If you don't have a class set of pentominoes, you can print out a set from Scholastic HERE. This version includes square units, which would be great for differentiation, too. Otherwise, I would recommend a plastic class set of them from Amazon or the teacher store because they are wonderful and have many applications!
And a little disclaimer before I post the lesson... have mercy on my poor quality pictures! These were taken on the fly before I erased it for the next topic. ;)
We started out with this stair step piece because I think it really clearly shows how you can use one measure to figure out others. First, we determined that the three measures on top (the ones I have highlighted in blue) were the same. Using that information, we could determine the measure of the bottom two lines. This clicked INSTANTLY for most kids!
We only did ONE example (above) together before I sent them off to begin looking at the shapes in their pack. I wanted them to really get into the activity and make the discoveries for themselves. They traced the pentominoes on graph paper and used the units on the graph paper as well as measuring them to find actual measurements.
For kids who quickly grasped the concept, I had them do some exploration with area as well. In the example the student is working on above, once she figured out the area of one of the squares in her cross, she could easily figure out the area of the entire shape.
Before they left, they had to answer the question above as an exit ticket. I wish I had taken pictures of some of their answers, because they were wonderful!
Not only was this a great lesson, but when I do it again, I will do it as part of their interactive notebook and make the exit ticket their reflection piece at the end. This would have been very valuable for them to refer back to!
If you are looking for more perimeter and area ideas, I have a HUGE blog post on it HERE!