About the Window Casing Calculator
Interior window casing is ordered by the linear foot, but the amount actually needed always exceeds a window's raw opening perimeter because of the extra length lost at each mitered corner. This calculator estimates total casing footage across multiple windows at once, building that miter allowance directly into the result.
How It Works
Enter the number of windows being trimmed along with their average width and height in inches. The calculator converts each dimension to feet, sums two widths and two heights for the raw perimeter, then adds 15 percent to account for the extra length each of the four 45-degree miter corners consumes. That per-window figure is then multiplied by the window count for a project total.
Formula & Methodology
By hand, start from the window's opening dimensions in inches, convert each to feet by dividing by 12, then add two widths and two heights the way you would for any rectangle's perimeter. The 15 percent addition isn't an arbitrary safety margin - it approximates the extra length each casing leg needs so its mitered end still reaches the full corner after the 45-degree angle is cut away, repeated across all four corners of the opening.
Examples
8 windows, 36 in x 48 in
At the default 8 windows sized 36 by 48 inches, or 3 by 4 feet, raw perimeter per window is 2(3) plus 2(4), or 14 feet, adjusted to 14 times 1.15, or 16.1 feet, for a project total of 8 times 16.1, which comes to 128.8 linear feet.
5 windows, 24 in x 36 in
For 5 smaller windows sized 24 by 36 inches, or 2 by 3 feet, perimeter is 2(2) plus 2(3), or 10 feet, adjusted to 11.5 feet, giving a total of 5 times 11.5, or 57.5 linear feet.
Advantages
- Bakes the miter allowance in automatically, so the footage total already reflects real cutting waste instead of just the flat window perimeter.
- Scales instantly across an entire house's worth of windows once an average opening size is known, without adding up each window by hand.
- Makes it easy to compare material needs between different window sizes or counts before finalizing a trim order.
Common Mistakes
- Ordering exactly the raw perimeter total and running short once actual mitered pieces are cut from it.
- Using one average width and height across windows that vary considerably in size, which skews the total away from what the largest or smallest openings really need.
- Forgetting that casing is typically sold in fixed-length sticks, so the calculated linear footage still has to be rounded up to whole sticks when placing an order.
Edge Cases to Watch For
- The 15 percent miter allowance is a flat estimate that doesn't change with casing width, even though a wider trim profile technically loses slightly more length per miter cut than a narrow one.
- Windows of noticeably different sizes should be run through the calculator in separate batches rather than averaged into one width and height, since averaging can understate what the largest openings need and overstate what the smallest ones need.
- The formula covers a standard four-sided rectangular casing only, with no stool or apron trim included separately.
Common Use Cases
- Trim carpenters or DIYers estimating material for a whole-house window casing installation before ordering stock.
- Homeowners pricing out a trim upgrade project before requesting quotes from contractors.
- Remodelers standardizing casing material orders across multiple rooms with similarly sized window openings.