Round 19.875 to 20. 20 is the new diameter. Circumference= Pi times the square of the radius.(Pi r squared). If radius is half of diameter you must divide 20 (The diameter) by two to get the radius of 10. Multiply 10x10 to get 100. Then multiply 100x3.14 to get 314. The circumference is 314 inches. This question, however, is asking if the answer in feet. Divide 314 by 12 to get 26.1666.