Short answer: a frustum with bottom radius 3, top radius 1 and height 4 has volume 54.45 cubic units — 54,454 litres if those units are metres — a slant height of √20 ≈ 4.472 and a curved surface of about 56.2 square units.
Why the cylinder formula is wrong here
πr²h assumes the same radius all the way up. A tapered vessel holds less than its bottom radius suggests, and the extra Rr term in the frustum formula is exactly the correction. A 10-litre pot guessed as a cylinder can come out closer to 7.
Worked example
A bucket 30 cm across the bottom, 24 cm across the top and 35 cm tall: R = 0.15 m, r = 0.12 m, h = 0.35 m gives V = 0.0199 m³ ≈ 19.9 litres. Adding the two circular faces gives the sheet metal needed, which is why the surface areas are printed alongside.