Baseboard is bought by length, so the whole job starts with a tape measure: add up the walls, subtract the openings, and turn the total into pieces or linear feet. Here is the formula, a chart for common room sizes, and worked examples.
The quick answer
Linear feet of baseboard = the room’s perimeter minus the doorways and other gaps where no trim runs. Retailers sell baseboard in fixed lengths – commonly 8, 12 and 16 foot pieces – or by the linear foot. Pieces needed = total linear feet ÷ the piece length, rounded up. For miter cuts and mistakes, a 10 percent example allowance works like this: multiply the linear feet by 1.10 first, then divide and round up.
The formula
Total linear feet = wall 1 + wall 2 + … – doorways and gaps. Pieces = linear feet ÷ piece length, rounded up. With an allowance, apply it before rounding: pieces = linear feet x 1.10 ÷ piece length, rounded up.
Pieces by room perimeter
Perimeter totals below already subtract nothing – take doorways off your own measurement first. Exact counts divide by the piece length; the allowance columns multiply by 1.10 before dividing and rounding. The 10 percent is an example, not a universal waste rate.
| Linear feet needed | 8 ft pieces (exact / with 10%) | 12 ft pieces (exact / with 10%) | 16 ft pieces (exact / with 10%) |
|---|---|---|---|
| 40 | 5 / 6 | 4 / 4 | 3 / 3 |
| 48 | 6 / 7 | 4 / 5 | 3 / 4 |
| 54 | 7 / 8 | 5 / 5 | 4 / 4 |
| 70 | 9 / 10 | 6 / 7 | 5 / 5 |
| 80 | 10 / 11 | 7 / 8 | 5 / 6 |
Worked examples
12 x 12 bedroom
Perimeter: 48 linear feet. Subtract a 30 inch doorway (2.5 ft): 45.5 linear feet. With 12 foot pieces: 45.5 ÷ 12 ≈ 3.8, so the count is 4 pieces. With the 10 percent example allowance: 45.5 x 1.10 ≈ 50.1, and 50.1 ÷ 12 ≈ 4.2, so 5 pieces. These are length-based counts, not a guaranteed layout – where the joints land depends on your walls and cut plan.
15 x 20 living room, two doorways
Perimeter: 70 linear feet. Subtract two 36 inch doorways (6 ft): 64 linear feet. With 16 foot pieces: 64 ÷ 16 = 4 pieces exactly, and the 10 percent example allowance takes it to 5 pieces (64 x 1.10 = 70.4, 70.4 ÷ 16 = 4.4).
Buying by the linear foot
Some suppliers cut or price baseboard by the linear foot. Then the math stops at the total: a hallway with 30 linear feet of wall needs 30 linear feet of trim, or about 33 with the 10 percent example allowance.
Tips before you buy
- Measure each wall at floor level. Rooms are rarely perfectly square, and the perimeter off a floor plan can miss bump-outs and closets.
- Subtract only real gaps: doorways, cased openings, and spots where cabinets or built-ins cover the wall. Leave everything else in.
- The 10 percent figure is an example allowance, not a universal waste rate. Miters at every corner eat length; rooms with many corners or angled walls can need more.
- Longer pieces mean fewer joints but more offcut waste on short walls. Mixing lengths to fit your wall dimensions is normal.
- Height and profile change the price, not the length. Door casings, window trim and shoe molding are separate pieces with their own math.
Sources
- Home Depot baseboard category – materials and sold lengths across the range
- Example 12 ft baseboard product listing – fixed-length pieces behind the piece-count math
Related calculators and guides
- Square footage calculator – measure the room first
- How many gallons of paint do I need for a room
- How many boxes of laminate flooring do I need
- How many sheets of drywall do I need for a room
Estimates only. This is general guidance, not professional advice. Confirm details with your supplier, installer or local building department.