When hiring a contractor to replace your windows, DO NOT MEASURE THEM YOURSELF. You can give him measurements to the nearest inch with the very clear indications that these are "approximate measurements only and not to be used for ordering the window".
WHY HAVE A CONTRACTOR TAKE HIS OWN MEASUREMENTS?
There is a lot of room for confusion on measurements, and for error. Some measure the inside of the finished frame, what you see on the wall; others measure the rough opening of the wall where the window will go; others measure the outside dimensions of the existing window -- and that could even be the wrong size if it was poorly measured to start with. If you provide the ordering measurements, you are responsible if it doesn't fit. It you leave it to the contractor to take their own measurements, they are responsible for all costs when a window just won't go in.
WHEN YOU DO WANT TO MEASURE YOURSELF
A new window is always rectangular, but the hole in the wall where you want to install it could be just about any shape and if you don't take your measurements carefully, the new window may not fit into the wall. Usually that is less of a problem when you are talking about a basement window through a concrete wall as unless it is all broken up, there is not much chance for house movement.
Start by measuring the width, but measure it all the way from the top to the bottom. That will tell you if the sides are properly parallel, or perhaps tapered, or maybe there is a bulge in the middle that you need to compensate for. Taking just one width measurement could get you a window that is going to jam into the wall. You want to note the smallest width.
Then do the same thing for the height. Measure all along from the left to the right to see if the height is the same all along. Again, take note of the smallest measurement.
Now both sides could be parallel but the whole thing tipped to one side so you need to measure both diagonals. If they are not the same, take note of both measurements. Your square window will end up with diagonals different than either of these measurement but having them noted down will allow you to reproduce the shape of the opening on a template to help you determine just what sized rectangle will fit properly into your wall.
One web visitor challenged the traditional idea that if the diagonals are equal, the hole is square. He is right; you could have a pyramid shaped window opening that had equal diagonals. If the height of both sides are equal, and the widths of the top and bottom are equal, and the diagonals are equal-- then you definitely do have a square opening.
One of the tricks for noting all the dimension details, such as for the brick and the trim pieces, is to take several photos of the area, maybe even leave your tape measure in the photo to give you a scale. Then print or photocopy large copies of the photos, return to the window and note down all the dimensions. When you have a photo rather than a sketch, nothing gets left out. Some of the little electronic notepads comes with cameras and allow you to draw right on the screen, giving you a complete dimensioned drawing right at the window. Some allow contractors to e-mail the drawings right back to the office, cutting down on time and errors.
When you install a window it is traditional to leave about half an inch all around between the window and the wall. This is for adjustment as well as insulation. So your window should be about 1 inch shorter and 1 inch narrower than the opening.
For the rest of the story on window replacement, click here.