Softonic

Crown Molding Calculator

Step 1 / 3 Pieces needed
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Enter your room length and width in feet plus the length of one molding piece, and this calculator returns the wall perimeter, the total length once a 10% waste allowance is added, and how many pieces of crown molding you need to buy. Crown molding runs around the top of every wall, so the amount you need follows the room perimeter rather than its floor area. Adding waste covers the off-cuts from mitered corners and trimming, which is exactly where most first-timers fall short and have to make a second trip to the store.

How to estimate crown molding

  1. 1

    Measure the room

    Enter the room length and width in feet. For a rectangular room the calculator works out the perimeter for you; for an L-shaped room, total the lengths of all walls and split them across the two fields.

  2. 2

    Set the piece length

    Enter the length of one stick of molding, commonly 8, 12 or 16 feet. The calculator divides the waste-adjusted total by this length to see how many pieces you need.

  3. 3

    Read the results

    You get the bare perimeter, the total length with 10% waste added for miter cuts, and the number of whole pieces to purchase, already rounded up.

How the crown molding estimate is calculated

Crown molding follows the top edge of every wall, so the length you need is the room perimeter, not its area. For a rectangular room the math is:

  • Perimeter = 2 × (length + width)
  • Total with waste = perimeter × 1.10 (a 10% allowance)
  • Pieces needed = round up( total with waste ÷ piece length )

The 10% waste factor covers the angled off-cuts at inside and outside corners, the short returns over doors and windows, and the occasional mis-cut. Pieces are always rounded up to the next whole stick because lumber yards sell molding by the full length, not by the inch.

Worked example

A 12 ft × 10 ft room with 8 ft sticks of molding:

  • Perimeter = 2 × (12 + 10) = 44 ft
  • Total with waste = 44 × 1.10 = 48.4 ft
  • Pieces needed = ceil(48.4 ÷ 8) = ceil(6.05) = 7 pieces

So you buy 7 eight-foot sticks, giving 56 ft of molding to cover a 44 ft room, a comfortable cushion for the corner cuts.

Room size vs. pieces (8 ft sticks)

Room (ft) Perimeter (ft) With 10% waste (ft) 8 ft pieces
10 × 10 40 44.0 6
12 × 10 44 48.4 7
14 × 12 52 57.2 8
16 × 14 60 66.0 9
20 × 16 72 79.2 10

Pitfalls to avoid

  • Forgetting the waste. Mitered corners waste several inches each. Buying exactly the perimeter length leaves you short, the 10% allowance fixes that.
  • Mixing feet and inches. Enter every measurement in feet. A length typed in inches will throw the piece count off badly.
  • Subtracting doorways. Crown runs above the openings, so do not deduct for doors or windows the way you would for baseboard.
  • Ignoring the grain match. If your molding has a strong profile, buy one extra stick so corner returns and patterns line up cleanly.

Frequently Asked Questions

Take the room perimeter, 2 × (length + width) for a rectangular room, then add about 10% for waste from the corner miter cuts. Divide that by the length of one molding piece and round up to get the number of sticks to buy.

Every inside and outside corner is cut at an angle, which wastes a few inches per joint, and short returns over doors and windows leave off-cuts too. A 10% allowance covers those losses plus the odd mis-cut so you do not run short mid-project.

Measure every wall that will carry molding and add the lengths together to get the true perimeter. Split that total across the length and width fields so the calculator works from the right perimeter, then read the pieces needed as usual.

The numbers you enter are sent to our server so the calculation can run. They are not stored or shared with anyone. In the step-by-step view they are also carried in the page link so your results can be shown.

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