Understanding your result
The headline is the trim run, the length of wall that gets baseboard and shoe, with the perimeter and the openings beneath it. The table gives the perimeter, the openings, the fixtures, the run for baseboard and shoe, and the run for tack strip.
The perimeter is the total length of wall. For a rectangle it is twice the length plus twice the width; for an L-shape, the same as its enclosing rectangle; for an irregular room, the total you enter. Closet walls are added if the closets get trim. The trim run is the perimeter less the doorways and the wall taken by fixtures. The tack strip run is the perimeter less the doorways only, because carpet runs to the wall even where a piece of furniture will stand.
None of these figures includes waste. The baseboard and quarter round calculator takes the trim run and adds the cutting waste, pieces and nails; the tack strip calculator does the same for carpet. The area of the same room is on the floor area calculator.
How we calculate this
The L-shape rule is geometry: removing a rectangular corner shortens two walls and adds two inner walls of exactly the lengths removed. Rooms with more than one notch or with angled walls are better measured wall by wall and entered as a total. The guide to measuring odd rooms has diagrams, and the expansion gaps guide explains why the trim must cover a gap at the wall for floating floors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Room | 14 × 12 ft (4.2 × 3.6 m) | A typical bedroom |
| Opening | 3 ft (0.9 m) | A standard interior door with casing |
| Fixtures | 0 | Enter cabinets, tubs and fireplaces against the wall |
| Waste | none | Added by the baseboard and tack strip calculators |
Assumptions last reviewed October 8, 2026.
The calculator does not know about wall returns, columns or radiators that the trim goes around; add their faces to the wall total. It treats every opening as the same width; if they differ much, enter the total opening width as fixtures and set the doorways to zero.
Two worked examples
An 18 × 14 ft bedroom with a closet
Two 3 ft doors, a closet adding 9 ft of wall.
- Perimeter: 2 × (18 + 14) = 64 ft; plus closet 9 ft = 73 ft
- Less openings: 2 × 3 = 6 ft
- Trim run: 67 ft; tack strip run 67 ft
Sixty-seven feet of baseboard, before cutting waste, is nine 8 ft lengths once the 10% is added. The closet’s nine feet are easy to forget and are a whole length on their own.
A kitchen measured wall by wall
Walls totalling 96 ft, four openings, 12 ft of cabinets against the walls.
- Perimeter: 96 ft
- Trim run: 96 − 12 − 12 = 72 ft; tack strip run 84 ft
The cabinets take 12 ft out of the baseboard but not out of the tack strip, which matters only if the kitchen were carpeted, which it should not be. For a kitchen the useful figure is the trim run.
Where to find your inputs
Walls. Measure each at floor level, corner to corner. For a rectangle, two dimensions are enough.
Openings. Count doorways and cased openings; measure any unusually wide ones.
Closets. The inside walls of each closet that will be trimmed.
Fixtures. The length of wall covered by cabinets, a tub, a fireplace or built-ins.
Common mistakes
- Using the area instead of the perimeter. Trim follows the walls.
- Forgetting closet walls. Three walls each, and they add up.
- Not subtracting doorways. Baseboard stops at the casing.
- Measuring an L-shape as two rooms. Its perimeter equals the enclosing rectangle’s.
- Adding waste here. The baseboard calculator adds it.
- Ignoring returns and columns. Every face the trim covers counts.
Questions people ask
- How do I calculate the perimeter of a room?
- Add the lengths of all the walls. For a rectangle, that is two times the length plus the width; a 14 × 12 ft room has a 52 ft perimeter. Subtract the doorways, about 3 ft each, for the baseboard run.
- What is the perimeter of an L-shaped room?
- The same as the rectangle that encloses it. Cutting a notch out of a corner moves two walls inward but does not change their total length. A 14 × 12 ft L-shape has a 52 ft perimeter whatever the size of the notch.
- Do I subtract doorways for baseboard?
- Yes, the full width of each opening, including the casing. A standard interior door opening is about 3 ft. Cased openings between rooms can be 4 to 8 ft.
- Do I include closets?
- If the closets get baseboard, add their three inside walls. A 6 ft wide, 2 ft deep closet adds 6 + 2 + 2 = 10 ft of wall.
- What about walls behind cabinets?
- Baseboard does not go where a cabinet, a tub or a fireplace sits against the wall. Enter that length as fixtures and it is subtracted from the baseboard run.
- How much extra trim should I buy?
- About 10% for the mitres and coped corners, which the baseboard calculator adds. This calculator gives the measured run without waste.