Crown Molding Calculator
Find out how much crown molding a room needs — total length with miter waste, the number of 8, 12 or 16 ft pieces, and corner blocks if you use them.
How much crown molding to buy
Crown is bought by the piece but planned by the perimeter: the room's four walls added up, plus the allowance miters eat. Every corner cut — and every scarf joint where two pieces meet mid-wall — throws away a few inches of profile, and a single miscut on the last corner is the most expensive mistake in the job. The calculator totals the length with waste and turns it into whole pieces at the length your store stocks.
Length to buy = room perimeter × (1 + waste) · Pieces = length ÷ piece length, rounded up
Example: a 12 × 10 ft room has a 44 ft perimeter. With a 10% allowance that is 48.4 ft of molding — 5 pieces at 12 ft each, for a room with 4 inside corners.
Corners decide the cutting plan
Inside corners are best coped — one piece runs square into the corner and the next is cut to nest over it — while outside corners are mitered and glued. Corner blocks sidestep both: the molding runs square into a decorative block at every corner, which removes the hardest cuts in the room and gives a deliberate, furniture-like look. If you use blocks, select them in the form and the count appears in the buying list — one per corner, inside and outside counted separately in the result.
| Room | Perimeter | Buy at 12 ft pieces (10% in) |
|---|
| 10 × 10 ft | 40 ft | 4 pieces |
| 12 × 10 ft | 44 ft | 5 pieces |
| 16 × 12 ft | 56 ft | 6 pieces |
| 20 × 14 ft | 68 ft | 7 pieces |
Long walls want the longest stock you can transport: every avoided scarf joint is a joint that cannot open later. When a wall is longer than the longest piece, plan the joint over a stud or a block, glue it, and pin both sides — and never land a joint in the middle of the room's focal wall if it can live over the door instead.
Frequently asked questions
What is the difference between inside and outside corners?
An inside corner is where two walls meet in a valley — the corner of every ordinary room. An outside corner wraps around a projection: a chimney breast, a bulkhead, an archway pier. Most rooms have 4 inside and no outside corners; the counts here default to that.
Should I cope or miter inside corners?
Coping is the professional default: a coped joint stays tight when the house moves seasonally, while a mitered inside corner opens a hairline gap every winter. Mitering is faster; coping is permanent. Corner blocks avoid the question entirely.
Why does the waste allowance matter so much for crown?
Because the cuts are long miters at compound angles, and each one is a chance to cut the mirror image of the piece you needed. Ten percent covers the offcuts; if it is your first crown job, one extra full piece is the honest allowance.
What about rooms that are not rectangles?
Add the wall lengths yourself and halve the sum cleverly: enter any two numbers whose doubled sum equals your true perimeter (a 50 ft perimeter could be length 15 and width 10). Alcoves add their inside walls to the perimeter; a chimney breast adds its two returns and face.
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