The perimeter is 20 in, and the area is 30.777 in sq. To get the perimeter, you just need to multiply the decagon side length by the number of sides, i.e., 10 × 2 in = 20 in. The area formula is more complicated, but approximately we have Area ≈ 7.694 × a², so, in our case, Area ≈ 7.694 ×4 in sq ≈ 30.777 in sq. Read more