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The store will usually have a deal showing you where to measure or how to convert your measurements to place an order. I just did a bunch and provided them with the "rough opening" size and they all come about a 1/2" shorter or under the opening size.

And yeah probably will be a special order.

I've had to deal with the same measuring headaches before replacing my own windows, and getting the "rough opening" right was what helped avoid a lot of frustration later. I realized pretty quickly that you usually do need something special ordered if your opening doesn’t match the common sizes, so don’t be surprised if you’re looking at a 60x42 custom order. Getting those diagonal and triple-point measurements can save you big time, especially in older houses where nothing is square anymore.

If you’re thinking about ways to add privacy or energy efficiency once the replacement is in, I’ve seen how switchable window film can be added to new or existing glass. Kind of cool if you want instant privacy without extra blinds, especially for street-facing rooms.