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Chair Rail Calculator

Enter the room length, width and total openings to get the chair rail length needed.

How it is calculated

The tool treats the room as a rectangle and measures the walls the rail will run along: footage = 2 × (length + width) − openings. length and width are the room's two wall dimensions, and openings is the combined width of doors, windows or other gaps the rail will not cross. The result has no unit of its own; it comes out in whatever unit you typed, so use feet or metres consistently.

Length and width must be above zero. Openings are optional and count as 0 if left blank, but they cannot be negative. If the openings are larger than the whole perimeter the result is shown as 0.00 rather than a negative number. No waste allowance is added, so rails bought in fixed lengths usually need extra for cuts and mitred corners.

Worked example

A room is 14 ft by 12 ft, with a 3 ft door and a 3 ft window opening that the rail will not cross, so openings = 6. The perimeter is 2 × (14 + 12) = 2 × 26 = 52. Subtracting the openings gives 52 − 6 = 46. The tool reports 46.00 ft of chair rail. Before buying, round up and add a little for waste, for example choosing enough standard lengths to cover 50 ft.

Try this example →

Common mistakes

  • Entering the area (14 × 12 = 168) in place of the two side lengths. Length and width are separate fields.
  • Entering the number of doors and windows in the openings field. It wants their total width, such as 3 + 3 = 6, in the same unit as the room.
  • Mixing units, such as a room in metres and the door width in feet or centimetres.
  • Subtracting openings the rail actually passes above. Only count the gaps the rail will not cross; a window that sits higher than the rail does not shorten it.
  • Buying exactly the result. Cuts, corners and mistakes need extra length, and the tool adds no waste.

About this calculator

Chair rail is a horizontal trim moulding installed partway up a wall — traditionally to protect plaster from chair backs, now mostly decorative. Estimating how much you need is a perimeter calculation: 2 × (room length + room width) gives the total wall perimeter, from which you subtract the width of any door and window openings the rail won't cross.

Getting this right before buying moulding matters because trim is typically sold in fixed lengths (6, 8, 10, or 12 feet), and underestimating means a return trip to the store while overestimating wastes material and money. DIYers, contractors, and interior designers use this calculation to plan the exact linear footage needed for a room's chair rail, baseboard, or crown moulding project.

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Comments (1)

  • Editorial team

    An L-shaped room is easier than it looks. For a room made only of right-angle corners, the perimeter equals that of the rectangle that would enclose it, so you can enter the enclosing length and width. A 14 by 12 room with 6 of openings gives 46.00, and with no openings it gives 52.00. Bump-outs such as a chimney breast add wall that this shortcut will not capture. What shape is the room you are trimming?

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