The complexity of the measurement Windows is that in most cases, to perform measurements without need of dismantling of the old window.

a Measurement window in a residential building without the "quarters"

For those who don't know what "a quarter", briefly explain that this term refers to the ledges on three sides of the window opening (top, left and right).
If the window openings in your home does not have "quarters" to determine the size of the future window will not make special work.
Measure the window width, subtract it from 4 to 8 cm, leaving a clearance of 2 to 4 cm on each side.
After measuring the height, from the values obtained taking 6 cm, leaving 3 cm on clearance, and 3 cm – wildcard profile.
For example, the width of the window opening in the house without the "quarters" is 120 cm and the height is 140 cm width and subtract 8cm (will take on a maximum) and height of 6 cm After performing the simplest of arithmetic operations the size of the future window should be 112 x 134 cm

Implementation of measurements in the house with "quarter"

In this case, you will need to measure the window opening inside and outside.
Start with measuring the window width (A) between the side walls of the quarters. Since entry window for exterior soffits is 1.5 to 2.5 cm on each side, to the received value should be added from 3 to 5 cm, respectively. This will be the desired width of future Windows. Let's denote it by the letter S. in Order to check the correctness of the calculations, you will need to measure the window from inside the room. We are interested in the width of the window between the inner slopes (B) and the width of the sill (In). If previously calculated window width more than B but less than B, so all measurements and calculations are done correctly.
Now go to the top. Measuring the distance (D) between the lower wall and the upper quarter of the window opening, first subtract 2 cm on the mounting gap (foam), and then that window came in the top quarter, adding between 1.5 and 2.5 cm (depending on the size of a quarter), and then minus 3 cm for the stand profile.Measuring the distance between the sill and the top wall, and knowing the thickness of the sill, you can check the accuracy of the measurements. The height of the future window should not exceed the value of D.
If the window openings in your home are of irregular shape, and the thickness of the window aperture in different places has different meanings, in this case it is better not to engage in independent measurements, and to use the services of a professional measurer.
Good luck to you! Let you will succeed!