’re using a different ratio. Only mix gas in a container meant to hold gasoline. If you’re using an old container, clean off the area around the lid so no dirt or grime gets into your gasoline.

If you have a jerry can that holds more than 1 gallon (3. 8 L), just multiply the number of gallons by 128. A 3-gallon can, for instance, has 384 fl. oz.

This also works with other mixtures, like 40:1. Just divide by 40 instead of 50. 128 fl. oz. is 3. 2 fl. oz. if you divide it by 40. This means you need 3. 2 fl. oz. of oil per gallon of gas for a 40:1 mixture. Use the same calculation for milliliters. If you have a 5-liter can, this is 5,000 ml. Divide 5,000 by 50 to get 100 ml of oil.

This calculation is for 1 gallon (3. 8 L) of gas at a 50:1 mixture, so adjust that if you use a different amount of gas or different mixture. In the US, mixing oil usually comes in small bottles measured to a 50:1 mixture for a gallon of gas. In this case, pour a whole bottle of oil in for every gallon of the mixture for easy measuring. Some bottles come in different concentrations as well. If you need a 40:1 mixture, look for a 40:1 oil bottle. This makes measuring much easier.

Don’t overfill the can. If you go over the fill line, your oil concentration will be too low. You could also use premium-level gasoline to protect your engine better. Don’t go below an 89-octane rating, though, or you could damage the engine over time.

Shaking in a circular motion is best. Shaking up and down could spill the gas out.