easy way to measure casement windows

Just measure the inside of the window jamb (what the window trim is attached to) to correctly size your insert. Some replacement windows come with an exterior perimeter foam strip or pad and require about a 1/4-inch deduction from your tight of exact measurements for proper compression of the foam while still being easy to caulk over.

- But, most other replacement windows require a 1/2-inch deduction to allow for shims and spray foam application. Measure in 3 places in both directions to ensure you're Deduction Measurement won't be pinched anywhere within the old jamb...pretty impossible for a window to be off the same or more than your deduction, but always make sure you'll be clear.