How to use the trim and molding calculator
Type each room's length and width, then add its doors and windows with their wall number, width and height. Walls are numbered 1 to 4 clockwise, starting with the top wall of the picture. Pick the trim you are putting in: baseboard, door and window casing, chair rail, picture rail, or wainscoting and shiplap. Choose the board lengths your store sells. The page works out every run, cuts the runs around the doors, packs the pieces onto boards so as little as possible is left over, and tells you which boards to buy. The Cut list tab shows every board and what comes off it. If a wall is covered by cabinets, tick it under No wall trim on, and the pieces on the next walls end there with a square cut. Nothing you type leaves your device unless you make a share link.
How it is figured
head casing, long point = opening width + 2 × reveal + 2 × casing width
leg casing, long point = opening height + reveal + casing width
Baseboard stops where it meets the door casing, so each door takes its opening plus the casing and reveal on both sides out of the wall. A wall with a door becomes two stretches, and each stretch is its own piece. Runs longer than your longest board are split into equal pieces joined with a scarf joint. For the cut list, the pieces of one profile are sorted longest first, and each goes on the first board with room for it and the saw cut. The page tries each board length you picked, drops every board to the shortest length that holds its pieces, and keeps the plan that buys the fewest feet of board. It is a good plan, not a proven best one.
A worked example
A 14 by 12 foot room has a 32 by 80 inch door on wall 3 and two 36 by 48 inch windows, with 2 1/4 inch casing and a 3/16 inch reveal. Each side of the door takes 2 7/16 inches for casing and reveal, so the door takes 36 7/8 inches out of the 14 foot wall, leaving two stretches of 5 feet 5 9/16 inches. Baseboard is then 14 + 12 + 12 feet plus those two stretches, 48.93 linear feet in five pieces. The page buys one 16 foot and three 12 foot boards, 52 feet for 48.93 feet of baseboard. The casing comes to 48.06 feet in eleven pieces. That example is what the calculator shows when it opens, with 10 percent more boards added for mistakes.
Casing a door or window
Casing sits back from the jamb edge by a reveal, so the jamb shows as a thin line. Fine Homebuilding's mitered-casing project uses a 3/16 inch reveal, and the page starts there; you can change it. On mitered casing, the long point of each cut is the outside corner and the short point sits on the reveal line corner. A leg is cut with its long point at the top, so it is longer than the opening by the reveal and the casing width; the head is longer than the opening by two reveals and two casing widths. The Casing tab gives the long and short length of every piece for one door or window and where to mark each. The tape measure page shows how to read the marks.
Inside and outside corners
Most finish carpenters cope inside corners and miter outside corners. A coped joint follows the profile of the piece it meets and is less likely to open when the wood moves. An outside corner is two miters, each half the angle of the corner: 45 degrees on a square corner. The Corners tab gives the saw setting for any wall angle, and how far an outside miter runs past the wall's corner. For a long wall, join two boards with opposite 45 degree cuts over a stud, away from the middle of the wall and out of the eye line.
Where the numbers come from
The 3/16 inch reveal comes from Fine Homebuilding's mitered-casing project. The cope-and-miter practice, the scarf joint and the 10 percent for waste come from This Old House's baseboard guide. Chair rail height starts at 32 inches, about a third of an 8 foot wall, from Metrie's guide; the Journal of Light Construction's Katz gives 26 to 32 inches for classic rooms. Baseboard sizes (3 1/4 inch tall, 8 and 12 foot boards) are from Lowe's product pages; colonial casing at 2 1/4 and 3 1/4 inches is from dimensions.com. No building code sets any of these sizes: it is all design and habit. The page does not allow for the thickness of the trim at coped joints or the saw cut unless you type one.
What people use it for
- Baseboard, casing and chair rail for one room or a whole floor.
- Shiplap or tongue and groove wainscoting, by board and by wall.
- Picture rail in an older house.
- Planning the cuts so the offcuts are long enough to use.
- Checking an estimate from a contractor.
Common mistakes
- Not taking out the door casing. Baseboard ends at the casing, not at the jamb.
- Forgetting the reveal. It adds about 3/8 inch to a head casing.
- Measuring corner to corner at the floor only. Walls are not always straight. Cut a little long and trim to fit.
- Splicing in the middle of a wall. Put the joint on a stud, away from the center.
- Buying exactly what the list says. Add a spare board; the page does it at 10 percent until you change it.
Questions
How much baseboard do I need?
Add the walls, take out each door's opening and the casing on both sides, and add a spare for mistakes. The page does this per wall and packs the pieces onto boards.
Does it include crown molding?
No. Crown has compound cuts, so it has its own page: the crown molding angle calculator.
Should I cope or miter?
Cope the inside corners and miter the outside ones. The Corners tab shows both.
Does a building code say how big trim must be?
No. Trim sizes and heights are design choices. The page names no code.
Can I use metric?
Yes. Press Metric to type meters, centimeters and millimeters. Board lengths still show the common 8, 10, 12 and 16 foot sizes with the meters beside them. The page opens in the system your browser's region uses and remembers your choice.
Can I share my result?
Yes. Share makes a link with a picture, a link you can copy, and a short video. The link keeps the rooms and trim choices for 90 days, never a price.