An oval looks like a circle that has been stretched out. Instead of having a constant distance from the centre to the sides, the distance varies between a minimum and maximum distance. The circumference is the area around the shape, similar to the perimeter. To find the exact circumference of an oval, you need calculus. However, you can approximate the circumference using a simpler formula.
Measure the short radius and the long radius. The short radius is the shortest straight line distance between a point on the oval and the centre of the oval; the long radius is the longest straight line distance between the centre of the oval and a point on the oval.
Double both the long radius and the short radius. For example, if your long radius is 5 and your short radius is 3, you get 10 and 6.
Square the results from Step 2. Continuing the example, you get 100 and 36.
Add the result from Step 3. Continuing the example, add 100 and 36 to get 136.
Divide the results from Step 4 by 2. Continuing the example, divide 136 by 2 to get 68.
Calculate the square root of the result from Step 5. Continuing the example, the square root of 68 is about 8.25.
Multiply the result from Step 6 by 2? to find the circumference. (Pi can be approximated as 3.14.) Continuing the example, the approximate circumference equals 25.905.