Many shippers make mistakes with freight class. These mistakes can cost money. A Freight Class Calculator helps avoid them, but only if used correctly. One common mistake is wrong measurements. If you measure size or weight incorrectly, the class will be wrong. Always measure carefully. Another mistake is guessing the class. Some people reuse old classes for new shipments. But small changes in size or weight can change the class. A Freight Class Calculator should be used each time.