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How much crown molding or coving do I need?

From the walls along the ceiling to whole lengths, and why cutting each wall into equal pieces can take more than the perimeter suggests.

Three numbers decide it

The number of lengths of crown molding, also called coving, depends on three things: how long each wall is where it meets the ceiling, how long one piece is as sold, and what happens at the corners. The total perimeter alone is not enough, because every wall is cut from whole pieces, and the planner never joins an offcut from one wall into another. The edge of a flat ceiling runs around the same walls as the floor, and doors and windows are not deducted: the coving runs over them.

Step by step

  1. Measure each wall along the top, or use the room shape you already entered in step 1 of the planner: a rectangle has four walls, an L-shape six. How to measure a room shows how to measure an L-shape as an overall rectangle with a corner cut out.
  2. Count the corners. A rectangle has 4 inside corners and no outside corner. An L-shape has 5 inside corners and 1 outside corner, where the two sides of the cut-out meet.
  3. Choose how the corners are made: Mitered corners, cut from the lengths themselves, or Corner pieces, ready-made blocks the lengths butt into.
  4. Take two numbers from the label: the Length of one piece and, for mitered outside corners, the Projection onto the ceiling.
  5. Work out each wall’s cut length. With mitered corners, add the projection once for every end of the wall that meets an outside corner. With corner pieces, the cut length is the wall itself; the width of a corner piece is not subtracted, which gives a little more coving, never less.
  6. Cut each wall into equal pieces. Pieces = cut length ÷ length of one piece, rounded up; each piece is the cut length divided by that number. A 14 ft wall from 8 ft lengths is 2 pieces of 7 ft, not one of 8 ft and one of 6 ft.
  7. Fit the pieces into lengths. Pieces of the same size share lengths, as many whole pieces per length as fit. Pieces of different sizes never share a length, so their offcuts are not reused.
  8. Add the spare lengths and round up to packs. Lengths to buy = lengths needed × (1 + your spare %), rounded up, then whole packs if the coving is sold in packs. The planner starts at 0% spare lengths.

Worked example in feet: one room, three piece lengths

An invented living room of 14 × 12 ft, with mitered corners. Its walls are 14, 12, 14 and 12 ft, 52 ft in all. All four corners are inside corners, so no projection is added. The piece lengths below are examples; use the length on the label of the coving you can buy.

Piece length Pieces Lengths 52 ÷ length
8 ft 4 × 7 ft, 4 × 6 ft 8 6.5, so 7
12 ft 4 × 7 ft, 2 × 12 ft 6 4.33, so 5
16 ft 2 × 14 ft, 2 × 12 ft 4 3.25, so 4

The offcuts are 12 ft, 20 ft and 12 ft. They are not reused.

With 8 ft lengths, a 7 ft piece fills one length and leaves 1 ft, too short for anything else, so 8 lengths are needed where the perimeter suggests 7. With 12 ft lengths the two 12 ft walls take exactly one length each; the planner then shows Whole lengths with nothing to spare for the end cuts for those walls and suggests one spare length.

Worked example in metric: an L-shaped room

An invented dining room of 4.6 × 3.8 m with a 2.4 × 1.4 m corner cut out. Clockwise from the top left, its walls are 2.2, 1.4, 2.4, 2.4, 4.6 and 3.8 m: 16.8 m, the same as 2 × (4.6 + 3.8). Walls 2 and 3 meet at the outside corner. The coving comes in example lengths of 2.4 m with an example projection of 90 mm.

Mitered corners. Walls 2 and 3 each get 0.09 m: 1.49 m and 2.49 m. The cut lengths and pieces are:

  • Wall 1, 2.2 m: 1 piece; wall 2, 1.49 m: 1 piece; wall 3, 2.49 m: 2 pieces of 1.245 m.
  • Wall 4, 2.4 m: 1 piece, an exact fit; wall 5, 4.6 m: 2 pieces of 2.3 m; wall 6, 3.8 m: 2 pieces of 1.9 m.

None of these pieces fits twice in one 2.4 m length, so every piece takes a length: 9 lengths. With 10% spare lengths that is 9 × 1.1 = 9.9, so 10 lengths; sold in packs of 4, that is 3 packs with 2 spare lengths in the packs.

Corner pieces. Every wall is cut as measured. Wall 3 is now 2.4 m, one piece like wall 4, so the room needs 8 lengths, plus 5 inside and 1 outside corner pieces. The 90 mm at the outside corner is the difference between one piece and two on wall 3.

Adhesive and filler follow the glued length, the 16.8 m of the walls, whichever corners you choose; the projection is a cutting allowance, not glued length.

  • Adhesive at an example 6 m of coving per tube and 10% extra: 16.8 × 1.1 = 18.48 m, 18.48 ÷ 6 = 3.08, so 4 tubes, with 5.52 m (0.92 tube) left over.
  • Filler along both edges is 2 × 16.8 = 33.6 m of bead. A 5 mm fillet uses 12.5 mL per meter: 420 mL, or 462 mL with 10% extra, so 2 cartridges of 300 mL. The bead arithmetic is in how much caulk do I need.

Why the planner buys more than the perimeter suggests

The perimeter divided by the length of one piece assumes that every offcut can be joined into the next wall. The planner does not plan joints like that: it counts equal pieces per wall and never reuses an offcut between pieces of different sizes. It still shows the simple division as a check: If every offcut could be reused: 8 lengths (linear estimate, a check only) for the mitered L-shaped room, where it buys 9. Without spare lengths, the equal-piece count is never below that check. The result also lists a Cut list of equal pieces (not an optimized cutting plan).

The spare lengths and the packs come after this count, never rounded down, in the order explained in how pack rounding works.

Common mistakes

  • Dividing the perimeter by the length of one piece. In the 14 × 12 ft room with 8 ft lengths that gives 7; the walls need 8.
  • Taking off doors and windows. The coving runs above them. Only a stretch where the coving really stops, such as a cabinet up to the ceiling, is left out, by typing runs.
  • Forgetting the projection at an outside corner. Without it, wall 3 of the L-shaped room looks like one 2.4 m length; with mitered corners it needs two pieces.
  • Measuring typed runs at the short point. A run is measured as the piece is cut, at its long point. Nothing is added to typed runs.

Where to plan it

In step 2 of the room refresh planner or any of the indoor calculators, open Plan other materials, then Coving. Use Add coving for the lengths, Add corner pieces with Used for set to inside or outside corners, and Add coving adhesive with Coving one tube fixes from its label. For each room, choose the coving, Where it goes (Around the whole ceiling (walls of this room) or Only these runs) and Corners. The filler is in Sealant and filler, as Along both edges of the coving. The board and batten calculator has the same group for rooms with paneling.

The planner only counts. Working at height or from a ladder: work safely and follow the ladder maker’s instructions and the product label. Very old textured ceilings or ceiling coatings can contain asbestos in some countries: do not sand or scrape them before checking.

Frequently asked questions

Do I take doors and windows off the length?

No. Crown molding runs along the top of the wall, above the doors and windows, so the planner never deducts them. If a wall has a stretch without coving, for example where a tall cabinet reaches the ceiling, choose Only these runs and type the stretches you will cover, each measured as the piece is cut, at its long point.

What is the projection, and when do I need it?

It is how far the coving reaches onto the ceiling, printed on the label. The planner needs it only for mitered outside corners: at each end of a wall that meets an outside corner it adds the projection once, so the piece is long enough for the cut. Inside corners and corner pieces add nothing. In the L-shaped example above, the 90 mm projection turns the 2.4 m wall into a 2.49 m cut, which no longer fits in one 2.4 m length.

My room is drawn as several rectangles. Can the planner still count coving?

Only once it knows the walls. Several rectangles give the floor area but not the single walls, so step 1 shows Enter walls one by one for a room with coving around the ceiling. A walls list does not say which walls meet at an outside corner, so with mitered outside corners the planner asks you to enter the coving as runs measured at the long point (Only these runs) or to choose corner pieces.

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