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Volume measures how much space an object occupies. Sometimes you might hear questions like "what is the capacity of a box?" or "how much can the box hold?" You can assume that these questions will need a volume to be calculated.
Note: To be totally smart, volume and capacity aren't always the same  think of a box with really thick sides!
When your child starts working with area and perimeter he or she will usually work with 2 dimensions  squares, rectangles, triangles, etc. that are shown on paper as flat  there is no depth, or 3rd dimension. Working with volume does involve 3 dimensions. Ensure your child is aware of this and does not think of the cubes, and other 3D shapes shown on paper as just being another "shape on the page." Show them real boxes, and show how these can be drawn (or represented) on a two dimensional piece of paper. In other words, make sure the connection between what's on paper and what it represents in the real world is made.
Volume is measured in cubes (or cubic units).
How many cubes are in this rectangular prism (cuboid)?
We can count the cubes although it is quicker to take the length, width, and height and use multiplication. The rectangular prism above has an volume of 48 cubic units.
The volume of a rectangular prism is = length x width x height
We need to do two multiplications to work out the volume. We calculate the area of one face (or side) and multiply that by its height. The examples below show how there are three ways of doing this.
Area = 6 x 4 = 24 Volume = Area x 2 Volume = 24 x 2 = 48 cubic units 

Area = 6 x 2 = 12 Volume = Area x 4 Volume = 12 x 4 = 48 cubic units 

Area = 4 x 2 = 8 Volume = Area x 6 Volume = 8 x 6 = 48 cubic units 
Notice how we get the same answer no matter what side we use to find an area.
Be sure your child is not confused by the use of volume as used when talking about loudness.
There are very big differences between units of measurement for volume. For example, there are 100 centimeters in 1 meter but there are 1,000,000 (yes, 1 million) cubic centimeters in a cubic meter.
Why the big difference? Because in volume we have not just length; we have length, width, and height. The sugar cube example below shows this.
How much sugar? 1 m^{3} or 1,000,000 cm^{3}  
Think of filling a very big box (it would be 1 meter wide, 1 meter, long, and one meter high) with sugar cubes (with each side 1 centimeter).  
Step 1: one row along the bottom of the box  that would be 100 sugar cubes 

Step 2: cover the rest of the base of the box  that would give a total of 100 rows each with 100 sugar cubes. 100 x 100 = 10,000 sugar cubes at the bottom of the big box. 

Step 3: Repeat this 99 times until there are layers of 10,000 cubes stacked 100 deep. 10,000 x 100 = 1,000,000 sugar cubes 

There are 1,000,000 cm^{3} in 1 m^{3}  be careful not to have too much sugar! 
There are other units for measuring volume; cubic inches, cubic feet, cubic yards are all units used for measuring volume. Milliliters, liters, gallons are also used especially when measuring liquids.
Don't forget the wee 3 
We write cubic sizes using a small ^{3} next to the unit. We write mm^{3}, cm^{3}, m^{3} , km^{3}, cm^{3} We can say "85 centimeters cubed" or "85 cubic centimeters" 
Volume = Length x Width x Height Volume = 12 cm x 8 cm x 6 cm 

Volume = Length x Width x Height Volume = 20 m x 2 m x 2 m 

Volume = Length x Width x Height Volume = 10 m x 4 m x 5 m 
Use the worksheet below to practice calculating volumes.
You will get other geometry worksheets on perimeter, area, and more here.
Do not reinforce the belief that a person can simply not be good at math. Saying things like "I was never any good at math" just supports the misconception that a person is either born with, or without an aptitude for math and nothing can be done to change that.