Convert fractions into decimals when you are dealing with measurements. For example, instead of writing "1 3/4 meters," write "1.75 meters." Instead of writing "2 2/3 liters," write "2.67 liters."
Change the unit of measurement if it will make the number more understandable to the reader. For example, if you are converting 1/1000 of a meter to a decimal, you could write ".001 meters." However, in most cases, it would make more sense to write "1 millimeter."
Leave the fraction as a fraction if you are using it in certain mathematical equations that involve other fractions. Many values, such as "1/3 meter," do not have an exact decimal equivalent, and changing the value to a decimal inside the formula will throw your answer off.
Leave the fraction as a fraction if you are using it in a casual statement. Writing, "He drank one and a half liters of water after running the race" usually makes more sense than writing, "He drank 1.5 liters of water after running the race."