Crown Molding Length Calculator
Crown molding finishes a room where the wall meets the ceiling, and because it runs continuously around the top of the room, working out how much to buy comes down to the perimeter plus a margin for cutting. This calculator does both. Enter the room length and width and a waste allowance as a percentage, and the tool adds twice the length and twice the width to find the perimeter, then increases it by the waste allowance to give the linear feet of crown molding you should order. Unlike baseboard, you usually do not subtract for doors and windows, because crown sits at the ceiling above them and runs unbroken around the room. The waste allowance matters because corners are joined with angled miter or cope cuts whose scrap cannot be reused, and matching profiles between lengths wastes a little more; 10 percent is a common figure, more for rooms with many corners. Every input is editable, so you can model a different room or a layout with extra corners. The result already includes your waste allowance; round up to whole boards when buying. Every figure is computed deterministically from the formula below, with a worked example that reconciles exactly to the calculator's defaults.
Crown molding length is the room perimeter plus a waste allowance: 2 x (length + width) x (1 + waste%). A 12 x 14 room has a 52 ft perimeter, and with a 10% allowance you need 57.20 linear feet of crown molding.
Crown molding length formula
Perimeter = 2 x ( length + width )
Crown molding length = perimeter x ( 1 + waste percent )
length, width = room dimensions in feet
waste percent = allowance for miter cuts, as a decimal
Add twice the length and twice the width for the perimeter, then increase it by the waste allowance. The result is the linear feet of crown molding to order.
Worked example
Suppose a room is 12 by 14 feet and you allow 10 percent for miter-cut waste.
- Perimeter = 2 x (12 + 14) = 2 x 26 = 52 ft
- Waste added = 52 x 0.10 = 5.20 ft
- Crown molding length = 52 x 1.10 = 57.20 ft
You need 57.20 linear feet of crown molding. These are the calculator's default inputs, so the result above matches the widget exactly.
Crown molding by room size
Length needed at a 10 percent waste allowance for a few room sizes.
| Room (ft) | Perimeter | With 10% waste |
|---|---|---|
| 10 x 10 | 40 ft | 44.00 ft |
| 12 x 14 | 52 ft | 57.20 ft |
| 14 x 16 | 60 ft | 66.00 ft |
| 16 x 20 | 72 ft | 79.20 ft |
Crown runs above doors and windows, so the full perimeter applies. Round up to whole boards.
Crown molding length calculator: frequently asked questions
How much crown molding does a room need?
Crown molding runs around the full top perimeter of the room, so find the perimeter by adding twice the length and twice the width, then add a waste allowance for miter cuts and offcuts. Crown molding length = perimeter times (1 plus waste percent). Unlike baseboard, you usually do not subtract for doors, because crown sits at the ceiling above them.
Why add a waste allowance?
Crown molding meets at corners with angled miter or cope cuts, and the angled scrap from each cut cannot be reused. Matching profiles between lengths also wastes a little. A 10 percent allowance is common, and more for rooms with many corners or where you must match a pattern. Adding waste avoids running short and ensures clean corners.
Do I subtract doors and windows for crown molding?
Usually no. Crown molding installs where the wall meets the ceiling, above the height of doors and windows, so it runs continuously around the room. The full perimeter applies. Only subtract a length if a wall is open to the ceiling with no surface for the molding, such as a wide cased opening to another room.
What is a linear foot of molding?
A linear foot is one foot of length measured straight along the molding, regardless of its profile or width. Crown molding is sold in fixed lengths or by the linear foot, so the total you need is expressed in linear feet. The decorative profile does not change the length measurement.
How do I handle a room with more than four corners?
Each extra corner adds miter cuts and a little more waste. For an L-shaped or multi-corner room, measure the true perimeter by adding all the wall lengths around the top of the room, then apply the waste allowance. Increase the waste percent slightly for rooms with many corners to cover the additional cuts.
Official sources
- Length and perimeter measurement reference: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.