To accurately calculate the size of the door, namely its width. If the opening does not exceed 90 cm, the door will need a single. In the presence of an aperture greater than 100 cm is necessary to choose a folding or sliding door, but may have to narrow the doorway.
To make an accurate calculation of the amount that would be required for the purchase of doors. Better will be if you purchased all the required number of doors at once and from one manufacturer. Doors of different shades and brands gathered in one apartment, will look weird.
To calculate the amount, you need to decide what material door you want and can buy. Doors are expensive, but durable. Doors from MDF wood cheaper. They are not afraid of wet or dry climate. The cheapest model with a honeycomb structure. They are lightweight, so they don't generate problems when transporting, but the sound insulation of such doors is poor.
Calculations for the purchase of doors to be multiplied by 30-40%. This is done in order to leave money on the trim, sills, box, accessories and all other accessories that sellers doors often do not include the cost of the door.
Before buying the doors you should carefully read all certificates for the selected products. Door "backyard" production can add to your future problems.