To get correctly sized exterior shutters or storm windows, you have to provide the manufacturer or dealer with a precise set of window measurements. The way to do that is to take each opening's measure at several points—old windows are seldom square. Use a folding rule with a slide extension; it's much more accurate for finding interior dimensions than a tape measure. Here's what to do.
1. Unfold the rule to the approximate width of the opening and set the folded end against the inside edge of the casing, as shown. Slide out the extension to touch the opposite casing's inside edge. Add the extension's measure to the number at the other end of the rule.
2. Measure the opening horizontally at the top, middle, and bottom, and vertically at the sides and in the middle. Use the smallest of the three measurements to ensure that the storm window or shutters won't be too big to fit in the opening. When you place an order, emphasize that you are providing actual measurements; the manufacturer will then know to take into account any clearances that might be needed to fit a unit into your opening. That way, there won't be any surprises on installation day.
To check whether an opening is square, measure diagonally from each top corner to the opposite corner on the bottom. If the two measures are different, then the opening is racked, a fact that the window or shutter maker should be made aware of.