To calculate the area of a room is rectangular, you need to multiply its length and width. The values of width and length shall be expressed in meters. In this case, the value of the area of the room would make in square meters.
If the room has additional niches or, conversely, coming into the room tabs on plumbing constructions and other premises, the area of such parcel shall be measured separately and added (or subtracted) to the area of the room.
If the room is not square, you can break it into rectangles and folded the square of each of them
To measure the length and width of the room is a conventional or electronic roulette (electronic rangefinder). For more accurate measurement results it is advisable to make two measurements - at the beginning of the wall and the end wall. If the measurements are slightly different, the arithmetic mean – both numbers are added up and split in half.
When using an electronic (laser) roulette you should pay special attention to the direction of the beam was exactly perpendicular to the wall. Even a small deviation of 5-10 degrees can "zoom" the area of a room per square meter.
If the room area is calculated for installation of infrared heating film, of the results obtained is subtracted the area of the "heavy" furniture – cabinets, walls, sofas, beds (if they are not on legs).
If the room has a more complex form, i.e. not broken into rectangles, the more difficult sections can be divided into triangles and sectors of a circle. To calculate the area of triangular sections is easier to use Heron's formula:

S=√(R(R-a)*(R-b)*(p-C)), where R - properiter triangle, p=(a+b+C)/2, a, b, C be the lengths of its sides.

To calculate the area of sectors formula area of a circle. (PR Kvadrat), how many degrees in the sector.