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Window trim calculator
Put your window count, window width, window height, piece length and waste in: 6 windows at 36 in by 48 in need 1008 in of trim — 11 pieces exact, 13 pieces with 15% waste.
★ CONSTRUCTION · WINDOWS · TRIM
Window Trim Calculator: 13 Pieces for 6 Windows, 48 in
On the defaults — 6 windows, 36 in by 48 in, 8 ft pieces, 15% waste — you need 13 pieces of window trim. This is the window trim tool: count the windows once, allow for the mitres, and order the pieces once — a half-trimmed room waits for no second delivery.
What does window trim needed 13 pieces mean?
It means thirteen pieces trim six windows once cutting waste is allowed for. Each window needs two side pieces of 48 inches, one top piece of 36 inches and one sill piece of 36 inches, which is 168 inches per window; six windows need 1008 inches, or 84 feet, of trim. With 15% waste that becomes 1159.2 inches, and at 8 feet (96 inches) per piece the exact count is 11 pieces, rounded up to 13 whole pieces because pieces are sold by the piece, not by the fraction. The waste pieces are not spare stock exactly — they are the pieces the job consumes in mitre cuts and breakage, so a house trimmed without waste runs short at the last window.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Trim per window | 2 × (width + height) | 168 in — 2 × (36 + 48). |
| Total trim | windows × trim per window | 1008 in — 6 × 168, or 84 ft. |
| Pieces | total trim with waste ÷ piece length, rounded up | 13 pieces — 1159.2 ÷ 96, rounded up. |
Three things to check before you trust the number
First, check the piece length you are actually buying: 8 ft is the standard stock length, but a 12 ft piece cuts this job to 9 pieces with waste, and fewer joints too. Second, remember the count follows the windows, not the rooms: six windows need six sets of trim even in three rooms, and a window with no trim at all still needs a full set if it is cased. Third, judge the waste honestly: trim is cut at angles, so 15% fits a house with standard windows, but windows with sills that run long or with picture rails can want 20%, since every extra corner consumes trim. Window sashes and glass are counted separately — see the door trim calculator for the matching door trim.
Worked example
6 windows, 36 in wide, 48 in high, 8 ft pieces, 15% waste. Trim per window: 2 × (36 + 48) = 168 in. Total trim: 6 × 168 = 1008 in, or 84 ft. With waste: 1008 × 1.15 = 1159.2 in. Pieces: 1159.2 ÷ 96 = 12.075, rounded up to 13 pieces. In practice that means buying 13 pieces, and expecting the last pieces to go on the mitres and the breakage. If the same windows used 12 ft pieces instead, the need would fall to 9 pieces with waste, but the windows trimmed would be identical.
Every branch, checked
With 8 windows instead of 6, the trim rises to 1344 in and the need to 17 pieces with waste. With 60 in windows, the trim rises to 1152 in and the need to 14 pieces with waste, since height moves the trim twice per window. With 12 ft pieces, the trim stays 1008 in but the need falls to 9 pieces with waste. With 0% waste, the trim stays 1008 in and the need falls to 11 pieces. Window count, window height, piece length and waste each move the count on their own, so a wrong piece length is as costly as a wrong window count.
The reverses, worked backwards
The window count behind 13 pieces is 6: at 4 windows the need is 9 pieces (672 in trim, 56 ft, 7 pieces exact, 8 ft pieces, 15% waste); at 8 windows it is 17 pieces (1344 in trim, 112 ft, 14 pieces exact, 8 ft pieces, 15% waste). The piece length behind 13 pieces is 8 ft: at 6 ft the need is 17 pieces (1008 in trim, 84 ft, 14 pieces exact, 6 ft pieces, 15% waste); at 12 ft it is 9 pieces (1008 in trim, 84 ft, 7 pieces exact, 12 ft pieces, 15% waste). The waste behind 13 pieces is 15%: at 0% the need is 11 pieces (1008 in trim, 84 ft, 11 pieces exact, 8 ft pieces, 0% waste); at 30% it is 14 pieces (1008 in trim, 84 ft, 11 pieces exact, 8 ft pieces, 30% waste). Each reverse moves one input and holds the rest, so the step is exactly one window, piece or waste change.
What the defaults assume
The defaults assume 6 standard windows, 36 in wide and 48 in high, 8 ft trim pieces, and 15% mitre and breakage waste. They do not assume window sashes, glass, jambs or fasteners — a window that needs a new jamb counts its jamb separately, and the fasteners are bought by the box, not by the piece.
Who uses this
- A homeowner trimming six windows: the 13-piece order settles the lumber-yard call before the first piece is cut.
- A DIY builder finishing a living room: the same maths on the windows alone gives the pieces, not a whole house.
- A contractor quoting a job: window count, window size, piece length, waste — the quote follows the same steps, and the waste line shows the client why.
- A landlord budgeting a unit refit: the window count off the plans, multiplied out, gives the piece line on the refit budget.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the piece length and the waste allowance. Real windows vary in width and height, and the piece on the shelf overrides any planning figure; a window count that misses an uncased opening under-orders, and a piece length read off a different product over-orders. Treat 13 pieces as the planning order for 6 windows at 36 in by 48 in at 8 ft pieces, and raise the waste to 20% for windows with long sills or picture rails.
Cite this calculator
Bright Side Kit, “Window Trim Calculator: 13 Pieces for 6 Windows, 48 in”, https://brightoolbox.com/window-trim-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (6 windows, 36 in × 48 in, 8 ft pieces, 15% waste) so the 13-piece result can be checked.
Frequently asked questions
How many trim pieces do I need for 6 windows?
13 pieces at 8 ft per piece with 15% waste. The trim is 1008 in; with waste that is 1159.2 in, and 1159.2 ÷ 96 = 12.075, rounded up to 13.
How do I work out window trim from a window count?
Two sides plus a top and a sill per window: 2 × (width + height), times the window count, plus waste, divided by the piece length, rounded up. Six 36 in by 48 in windows need 1008 in, or 13 pieces at 8 ft with 15% waste.
How much waste should I allow for window trim?
15% for standard windows, 20% for windows with long sills or picture rails. Waste is the mitre cuts and breakage, not spare stock.
Do I count a window that already has trim?
No — count only the windows you are trimming. A window with sound trim already in place needs no new pieces, and counting it over-orders.
Does this count the window jamb too?
No — this calculator counts casing trim only. Jambs, sashes and glass are counted separately, by the window.
Should I buy the exact pieces or round up?
Round up: pieces are sold by the piece, and the spare pieces are the mitre stock. A house trimmed to the exact piece leaves nothing for a miscut corner.