Crown molding is measured like any other trim — perimeter divided by stick length — but the waste factor is genuinely different, and the reason is that no corner in a real house is ninety degrees.
Why the waste is higher than baseboard
Baseboard corners are a simple mitre in one plane. Crown sits at an angle to both the wall and the ceiling, so every corner is a compound cut — two angles at once.
Compound cuts have to be right in both planes simultaneously, and the first attempt at each corner is often a test piece. Twelve percent waste is normal for baseboard; fifteen is realistic for crown, and on a room with several outside corners it can run higher.
Outside corners are the expensive ones. An inside corner can be coped, which is forgiving. An outside corner has to be mitred, and a mitre that is one degree off shows as a visible gap from across the room.
Cope inside corners, mitre outside ones
The traditional method for inside corners is coping — one piece runs into the corner square, the second is cut to the profile of the first with a coping saw.
The reason is not tradition, it is movement. A coped joint stays closed when the house shifts with the seasons; a mitred inside corner opens into a visible V. Since almost no corner is exactly ninety degrees anyway, coping also absorbs the error.
Coping takes longer and it is a skill. If you are mitring inside corners instead, add caulk to your materials list and accept touching them up later.
Spring angle, and why it matters for buying
Crown is installed at a spring angle — usually 38 or 45 degrees — which is the tilt it sits at between wall and ceiling.
That angle determines your saw settings, and it also means a given nominal size of crown covers more wall and ceiling than its face width suggests. Check which spring angle your profile is before cutting anything.
Mixing two profiles in one room almost never works, which is the real reason to buy a spare stick. Profiles are discontinued, and a second batch from the same supplier can differ slightly.
Long walls and scarf joints
Where a wall is longer than your longest stick, the joint should be a scarf — two 45 degree cuts overlapping — not a butt joint. A scarf joint stays visually closed as the wood moves; a butt joint opens into a dark line.
Put scarf joints away from the room's focal point and away from direct light. Standard lengths run 8, 12, 14 and 16 feet, so buy for the longest walls first.
Measuring the perimeter
Measure wall to wall along the ceiling, not along the floor, and include closet walls and any bump-outs. Doorways do not get subtracted the way they do for baseboard, because crown runs above them.
If the room has a soffit, a tray ceiling or a bulkhead, measure its perimeter separately and add it. Those features add substantial footage and are easy to forget until the delivery arrives short.