Tell students to write the known ratio as a fraction. For instance, to change 345 centimeters to meters, students write what they know. Ask the class to look at a meter stick to determine how many centimeters are in one meter. They discover 100 centimeters is equivalent to one meter. Instruct them to write what they know as a fraction, the ratio is written 100/1.
Write an equal sign next to the ratio and continue writing the proportion with information known; 100/1 = 345/x. The unknown is represented by the variable 'x'. You do not know how many meters 345 centimeters is equivalent to at this point.
Demonstrate how to multiply the cross-products of the proportions. Your equation is 100x = 345 x 1. Both sides of the equation need to be equal, 100x = 345. Divide 345/100 = 3.45 to solve for the answer. The number of meters equivalent to 345 centimeters is 3.45. The same process can be followed for changing inches to feet, gallons to cups, inches to centimeters, or any measurements you need to convert to another unit of measure.
Provide hands-on activities dealing with other units of measure. Use a teacher-generated worksheet so the students complete conversions that are found in the classroom -- the height of the door from inches to feet, the amount of water in the vase from cups to gallons.