Window Size Calculator

The rough opening is the framed opening in the wall where the window will be installed, while the unit size is the actual window dimensions. The rough opening is typically larger to allow for shimming and adjustment during installation.

Standard shim space is typically 1/2 inch on all sides, but can range from 1/4 to 3/4 inch depending on construction tolerances and installation requirements. This space allows for proper leveling and squaring of the window.

Sealant gap is the space left between the window frame and surrounding materials for applying weatherproof sealant. Typically 1/4 to 3/8 inch, this gap ensures proper water and air sealing around the window perimeter.

Exterior casing is used for aesthetic purposes and to cover the gap between the window frame and siding. It's commonly used with wood siding, stucco, or brick applications where a finished appearance is desired.

For replacement windows, measure the existing rough opening width and height at three points (top, middle, bottom for width; left, center, right for height). Use the smallest measurement and subtract 1/2 inch for shim space.

Sill nosing is the protruding edge of the window sill that extends beyond the wall surface. It helps direct water away from the building and is typically required for proper water management, especially in areas with significant rainfall.

Yes, windows can be installed in oversized openings by using additional framing lumber to reduce the opening size, or by using larger trim pieces. However, there are practical limits to how much oversizing can be accommodated.

You'll need a quality tape measure, level, and square. For precise measurements, a digital caliper can be helpful for checking existing window dimensions. Always measure twice and record measurements to the nearest 1/8 inch.