It will be helpful to children to use the knowledge they gain in trying out these perimeter problems to develop a way (formula) to make the calculations quickly. Thus, while all perimeters can be calculated by adding the length of all sides together, for specific shapes there are quick formulas:
For a rectangle, width + length and multiply that number x 2 is often faster than adding all of the side measurements together. Another formula that is equally correct is “Double the length of one long side and double the length of one short side. Add those two numbers together.”
For any polygon whose sides are all of equal length, it is faster to multiply the number of sides by the length of one of them.