The Pyramid Inch is a little longer than the standard Inch, i.e. 2,54 cm x 1,0010601216 = 2,542692708864 cm.
1 Egyptian (at the time of the building of the Great Pyramid)Cubit equals 20,6065929(52,39623 cm)Pyramid Inches.
And the calculations is:
8th root of (10 to the power of 3 multiplied by Pi to the same power multiplied by 2 to the power of 20)