In the physical world around us, we come across the quantities such as time, distance, mass, area, volume, and so on. In a mathematics course, we are more interested in the units of measurements, which are used to describe the magnitude of each of these quantities.
Some units of measurement are given below:
Time 
Distance (Metric)  Distance (Customary)  Mass (Metric)  Weight (Customary) 
Second 
Millimeter 
Inch 
Milligram 
Ounce 
Minute 
Centimeter 
Foot 
Gram 
Pound 
Hour 
Meter 
Yard 
Kilogram 
Ton 
Day 
Kilometer 
Mile  Metric Ton 

Month 

Year 

Century 
While describing various physical quantities, it is best to use appropriate units of measure. Using other units of measure to describe a quantity gives either very small or very large number value. It becomes difficult to estimate the magnitude of the quantity, if inappropriate units are used.
Example 1:
Quantity 
Appropriate unit of measure 
Distance between two cities 
Miles or kilometers 
Height of a person 
Feet and inches, or centimeters 
Weight of a person 
Pounds or kilograms 
Height of a highrise building 
Meters or feet 
Height of a mountain peak 
Meters or feet 
Depth of a ocean 
Meters or feet 
Area of a football field 
Feet^{2} or meters^{2} 
Capacity of milk bottle 
Liters 
Size of a Paper 
Inches or centimeters 
Time taken to complete 100 meter race 
Seconds 
Time taken to travel between two cities by car 
Hours 
Example 2:
What is the appropriate unit of measure used to describe the width of a computer monitor?
Answer:
Imagine yourself measuring the width of a computer monitor with a tape measure. What is the approximate value you could get?
The width of computer monitor is usually between 10 inches and 20 inches.
We know that 1 inch = 2.54 centimeters. So, when these values are converted to centimeters, it is about 25 and 50.
Can you measure the width of the same computer in feet or meters?
Very small numbers (less than 1) are needed to express the width of the computer in feet or meters. So, it is not possible to accurately measure the width of computer monitor in feet or meters.
Therefore, the appropriate unit of measure to describe the width of a computer monitor is inches or centimeters.