Position the pumpkin on a flat surface so that it is resting on the blossom or bottom end of the pumpkin.
Measure the circumference around the widest part of the pumpkin using the tape measure. The circumference or ring formed by closing the tape measure around the pumpkin should be approximately parallel to the flat surface. Remove the tape measure and repeat the measurement two times. Calculate the average circumference by adding the three measurements and dividing by 3. For example, if the circumference measures 40.0 cm, 39.5 cm, and 40.5 cm upon taking three different readings, the average circumference is 40.0 cm = (40.0 + 39.5 + 40.5)/3 = (120/3).
Measure the diameter of the pumpkin. To measure the diameter, place the two pieces of cardboard on opposite sides of the pumpkin. The cardboard pieces should be held upright so that they are at 90 degrees to the flat surface. The two pieces of cardboard should be parallel to each other to obtain an accurate diameter measurement. Use the ruler to measure across the top of the pumpkin from one card to the other. Move the cardboard pieces clockwise through 90 degrees and repeat the diameter measurement. Find the average of the two measurements by adding them and then dividing by 2. The average measurement is the diameter of the pumpkin. For example, if the first measurement is 12.8 cm and the second measurement is 12.6 cm, then the average diameter of the pumpkin is 12.7 cm = (12.8 + 12.6)/2 = (25.4/2).
Divide the circumference of the pumpkin by its diameter. For example, dividing a 40 cm circumference of a pumpkin by the pumpkin's diameter of 12.7 cm yields a ratio of (40/12.7) = 3.15.