Type 2.54 into your calculator. One inch is equal to 2.54 cm. This is the basic conversion unit from which all other calculations can be performed.
Divide 2.54 by 16. The answer is 0.15875. So 1/16 inch---the lowest calibration on a standard ruler---is equal to 0.15875 cm, or, roughly, 1.6 mm (there are 10 millimeters to a centimeter).
Multiply 2.54 by 12. The answer is 30.48. This means that 30.48 cm is equal to 12 inches, or a "foot." Most standard rulers are 1 foot long.
Perform various multiplications and divisions to get the hang of converting standard ruler measurements to metric units.