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Start at the top of the frame interior and pull the tape measure from one side to the other. Next, move the tape measure down to roughly the center of the interior frame and measure from side to side again. Finally, use the tape measure to measure the bottom interior of the frame. Hold the tape measure so it’s parallel to the ground for all three measurements. After you’ve measured the interior frame’s width in the three spots, compare the measurements. The smallest number is the proper width for the blinds to ensure that they clear the sides of the frame. Pull the tape measurement from the top of the frame interior to the bottom on the left side. Next, move the tape measure over to roughly the center of the frame, and measure from top to bottom again. Finally, place the tape measure on the right side of the frame from top to bottom. Once you have the three height measurements, compare the figures. Use the smallest measurement as the blinds’ height to be sure that they’ll clear the opening from top to bottom.
Measure the width of the window in three places within the frame. Use the smallest measurement as the width. Run the tape measure from top to bottom in several spots. Use the smallest measurements as the height.