I always review with customers the proper way of measuring a window. For an inside mount,you measure from inside frame to inside frame in 3 places :top, center, and bottom for the width. For the height, you measure from top of the frame to the bottom of the sill in 3 places: left, center, and right. Now often times customers believe that their measurement of frame to frame in one place is true to the frame from top to bottom; however, this is inaccurate.
Many times in older windows they can and will bow, and become narrower. With using one measurement, there may be a decrease in the width and if left unknown when the window treatment is made it will not fit into the window properly, as the narrower portion will be too small for the window treatment to fit. I always ask the customer to double check their measurements and call back in as it is a custom product and I want to prevent any mistakes.
Another important factor is measuring separately. Often people conclude that if one window is at a certain size that the rest of them are all the same. Even a brand new window can be a quarter of an inch or an inch off from a brand new window in another room. It can vary from the structure of the house as well as the condition of the window.
After remeasuring I’ve often had customers who will call back or come back in with measurements and find that they indeed did have different sizes on windows they believed were “all the same”. Measuring properly and remeasuring for confirmation is always a great idea and I always recommend doing so with any custom window treatment. It takes a little extra time but definitely saves you from the hassle in the future!