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Measure the window openings from outside, not inside. A common mistake people make is to measure the windows inside for the replacements, but for glass block, you'll need to be outside. Since the windows featured here are built right into the foundation, the full width of the masonry opening is measured (as if the frame were already removed).
Measure the height of the opening from the top of the foundation wall to the wood or steel support, making sure the height includes the sill thickness, since the sill will be removed.
Tip: To see if a sill is removable, tap it. If it sounds hollow, it should come off easily. If a sill is permanent, just measure the height from the top of it.
The next step is to measure each remaining window, since the dimensions of many basement windows are different.
Once all measurements are verified and recorded, they're ready to take to a glass-block vendor. The retailer will help make sure you order the proper size — and remember to let them know if the windows in question will require any kind of vent openings.