Round pen sizing basics
Omni's round pen calculator is centered on one main question: once you know the desired pen size and your panel width, how many panels do you need for the enclosure?
Formula from the fetched source
P = D x pi / L
C = D x pi
Omni also notes typical panel lengths from 4 ft to 20 ft and recommends a diameter of at least 60 ft for horses.
Examples mirrored from Omni
The article shows that an 80 ft pen has a circumference of about 251.3 ft. With 8 ft panels, that corresponds to about 31.4 panels, so the practical layout rounds to 31 panels.