How to Find Missing Measures of Perimeter Find missing measurements. So before we begin, let’s review. The perimeter is the distance around the outside of a closed figure. Now, to find the perimeter of a polygon, we add the lengths of all its sides. So to find the perimeter would equal the sum of all its sides. Now, also mentioned, when you find the perimeter, you also have to mention the unit length. So always mention the unit length. So the unit length could be inches, feet, meters, miles, etc. Let’s take a look at our first one. Here, we need to find the missing measure. So here, we know our perimeter which is 60 and we don’t know our length. Now, this given figure is a rectangle with a width of 15 centimeters and the length is missing. Now, the perimeter of the given figure is given, 60 centimeters. So we’re going to use the formula for the perimeter. So we know what the perimeter of a rectangle which we’ll say is P = 2 x (L + W) where L is the length and W is the width of the rectangle. So now, we’re just going to plug in. So P is 60, so we have 60 = 2 x (L + 15). Now, we’re going to go ahead and solve. So what we’re going to do is we’re going to divide both sides by 2. Now, 60/2 is 30, and this canceled so now we’re left with L + 15. Now, I’m going to go ahead and subtract 15 from both sides and then I get 15 = L. So 15 = L or we can say L = 15. So the missing measure is 15 centimeters. Alright, let’s move on to our second figure. So here, it’s telling us that the perimeter of this figure is 20 meters and we are missing one of the sides. So the given figure is an irregular polygon with five sides. So it’s measuring 2 meters, 2.5 meters, 5 meters, 7 meters and S which we don’t know. So the perimeter of this irregular polygon is 20 meters. So we need to find for S. So remember that the perimeter of a polygon is the sum of all its sides. So what we’re going to do is we’re going to plug everything so we know that the perimeter which is 20 meters, and then we’re going to plug in all the sides. So we’re going to have 2 + 2.5 + 5 + 7 + S, because S is something that we don’t know. So now, I’m going to go ahead and solve for S. So if I add these up, I got 16.5 + S meters. Now, I’m going to go ahead and solve for S. So I’m going to subtract 16.5 from both sides because I want to get the S by itself. So when I do that, I get 3.5 = S or S = 3.5 meters. So the missing side for this figure is 3.5 meters. Some things to keep in mind and remember are that the perimeter is the distance around the outside of a closed figure and the perimeter is the sum of all the sum. So the perimeter of a polygon is the sum of all the sides. The perimeter of a rectangle with length with width is 2 x (L + W) where L is the length and W is the width. The perimeter of a square would be with the sides of S, so it’s a square with the sides of S, it would be 4 x S. We can use formulas and equate the known and unknown lengths just like what we did in our two problems here. Remember to always name your units, name the unit of length you use to measure your shape.