Flexi Says: The measurement system used by most scientists and engineers is the International System of Units, or SI. There are a total of seven basic SI units, or base units. These include units for length (meter) and mass (kilogram). SI units are easy to use because they are based on the number 10. Basic units are multiplied or divided by powers of ten to arrive at bigger or smaller units. Prefixes are added to the names of the units to indicate the powers of ten.