calculate the volume of hip roof

Hi, this is very simple job (obviously if you know how unlike me), i need to calculate the volume of a hip roof. I have attached architectural drawings which has measurements. The drawing show a hip to gable loft conversion, bottom right hand corner shows calculations for volume of hip to gable roof extension and volume of rear dormer. The bottom image in red shows the shape of the 2storey side/rear extension roof, I have split the roof in sections, and can calculate volume of roof slope A, B, C, and centre flat roof section, just need the formula to work out volume of the three hips but not sure what shape they are. I0