Area formula
A rectangle is length × width. Add every room. 12 ft × 10 ft is 120 sq ft. In metres, 3.6 m × 3 m is 10.8 m². Closets and hallways count if you are buying flooring for them.
Odd rooms: split into rectangles and add them as extra rows. For paint or flooring, many stores tell you to add 10% waste. This page reports raw area only.
Worked example
A room 12 ft by 14 ft is 168 sq ft. Two of those plus a 10×10 is 436 sq ft. In metres, 4.0 m × 3.5 m is 14 m².
Closets and bump-outs are extra rectangles. For paint or flooring, add waste (often 10%) rather than assuming the floor plan is the order quantity.
Common questions
How do I do a triangle?
Area = base × height ÷ 2. Enter that area as length = the area and width = 1, or split the room.
Square metres to square feet?
1 m² ≈ 10.764 sq ft. This page totals in the unit you picked; it does not mix units in one run.