Short answer: a cube of side 2 has a volume of 8 cubic units and a surface area of 24. A 1 × 2 × 3 box is 6, a cylinder of radius 1 and height 2 is 2π ≈ 6.28318531, and a sphere of radius 1 is 4.18879020.
Square units in, cubic units out
The shapes here are the ones with a closed formula. Anything irregular needs either a displacement measurement or a mesh, and this page does not pretend to integrate an arbitrary shape — the honest answer for a bucket that is not a cylinder is to measure the water.
Worked example
A cylindrical water tank with a 0.75 m radius and 1.2 m height: V = π × 0.5625 × 1.2 = 2.1206 m³, which is 2,120.6 litres. The surface area, 2π×0.75×(0.75+1.2) = 9.19 m², is what you need for the paint.