How the crown molding calculator works
Crown runs unbroken along the top of every wall, over doors and windows alike, so the quantity is simply the ceiling perimeter plus an allowance for the joints you will cut. The harder part is the angles. Crown sits tilted between wall and ceiling, so an inside or outside joint is a compound cut: the blade is swung (miter) and tilted (bevel) at the same time when the molding lies flat on the saw table.
Miter = atan(sin S ÷ tan(C/2)) · Bevel = asin(cos S × cos(C/2))Here S is the spring angle and C the angle between the two walls. For the nested method, where the crown is held upside down against the fence as if the fence were the wall, only a plain miter of 90° − C/2 is needed.
Worked example
A 4.5 m × 3.6 m dining room has a ceiling perimeter of 16.2 m. With a 10% allowance the order is 17.82 m, which is 7.4 lengths of 2.4 m, so buy 8. For a common 38° spring crown in 90° corners the saw is set to a 31.6° miter and a 33.9° bevel. In a 15 × 12 ft room the perimeter is 54 ft; plus 10% it becomes 59.4 ft, or five 12 ft pieces. A 45° coving in the same corners cuts at 35.3° miter and 30.0° bevel.
Saw settings for flat cutting
| Wall corner | 38° spring: miter / bevel | 45° spring: miter / bevel |
|---|---|---|
| 90° (square room) | 31.6° / 33.9° | 35.3° / 30.0° |
| 135° (bay window) | 14.3° / 17.5° | 16.3° / 15.7° |
| 120° (hexagonal bay) | 19.6° / 23.2° | 22.2° / 20.7° |
| 60° (sharp outside nose) | 46.8° / 43.0° | 50.8° / 37.8° |
Most compound miter saws have detents at 31.62° and 33.86° for exactly this reason. Check that your crown really is 38°: hold it in a framing square so both flats touch, and read the angle between the back and the vertical leg.
Sizes sold in shops
| Product | Typical face size | Lengths |
|---|---|---|
| Pine or poplar crown (US) | 2-1/4 to 5-1/4 in | 8, 12, 16 ft |
| MDF crown, primed | 3-5/8 to 7-1/4 in | 8, 12, 16 ft |
| Polystyrene or PU coving | 50–150 mm | 2 m |
| Gypsum plaster coving (UK) | 100 mm, 127 mm | 3 m, 4 m |
Advice before you start cutting
- Keep track of orientation. When the crown lies flat, the edge that touches the ceiling goes against the fence for one side of the corner and away from it for the other. Label test pieces "L inside", "R inside" and so on.
- Cut two short test blocks from an offcut and hold them in the actual corner. Ceilings sag and corners drift a degree or two, so adjust before touching a full length.
- Start with the longest wall so a miscut piece can still be reused on a shorter wall.
- Find ceiling joists or add a backing strip. Heavy wooden crown needs something to nail to at both edges; lightweight coving is glued with coving adhesive along both contact faces.
- Caulk the paint-grade joints, but not stained hardwood, where only tight cuts will do.
Frequently asked questions
How much crown molding do I need for a 12x12 room?
The perimeter is 48 ft; with 10% for cuts, order about 53 ft. That is five 12 ft lengths, or four 16 ft lengths if you can transport them.
What angle do I cut crown molding for a 90 degree corner?
Laid flat on a compound miter saw, 38° spring crown is cut at 31.6° miter and 33.9° bevel, and 45° crown at 35.3° miter and 30.0° bevel. Nested upside down in a miter box, both are a simple 45° miter.
What is the spring angle of crown molding?
It is the angle between the back of the molding and the wall when installed. 38° (sold as 52/38) is the most common for wooden crown, and 45° for many plaster and foam covings.
Do I include doors and windows when measuring for crown?
No deductions: crown runs along the top of the walls above every opening, so you measure the full ceiling perimeter.
How do I cut crown for a bay window?
Measure each corner with an angle finder. A 135° bay corner with 38° crown cut flat needs about 14.3° miter and 17.5° bevel; enter the measured angle and the calculator gives the exact settings.
Last reviewed: 2026-09-27 · How we calculate