Volume & surface area (prisms, cylinders, spheres, cones, pyramids)

Forgetting to divide the sphere formula by 2 for a hemisphere, or using $r^2$ instead of $r^3$.

Finding the volume by mistake, not halving the triangle area, forgetting the slant face, or forgetting one triangular end.

Not knowing $1000\ \text{cm}^3 = 1$ litre (and $1\ \text{m}^3 = 1000$ litres); dividing by 100 or 10000.

Writing $\tfrac43\pi r^2$ instead of $\tfrac43\pi r^3$. Volume needs the CUBE of the radius: a sphere is $\tfrac43\pi r^3$ and a hemisphere $\tfrac23\pi r^3$.

Using the full $\pi r^2 h$ for a cone, forgetting the $\tfrac13$. Cone $=\tfrac13\pi r^2 h$; pyramid $=\tfrac13\times$ base area $\times$ height — a third of the matching prism or cylinder.