A standard unit of measurement provides a reference point by which objects of weight, length or capacity can be described. Although measurement is an important part of everyday life, kids don't automatically understand that there are many different ways to measure things. A standard unit of measurement is a quantifiable language that helps everyone understand the association of the object with the measurement.
Your child might understand that the words “weight,” “height,” and “length” are associated with measuring, but it will take a while to understand that in order to compare and contrast objects or to build to scale, everybody needs the same starting point.
To begin, consider explaining to your child why a standard unit of measurement is necessary. For example, your child likely understands that he or she has a name, as do relatives, friends, and pets. Their names help identify who they are and show that they are a person. When describing a person, using identifiers, such as "blue eyes," helps specify the attributes of the person.
Objects also have a name. Further identification and description of the object can be achieved through measurement units.
A Measurement Experiment
You can use two objects at home to demonstrate this concept: a table and a book. Both you and your child can participate in this measurement experiment.
Holding your hand rigid, measure the length of the table in hand spans. How many of your hand spans does it take to cover the length of the table? How many of your child's hand spans? Now, measure the length of the book in hand spans. Your child may notice that the number of hand spans required to measure the table and book is different than the number of hand spans it took for you to measure the objects.
This is likely because your hands are different sizes, so you are not using a standard unit of measurement.
For your child’s purposes, measuring length and height in paper clips or hand spans, or using pennies in a homemade balance scale, may work well, but these are nonstandard measurements.
Once your child understands that hand spans are nonstandard measurements, introduce the importance of a standard unit of measurement. If your child is interested in how measurement works and in measuring things, you can introduce the vocabulary used for standard units of measurement.