The perimeter formula

Baseboard is sold by the linear foot (per foot of wall), usually in 8 ft or 12 ft boards. The starting number is simple: add the length of every wall in the room. For a rectangle that's 2 × (length + width) — a 12×16 room has 2 × 28 = 56 linear feet of wall.

If the room has an L-shape, an alcove, or a half-wall, just measure each straight run with a tape and add them up. Measuring wall by wall is always more accurate than assuming the room is a clean rectangle.

Subtracting doors and openings

Baseboard stops at door casings, so each standard interior door removes about 3 ft from your total (a 30–36 inch opening). If a door is wider — double doors, a slider — measure the actual opening. Fireplace surrounds and built-ins also skip baseboard; subtract those runs too. On a room with two doors, that's 6 ft off the perimeter before you buy a single board.

Linear feet by room size

Here are common room sizes with one door (subtract 3 ft) and a 10% waste allowance:

Room	Perimeter	After door (−3 ft)	With 10% waste
10×10	40 ft	37 ft	~41 ft
10×12	44 ft	41 ft	~45 ft
12×12	48 ft	45 ft	~50 ft
12×16	56 ft	53 ft	~58 ft
14×18	64 ft	61 ft	~67 ft
16×20	72 ft	69 ft	~76 ft

Worked example: a 12×12 room

• Perimeter = 2 × (12 + 12) = 48 ft
• Minus 1 door = 45 ft
• Add 10% waste = ~50 ft to buy
• 8 ft boards: 50 ÷ 8 = 6.25 → 7 boards
• 12 ft boards: 50 ÷ 12 = 4.2 → 5 boards

Common mistakes

• Forgetting doors. Ordering the full perimeter wastes 3 ft per door — on a whole house that's a full extra board or two.
• Not accounting for corners. Inside and outside corners eat material in miter cuts. The 10% waste allowance covers it; skip it and you'll run short at the last corner.
• Mixing board lengths on one wall. Long walls look better with 12 ft boards and fewer seams; short closets are fine with 8 ft cutoffs.
• Ignoring tall baseboard. 6–8 inch tall baseboard costs more per foot and takes more paint — the linear footage doesn't change, but the budget does.

FAQ