Baseboard & Trim Calculator
This counts lengths to buy, not the run divided by the board length. Two offcuts do not join into one board, which is why the honest number is usually one more than people expect.
Lengths to buy
5lengths
- Room perimeter
- 52.0 ft
- Less doorways
- 46.0 ft
- With waste
- 50.6 ft
- Total bought
- 60.0 ft
- Offcut
- 14.0 ft
- Includes a 10% allowance for mitres. The bare run is 46.0 feet.
- Doorways are taken off because the board stops at the architrave. Windows are not, because skirting runs straight under them.
- The count is lengths to buy, not the run divided by the length. Two offcuts do not join into one board, so a run needing 3.1 lengths needs four.
- Every external corner costs two mitres and there is no second chance at one, which is what the waste allowance is really for.
About the baseboard calculator
The run is the room perimeter less the doorways, because skirting stops at the architrave either side of an opening. Windows are not deducted — the board runs straight under a window, and taking the width off is a common way to end up short.
What makes this different from dividing the run by the board length is that a board is bought whole. A forty-six foot run in twelve-foot lengths is 3.8 lengths, and you cannot buy 3.8 or make the fourth out of two offcuts. It is four lengths, or five once you allow for mitres.
The allowance matters more here than on most materials because of how the waste happens. Every external corner is two mitres cut at forty-five degrees, and there is no second chance at a mitre — a cut that is a degree out shows as a gap and the board has to be recut shorter. Ten per cent is the usual allowance; a room with several external corners or a bay wants fifteen.
Longer stock is generally better where the room allows it. A wall covered in one length has no joint in it, and a scarf joint in the middle of a wall is visible under paint no matter how well it is filled.
What it works out
- Lengths to buy, not just the total run
- Doorways deducted, windows correctly left in
- Any stock length, in feet or metres
- Baseboard, skirting, coving, picture rail and dado
The formula
Lengths = ⌈((2 × (Length + Width)) − Doorways × Door width) × (1 + Waste) ÷ Board length⌉
Start with the perimeter — twice the length plus twice the width. A fourteen by twelve room is fifty-two feet round.
Take off the doorways. Skirting stops at the architrave on each side of an opening, so two three-foot doorways come off, leaving forty-six feet. Windows do not come off: the board runs straight underneath them, and deducting a window is a reliable way to be one board short.
Add the mitre allowance, then divide by the board length and round up. The rounding up is the part that matters. Forty-six feet plus ten per cent is 50.6 feet, which is 4.2 twelve-foot lengths — and you buy five, because the fifth cannot be assembled out of the offcuts from the first four.
That leaves you with more board than run, which is normal and not a mistake in the arithmetic. The offcuts are what you cut the short walls and the returns from.
- Length, Width
- The room, measured wall to wall at floor level.
- Doorways
- Openings the board stops either side of. Windows are not deducted.
- Door width
- The opening including its architrave. Thirty-six inches is standard.
- Board length
- The stock length your supplier sells, commonly 8, 12 or 16 feet.
- Waste
- The mitre allowance — every external corner is two cuts with no second chance.
A worked example
A fourteen by twelve foot room with two doorways, in twelve-foot lengths, with ten per cent for mitres.
That works out to 5 lengths.
- Room perimeter
- 52.0 ft
- Less doorways
- 46.0 ft
- With waste
- 50.6 ft
- Total bought
- 60.0 ft
- Offcut
- 14.0 ft
Questions
How much baseboard do I need for a 12 by 12 room?
The perimeter is 48 feet. With one doorway taken off that is 45 feet of run, and with a ten per cent mitre allowance you would buy five twelve-foot lengths.
Do I subtract windows from the run?
No. Baseboard runs straight under a window — only a doorway stops it, because the board dies into the architrave on each side. Deducting windows is one of the most common reasons people come up short.
Why does it say five lengths when the maths gives 4.2?
Because you cannot buy 0.2 of a board, and the leftover pieces from four boards will not join into a fifth. The count here is what to buy. The total length you end up with is always more than the run, and the difference is what the short walls and returns get cut from.
How much extra should I allow for mitres?
Ten per cent in a plain rectangular room, fifteen where there are several external corners, a bay or a chimney breast. External corners are the expensive ones: two mitres that have to meet exactly, and a recut always makes the board shorter.
Should I buy longer boards?
Where the room allows it, yes. A wall covered by a single length has no joint in it. A scarf joint mid-wall shows under paint however well it is filled, and it opens up as the timber moves through the year.
Does this work for coving and picture rail?
Yes — it is the same run and the same problem. Coving is worth allowing more for, because its mitres are compound cuts and a spoiled one is easier to make than a spoiled skirting mitre.
Should I cope internal corners instead of mitring them?
On internal corners a coped joint holds up better than a mitre, because it stays closed when the timber shrinks or the corner turns out not to be square. It uses the same amount of board, so the count here does not change.
What if my room is not a rectangle?
Measure the actual perimeter and enter a rectangle with the same total — a length and width that add to half of it. An L-shaped room also has two extra external corners, so use the higher mitre allowance.