I recently received a house design question from a reader asking if there was a formula for determining the proper window size (height and width) for a house. Here’s how I answered his question.

I don’t have any strict formula I use for determining window size. I depend on my eye. If you have elevations of your cabin drawn accurately to scale, simply try a few window sizes until you get ones that just look right. They should fill the wall but not overwhelm it. Windows in large rooms need to be wider than windows in small rooms. Generally, window size selection is a little like the story of the Three Bears. They shouldn’t be too big or too small. They need to be just right. If you don’t trust your instincts on this, ask for some other people’s opinions and factor those into your decision.

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Window proportions are a very subjective decision. The proportions of the wall, the style of the house, and personal preference are all important considerations.

Here are a few guidelines that might be helpful. Windows should have low enough sills so you can see out when seated. But if you get the glass too close to the floor, the Building Code may require tempered or safety glass in the windows. This will add costs. So check the local codes to be sure. They also should be tall enough so they look appropriate inside the rooms. If you have ten foot ceilings, you don’t want the top of the window to stop at seven feet off the floor. There would be too much empty space on the wall above. On the other hand, you don’t want the top of the window to be too high where it crowds or runs into the crown molding. Be sure to leave room for the window treatments you might have.

The proportions of height to width are also a subjective decision. This is often a trial and error process until things look right. If your windows will have muntins (grids) dividing the windows up into smaller panes, consider the proportions of those. Generally speaking, window panes that are taller than they are wide will look best. If the height and width are identical, they will look like they are wider than they are tall. This is a persistent optical illusion, so be aware. Once you determine the proportions of the window panes and the pattern of the window muntins or grids, try to keep these sizes and proportions consistent from window to window so your house will look cohesive and not seem like a collection of all sorts of windows.

The Building Codes require that at least one window in each sleeping room must meet certain clear opening requirements and have a sill height that’s not too high above the floor. These windows are required as a second “Means of Egress” or emergency escape, from the bedroom. This can be an important consideration when selecting window sizes.

The window sizes need to be considered from both the outside and the inside. I think that in most cases one size window will stand out as being just right and the decision will become easy.

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