How trim is counted
Trim is bought in lengths, not in feet, and that is the whole difficulty.
The perimeter tells you how much material ends up on the wall; the stock
length tells you how much you have to carry home.
perimeter = 2 × (length + width)
with waste = perimeter × (1 + waste)
lengths = ceil( with waste ÷ stock length )
Worked example: a 12 × 14 ft room
perimeter = 52 ft
with 10% = 57.2 ft
in 8 ft = 8 lengths (64 ft)
in 16 ft = 4 lengths (64 ft), but no mid-wall joints
Both options buy 64 feet. The difference is where the joints land: eight
short lengths put a joint in the middle of nearly every wall, four long
ones put them only at corners, where nobody looks.
Common mistakes
- Buying the perimeter. Fixed lengths and real walls never divide evenly.
- Stock shorter than the longest wall. It puts a visible joint mid-wall for no saving.
- Mitring inside corners. They open as the house moves. Cope them.
- Cutting the longest piece first. Cut the hardest wall first while you still have full lengths to make a mistake on.
- No allowance for learning to cope. The first few joints cost material.
The rest of the room
Trim and panelling usually happen in the same job: the
board and batten
calculator spaces battens so every bay matches, which matters most
where the trim meets them. Painting is easiest before the trim goes up,
and the paint calculator works
out the gallons.
Lengths of trim by room size, at 10% waste
Longer stock usually buys the same total footage with fewer joints, and a joint near a corner is far less visible than one in the middle of a wall.
Common questions
How much crown molding do I need for a 12x14 room?
The perimeter is 52 feet. With the usual ten percent allowance that is 57.2 feet, which is eight 8-foot lengths. Note that eight lengths is 64 feet — you are buying almost seven feet more than the room measures, and most of that surplus disappears into corner cuts and the ends you trim square.
Why can I not just buy the perimeter in feet?
Because trim is sold in fixed lengths and a wall does not care what those are. A 14 foot wall from 8 foot stock needs two pieces and leaves a 2 foot offcut, and that offcut is only useful if another wall happens to need a piece under 2 feet. Dividing the perimeter by the stock length is the arithmetic; buying whole lengths is the shopping list.
Should I cope or mitre inside corners?
Cope them if you can. A mitred inside corner opens up as the house moves and the joint shows as a dark line within a season; a coped joint hides that movement because one piece sits against the profile of the other. Coping wastes slightly less material too, since only one piece of the pair is cut at an angle.
How much waste should I allow for trim?
Ten percent is the working figure for a straightforward rectangular room, and this calculator uses it. Add more if the room has many corners, if the molding has a pattern that has to line up across a joint, or if you are new to coping — the first two or three coped joints usually cost a length between them.
What length stock should I buy?
Long enough to cross your longest wall in one piece if you possibly can. A joint in the middle of a wall is visible from across the room no matter how well it is cut, whereas a joint tucked near a corner is not. If the longest wall is 14 feet, 16 foot stock costs a little more and removes the worst joint in the room.
Does this work for baseboard and chair rail too?
Yes. The arithmetic is identical for anything that runs around a room in fixed lengths — baseboard, chair rail, picture rail, shoe molding. The only thing that changes is whether you deduct door openings, which for baseboard you should and for crown you should not.