Window trim & casing calculator
Add up how much casing to buy in linear feet from your opening count and sizes, with a waste allowance for miter cuts.
8 openings at 36 × 60 in is 128 linear feet of casing perimeter, or 140.8 lf with 10% for miter cuts and waste. Casing wraps the opening perimeter (all four sides, or three sides + a stool/apron); the calc uses the full perimeter as an upper bound.
Calculator inputs
Casing is the flat trim that frames a window on the inside (and sometimes the outside), and it is bought by the linear foot. The quantity is just the perimeter of every opening added up, converted to feet, with a slice added for the waste that miter cuts and short offcuts always generate. This calculator uses the full four-side perimeter as an upper bound — safe for a picture-frame casing, and comfortably generous if you run three sides with a stool and apron at the sill.
Enter how many openings share the same profile and their outside dimensions, pick a waste allowance, and you get the total linear feet to buy. Buy in the stock lengths your supplier carries and round up to whole sticks — you cannot splice a mitered return.
Formula
trim (lf) = openings × 2 × (width + height) ÷ 12 × (1 + waste)
The ×2×(w+h) is the perimeter of one opening in inches; ÷12 converts to feet; the waste factor covers miter cuts, offcuts and the odd bad stick.
Worked example
Eight openings, each 36 × 60 in, with a 10% waste allowance:
per opening = 2 × (36 + 60) ÷ 12 = 192 ÷ 12 = 16 lf
base = 8 × 16 = 128 lf
with waste = 128 × 1.10 = 140.8 lf
So buy about 141 linear feet, rounded up to whole sticks. If you run a stool-and-apron sill instead of a four-sided picture frame you will use a little less on the bottom leg — the full perimeter keeps you from coming up short.
Casing details that change the count
The perimeter math is the easy part; these decisions move the real quantity:
- Picture frame vs stool-and-apron. A picture-frame casing wraps all four sides equally. A traditional stool-and-apron sill replaces the bottom casing leg with a stool (the interior sill) and an apron beneath it — different parts, slightly less casing. The tool's four-side perimeter is a safe upper bound for either.
- Mitered returns and reveal. Casing is usually held back a small "reveal" from the jamb edge and mitered at the corners, which lengthens each leg slightly and produces offcuts — the reason for the waste allowance.
- Mixed sizes. If openings differ, run the calculator per size group and add the totals, rather than averaging — a few large picture windows can swing the number.
- Interior and exterior. If you are casing both faces, double the count or run it twice; exterior brickmould and interior casing are different profiles and priced separately.
Waste of 5–10% is the usual planning range; lean toward 10% when the profile is wide, the corners are all mitered, or the sticks are short. Always round the final figure up to the whole lengths your supplier stocks.
Reference table
| Situation | Suggested waste |
|---|---|
| Simple square profile, long sticks | 5% |
| Typical mitered picture-frame casing | 10% |
| Wide profile / many mitered returns | 10% |
Labeled planning typicals — round the final linear feet up to whole stock lengths.