About the Exterior Trim Calculator
The Exterior Trim Calculator converts a simple count of windows, doors, and building corners into a single linear footage figure for ordering trim boards. It is built for anyone planning to install or replace the flat trim that wraps window and door openings and runs up the corners of a house, whether that is a contractor pricing a siding job or a homeowner tackling a weekend repaint.
How It Works
You enter how many windows and doors need trim, how many building corners will get corner boards, and the wall height those corner boards must span. The calculator multiplies each window by a standard 14 linear foot trim perimeter, each door by 17 linear feet (covering both legs and the head casing on three sides), and each corner by the wall height you provide, then adds the three totals together for one combined linear footage number.
Formula & Methodology
The 14 ft and 17 ft per-opening figures are fixed averages built into the calculation: a standard window opening trimmed on all four sides works out to roughly 14 linear feet of board, and a standard door trimmed on its two legs and head (three sides, since the threshold isn't trimmed) works out to roughly 17 linear feet. Corner trim is handled differently since it isn't tied to a fixed opening size - it simply runs the full height of the wall at each corner, so that term is corners multiplied directly by the wall height field.
Examples
Typical Single-Story Home
With the defaults of 8 windows, 2 doors, 4 corners, and a 9 ft wall height, the calculator returns 112 ft of window trim (8 x 14), 34 ft of door trim (2 x 17), and 36 ft of corner trim (4 x 9), for a total of 182 linear feet.
Larger Two-Story Layout
For a home with 12 windows, 3 doors, 6 corners, and 10 ft walls, the totals become 168 ft for windows, 51 ft for doors, and 60 ft for corners, adding up to 279 linear feet of trim board needed.
Advantages
- Combines three separate trim categories, windows, doors, and corners, into one ordering total instead of three manual calculations.
- Lets a single wall height input adjust just the corner board portion, useful on homes with unusually tall or short walls.
- Gives a quick linear footage figure that translates directly into board counts once a specific board length is chosen.
Common Mistakes
- Leaving the wall height at its 9 ft default when the actual corner run is taller or shorter, which skews only the corner portion of the total.
- Applying the flat 14 ft or 17 ft averages to non-standard openings like bay windows, sliding doors, or oversized entries without adjusting for their larger perimeters.
- Ordering exactly the calculated total with no waste allowance, then coming up short after miter cuts and offcuts that can't be reused.
Edge Cases to Watch For
- Window and door totals use flat per-unit averages, so oversized picture windows, French doors, or unusually tall openings need manual adjustment rather than the calculator's 14 ft and 17 ft defaults.
- The wall height field only affects corner board length and defaults to 9 ft, so it should be updated to match your actual eave height or the corner totals will be off.
- No waste factor is built into this calculator's total, so cut losses from miters and board-length mismatches need to be added separately before ordering material.
Common Use Cases
- Contractors preparing a material list and rough quote for an exterior trim replacement or new siding installation.
- Homeowners planning a DIY trim repair or repaint project who need to know how many boards to buy.
- Lumber yards or suppliers estimating order size for a trim package based on a customer's opening counts.