I draw a simple cylinder radius=10, height=20. I make it into a group and show the entity info box, the volume Sketchup calculates for this cylinder is = 6211.66 mm³

Enter the data cylinder into Excel and use the V=PI x R2 x H formula and the volume computes = 6283,19

It probably has to do with the fact that SU doesn’t draw actual arcs or circles. They are segmented. If you increase the number of segments in the circle the volume will get closer to the exel number.

Thank you Shep, that did the trick. I redrew the cylinder with 1000 segments and the volume came close enough for me: 6283.14 mm³ vs 6283.19 from Excel!