Paint & Walls
Baseboard & Trim Calculator
Linear feet of baseboard, casing and crown moulding, converted to standard board lengths.
Inputs
Results
Adjust the inputs to see your result.
About this calculator
Trim is bought in fixed lengths — typically 8, 12 or 16 ft — so the question is not just total linear feet but how your wall runs divide into boards with the fewest joints. This calculator deducts door and window casing spans and converts the result into a board count with a miter-cut allowance.
What this number means
Trim is bought in boards, not feet, so the honest output is a board count — and board length is a design decision as much as a maths one. Every joint in a baseboard run is a scarf cut that has to be glued, filled and sanded; on a 15 ft wall, three 8 ft boards mean two visible joints, while a single 16 ft board means none.
The 5% allowance in the formula is not cutting sloppiness — mitered outside corners consume length at 45°, inside corners are coped, and one miscut board per room is close to a law of nature. The calculator builds it in so your count includes the spare.
The formula
- Room perimeter = 2 × (Length + Width)
- Net run = Perimeter − (openings × opening width)
- Buying length = Net run × 1.05 (miter and cutting allowance)
- Boards = Buying length ÷ stock board length, rounded up
Worked example
A 12 × 15 ft living room, 12 ft boards. Two standard 3 ft doorways interrupt the run.
- Perimeter = 2 × (12 + 15) = 54 ft
- Net run = 54 − (2 × 3) = 48 ft
- With the 5% miter allowance: 48 × 1.05 = 50.4 ft
- Boards = 50.4 ÷ 12 = 4.2 → 5 boards — four to run, one spare for miters and the inevitable miscut
Assumptions and limitations
- The count treats the room as one rectangular run. Closets you plan to trim, fireplace hearths and built-ins change the run — measure those walls separately and adjust the openings.
- Crown moulding and door/window casing use the same board-count logic, but their openings differ: casing is measured per door or window (two legs plus a head), not as a perimeter run.
- Pre-primed MDF and finger-joint pine come in the same stock lengths; solid hardwood trim is often sold by the linear foot instead — the buying length in the breakdown is the figure to order by in that case.
- Outside corners consume extra length at the miter; a room with a chimney breast or bay has more than four. The 5% covers a standard room — add a board for complex shapes.
Frequently asked questions
The perimeter is 54 ft. Deduct your doorways (about 3 ft each) and add 5% for miters: with two doors that is 50.4 ft to buy, which is 5 boards of 12 ft. The calculator personalises this for your room, openings and board length.
It runs under windows without interruption, so do not deduct them. It stops at door casing, so doorways and cased openings are deducted. Built-in bookcases and fireplace hearths also interrupt the run — treat wide ones as openings.
Longer boards mean fewer joints to hide, and joints are what make DIY trim look DIY. Buy 16 ft for any wall longer than 12 ft if you can transport it; use 12 ft as the general default and 8 ft only for small rooms, closets and bathrooms.
Because miters consume length geometrically: an outside corner cut at 45° uses more board than the wall it wraps, inside corners are coped one board at a time, and one board per room typically gets cut wrong or arrives with a damaged end. Five percent is the trade allowance for exactly this.
For crown moulding, yes — it follows the full perimeter with no door deductions, so set openings to 0. For door and window casing, measure each opening separately (two side legs plus a head piece, each with a miter) rather than as a perimeter run.