Crown Molding Calculator
Find the linear feet of crown molding to buy for a room from its perimeter and a waste allowance, plus the number of 8-ft pieces.
How it works
perimeter = 2 × (length + width) · buy = perimeter × (1 + waste%) · pieces = ceil(buy ÷ stock length)
Crown molding wraps the top of every wall, so the base figure is the room perimeter — unlike baseboard, doors and windows do not interrupt it. Crown is trickier to fit than flat trim because each corner is a compound (coped or mitred) cut, and a spoiled cut on an expensive profile is common, so a 10–15% waste allowance is genuinely used up rather than optional. A standard rectangular room has four inside corners; bays, soffits and returns add more.
Worked example
A 15 × 12 ft room has a 54 ft perimeter. At 10% waste that is 54 × 1.10 = 59.4 linear feet — eight 8-ft pieces or five 12-ft pieces — to trim four inside corners.
Frequently asked questions
How much waste for crown molding?
Ten percent for a simple rectangular room, and 15% or more if there are many corners, a coffered or tray ceiling, or you are new to coping crown. Compound corner cuts are unforgiving, and a wasted piece on a long or detailed profile adds up fast.
Should I cope or mitre inside corners?
Coping — cutting one piece to the profile of the other — gives a tighter joint that stays closed as the wood moves and the house settles, which is why finish carpenters prefer it inside. Mitres are quicker but tend to open up over time. Outside corners are always mitred.
Do windows and doors reduce crown molding?
No. Crown runs along the ceiling line, above every opening, so the full room perimeter applies with nothing subtracted — the opposite of baseboard, where you take out the door widths.
Related calculators
Estimate for planning. Add a waste margin and confirm against your supplier specs and local building codes.