who knows the capicity of 1/2 inch pex pipe per 100ft?

Do you want exact? Cubic inches or gallons?

Since the ID of 1/2 PEX is not really 1/2 inch I figured it using a more exact number which is around 15/32 diameter.

206.98 cubic inches or .896 gallons is real close for 100 feet of 1/2 pex

pi x r squared x length in inches = cubic inches
231 cubic inches = a gallon

very nice