Convert area units between units, using exact SI factors.
One hectare is 10,000 square metres — a plot of 100 × 100 metres. An are is a hundredth of a hectare, or 100 square metres. The BIPM SI Brochure accepts the hectare for use with SI, although the SI unit of area is the square metre.
Because area units scale by the square, not linearly. A metre is 100 centimetres, and 100 squared is 10,000. The same holds for volume, where the factor is cubed. This is the most common error when converting areas into SI units to order materials.
An acre is 4,046.86 square metres, roughly 0.4 hectares. Historically it was the area one person with an ox team could plough in a day. The acre is not an SI unit and does not appear in the BIPM brochure, but it still governs land records across the English-speaking world.
Divide the room into rectangles and triangles, work out each area separately and add them. Subtract alcoves and projections. When ordering materials, add 5–10% to the resulting SI square metres for cuts and breakage.