Roofers measure, order and price by the square. A roof with 2,400 sq ft of surface is 24 squares.
Measure the real surface, not the footprint. A sloped roof has more surface than the plan view shows. Use the slope from the roof plan or the sections.
Add for waste at hips, valleys and edges. The amount depends on the roof shape and the material.
Siding is sometimes sold by the square too. Check the unit on every supplier quote so squares and square feet do not get mixed.