Objects can be measured in inches, feet, centimeters, or meters. Knowing approximate conversions (12 inches = 1 foot, 100 cm = 1 meter) allows flexible measurement. Choosing an appropriate unit depends on the object's size.
You already know how to measure using standard units with a ruler. Now comes an important insight: the same object can be measured in multiple units, and each unit gives a different number for the same physical length. A door might be 80 inches tall, or about 6 feet 8 inches tall, or roughly 203 centimeters tall. All three describe the same door. The number changes, but the actual height does not.
This reveals a key idea: larger units produce smaller numbers, and smaller units produce larger numbers. A desk that is 4 feet wide is also 48 inches wide — the number jumped from 4 to 48 because inches are much smaller than feet, so you need many more of them to cover the same span. When you switch from centimeters to meters, the same principle applies: a 150-centimeter bookshelf is only 1.5 meters tall because a meter is 100 times longer than a centimeter.
Choosing an appropriate unit is a judgment skill. You would not measure the width of your fingernail in feet, nor the length of a road in inches — the numbers would be awkward (a tiny fraction, or an enormous count). The right unit produces a number that is easy to work with and easy to communicate. In the customary system, inches work well for small objects, feet for room-sized things, and yards or miles for distances. In the metric system, centimeters suit small objects, meters suit rooms and people, and kilometers suit large distances.
The conversions you need at this stage are anchors, not a long list. Know that 12 inches make 1 foot, 3 feet make 1 yard, 100 centimeters make 1 meter, and 1000 meters make 1 kilometer. With these four equivalences and the reasoning above — big unit = smaller number — you can navigate most unit questions you will encounter.