To calculate the square meterage of a rectangular area, multiply the height in meters by the width in meters. The resulting number represents how many square meters of geometric space are contained in an area, including all contained space. This is a multi-dimensional area calculation.
When calculating the area of a room, begin with the shapes that compose it. Note these and reconstruct the room as the sum of these shapes, such as a large rectangle with an attached square and small triangle. Measure the height and width of each of these and multiply the two figures. This will provide a set of individual square meterage figures that you can then add together to find the entire room's sum.