Sometimes, especially when you are traveling overseas where the metric system is the measurement system of choice, your items will be measured in milliliters instead of ounces. To fit within the guidelines of the 3-1-1 you will need to convert the milliliters into ounces. This is fairly simple to do. Simply multiply the number of milliliters by 0.0338 to find the number of ounces. In general, 100 milliliters equals 3.38 ounces, so an item that is 100 milliliters or less would fit within the guidelines.