HST-mkiii 18 Sd = 176.71 in^2 x 2.54 x Pi = 1,410 cm^2.
Hmm, [176.71*4/pi]^0.5 = 15" dia. x 2.54 = 38.1 cm dia.
38.1^2 x pi/4 = 1,140.062 cm^2; or if wanting to get it quick/easy like me, then recommend this nifty little program or online converter: Convert for Windows – joshmadison.com