How it works
The calculator finds the perimeter at ceiling height, subtracts the stretches where the crown stops, adds your allowance for each corner and the waste percentage, then divides by the length of one piece.
Perimeter = 2 × (Length + Width) or entered directly
Run = Perimeter − Stretches without crown
Length to buy = (Run + (Inside corners + Outside corners) × Extra per corner) × (1 + Waste ÷ 100)
Pieces = ⌈ Length to buy ÷ Piece length ⌉
⌈ ⌉ means rounding up to the next whole piece. Rounding happens only at the end.
Worked example in words
A 15 × 12 ft room has a ceiling perimeter of 2 × (15 + 12) = 54 ft. With no cabinets to the ceiling, the whole 54 ft gets crown. Four inside corners with 6 in added at each add 2 ft, for 56 ft. In 16 ft pieces that is 56 ÷ 16 = 3.5, so you buy 4 pieces. Without the corner allowance it would be 54 ÷ 16 = 3.4 – still 4 pieces, but with less margin for a recut.
Corners at the ceiling
Crown molding meets the wall and the ceiling at an angle, so every corner is a compound cut. At an inside corner many installers cope the joint – the first piece runs into the corner square, and the second is cut to the profile of the first so it fits over it. At an outside corner the two pieces are mitered. Each of these cuts uses a little length, and getting one wrong means cutting a new end – sometimes from a new piece if the remaining length is too short. That is why the calculator counts corners separately and lets you add a length per corner.
The allowance per corner starts at 0 because there is no one figure that fits every profile and every installer. Choose a figure from your own experience, or leave it at 0 and add a waste percentage instead.
Planning the pieces wall by wall
The total-length estimate is a minimum. A piece cannot bend around a corner, so each wall is covered by its own pieces, and joints on long walls need pieces that meet neatly. For a better count, write down each wall’s length, decide which piece covers it, and see which offcuts are long enough for a short wall elsewhere. If the result here is close to a whole number of pieces, one extra piece is often cheaper than an extra trip to the store.
What to measure
Measure at the ceiling, not the floor: bulkheads, soffits, beams and built-in cabinets change the run. For rooms that are not rectangles, add up every wall segment and enter the total as the perimeter, and count the inside and outside corners as you go.
Worked example
Inputs
- Measure by
- Room length × width
- Room length
- 15 ft
- Room width
- 12 ft
- Stretches without crown
- 0 ft
- Inside corners
- 4
- Outside corners
- 0
- Extra length per corner
- 6 in
- Piece length
- 16 ft
- Waste allowance
- 0 %
Result
- Ceiling perimeter
- 54 ft
- Crown run
- 54 ft
- Corners
- 4
- Length to buy
- 56 ft
- Pieces
- 4
Common mistakes
- Subtracting doors and windows. Unlike baseboard, crown molding at the ceiling usually runs over doors and windows, so only subtract stretches where it really stops.
- Ignoring the corners. Every inside and outside corner is a cut on two pieces, and crown is cut at an angle on two planes at once – a recut corner can use a lot of length.
- Counting only total length. A piece cannot turn a corner, so offcuts from one wall are often too short for the next one; plan the pieces wall by wall.
- Measuring the floor instead of the ceiling. Bulkheads, soffits and built-ins change the run at ceiling height.
- Buying pieces too long to carry into the room. Check that they fit through doors and up stairs, or plan a joint on a long wall.
Assumptions and limits
- In room mode, the room is a rectangle; the perimeter is 2 × (length + width). For other shapes, enter the perimeter at ceiling height.
- Stretches without crown are subtracted as one total length; doors and windows are not subtracted.
- The corner allowance is added once per inside or outside corner, then the waste percentage is added to the total.
- Pieces are counted from total length and rounded up to whole pieces. Individual cuts, scarf joints and which offcuts can be reused are not planned.
- Nails, adhesive, caulk, backing blocks and paint are not included.
Frequently asked questions
How do I calculate how much crown molding I need?
Add up the length of every wall at the ceiling, subtract only the stretches where the crown stops, then divide by the length of one piece and round up. For a 15 × 12 ft room that is 54 ft, or 4 pieces of 16 ft – or more, if you add an allowance for the corners.
How much extra crown molding should I buy for corners?
There is no single published figure. It depends on the profile, the size of the room and how many corners you have done before. Enter your own extra length per corner, or a waste percentage, and compare the piece counts. The safest check is to plan the pieces wall by wall.
What is the difference between inside and outside corners?
An inside corner is where two walls meet facing into the room – a plain rectangular room has four. An outside corner points into the room, like the corner of a chimney breast. Inside corners are often coped – one piece is cut to the profile of the other – and outside corners are mitered.
Do I subtract doors and windows for crown molding?
Usually not. Crown molding sits at the ceiling line and runs over door and window openings. Subtract only stretches where it stops, such as cabinets that reach the ceiling.
Can I use the baseboard calculator instead?
For the length, the method is the same, but baseboard stops at every doorway while crown usually does not, and crown corners are harder to cut. This page starts with no doorway gaps and lets you add corners and an allowance per corner.
Sources
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 3, 2026