How it works
The calculator lists every piece of casing, adds the miter and reveal to each one, then fits the pieces into the stock length you buy.
Door leg = Height + Reveal + Casing width + Extra
Door head = Width + 2 × Reveal + 2 × Casing width + Extra
Window leg = Height + 2 × Reveal + 2 × Casing width + Extra (with a stool: Height + Reveal + Casing width + Extra)
Window head and bottom = Width + 2 × Reveal + 2 × Casing width + Extra
Total = Σ pieces · Stock pieces = pieces fitted longest first into stock lengths, no joint inside a piece
Each door side gets two legs and a head, so a door cased on both sides has four legs and two heads. A window with picture-frame casing has two legs, a head and a bottom piece; a window on a stool has two legs and a head.
Worked example in words
Two 36 × 80 in doors are cased on both sides with 2 1/4 in casing and no reveal. Each leg is 80 + 2 1/4 = 82 1/4 in and each head 36 + 2 × 2 1/4 = 40 1/2 in, so the doors need eight legs and four heads. Two 36 × 48 in windows with picture-frame casing each need two legs of 48 + 4 1/2 = 52 1/2 in and a head and bottom of 40 1/2 in. That is 20 pieces and 1,192 in, or about 99 1/3 ft of casing.
In 7 ft (84 in) stock, every 82 1/4 in leg takes its own piece. Each 52 1/2 in window leg also takes a piece, and its 31 1/2 in offcut is too short for a 40 1/2 in head. The eight 40 1/2 in heads and bottoms fit two to a piece. That makes 8 + 4 + 4 = 16 pieces – more than the 99 1/3 ft ÷ 7 ft = 14.2 that total length alone would suggest.
Why the piece count is higher than total length suggests
Casing is bought in fixed lengths, and a leg or head should be one unbroken piece. Short offcuts – the ends left after cutting a long leg – are often too short for the next piece. The calculator therefore fits pieces into stock lengths one by one, longest first, and only reuses an offcut when a whole piece fits in it. Saw kerf is ignored, so if a piece fills a stock length almost exactly, check it against the real board.
Measuring openings
Measure where the casing will start: the edge of the jamb for doors, and the jamb or extension jamb for windows. If you set the casing back to show a reveal, enter it – a common look leaves a narrow strip of the jamb edge visible, but there is no fixed size. Openings of different sizes can be worked out one size at a time.
Worked example
Inputs
- Doors
- 2
- Door opening width
- 36 in
- Door opening height
- 80 in
- Cased sides per door
- Both sides
- Windows
- 2
- Window opening width
- 36 in
- Window opening height
- 48 in
- Window trim style
- Casing on four sides (picture frame)
- Casing face width
- 2 1/4 in
- Reveal
- 0 in
- Extra length per piece
- 0 in
- Stock length
- 7 ft
Result
- Casing pieces to cut
- 20
- Longest piece
- 82.25 in (6.85 ft)
- Total casing length
- 99.33 ft
- Stock pieces to buy
- 16
Common mistakes
- Measuring the door instead of the opening. Casing is sized to the jamb opening; the door slab is smaller.
- Forgetting the miter. A head piece with two 45° miters is longer than the opening by twice the casing face width, plus the reveal on each side.
- Counting total length only. A leg cannot be spliced from two short offcuts without a visible joint, so every side needs one piece long enough – check that the longest piece fits your stock length.
- Forgetting the second side of interior doors. Most interior doors are cased on both faces of the wall.
- Leaving out the stool and apron on windows trimmed with a stool. They are separate parts and are bought separately.
Assumptions and limits
- Openings of one size are entered together; for different sizes, run the calculator once per size and add the stock pieces up.
- Door legs are cut square at the floor and mitered at the top; door heads and window heads and bottoms are mitered at both ends at 45°.
- With a stool, the window legs stand on the stool and are mitered only at the top.
- Each mitered end adds one casing face width; the reveal is added on each side of the opening that the piece spans.
- Pieces are fitted into stock lengths longest first with no joint inside a piece. Saw kerf, defects and the grain or color match between pieces are not considered.
- Plinth blocks, rosettes, back bands, stools, aprons, nails and caulk are not included.
Frequently asked questions
How do I calculate how much door casing I need?
For each side of a door you need two legs and one head. A leg is the opening height plus the reveal and one casing width for the miter at the top; the head is the opening width plus two reveals and two casing widths. For a 36 × 80 in opening and 2 1/4 in casing that is two 82 1/4 in legs and a 40 1/2 in head – about 17 ft per side.
How much longer than the opening is a mitered casing piece?
At a 45° miter the long point of the piece sticks out by the face width of the casing. A piece mitered at both ends is therefore two face widths longer than the opening, plus the reveal on each side.
Will one 7 ft piece cover a door leg?
Only if the leg is no longer than 84 in. An 80 in opening with 2 1/4 in casing needs 82 1/4 in, which fits; with wider casing or a reveal the margin shrinks. The calculator stops with a message if your longest piece is longer than the stock length.
Can I join two pieces of casing on one side?
You can with a scarf joint, but it shows on a short leg or head. This calculator assumes each side is one piece and fits only whole pieces into each stock length.
How do I trim a window with a stool?
Choose “casing on three sides with a stool”. The calculator then counts two legs that stand on the stool and one mitered head per window. Buy the stool and the apron under it separately.
Sources
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026