Volume and surface area
Volume fills a solid and uses cubed units; surface area covers its outside and uses squared units.
A worked example
A cylinder of radius 6 cm and height 10 cm has volume π x 6² x 10 = 1 131 cm³.
The mistake people actually make
If the question gives a diameter, halve it before squaring. A cone holds one third of the cylinder with the same base and height.