How much baseboard do I need?

Corners are where the material goes

The linear footage is simple: the room perimeter, minus the width of every doorway. A 12 by 15 foot room has a 54 foot perimeter, and two 3 foot doorways bring it down to 48 feet of trim.

Turning that into pieces is where judgement comes in. Baseboard is sold in fixed lengths — commonly 8, 12 or 16 feet — and the count here is the total divided by the length you can buy. But trim is not like flooring: you cannot butt two short pieces together in the middle of a wall and expect it to look right. Wherever possible each wall wants a single piece, which means a 14 foot wall needs a 16 foot stick and the offcut is only useful if another wall happens to be under 2 feet.

That is what the waste allowance is really covering. Every inside corner is a coped or mitred joint, every outside corner is a mitre, and a mitre cut a degree off wastes the end of the piece. Ten percent is fine for a plain rectangular room; go to 15% once the room has more than four corners, or if you are coping the inside joints by hand.

One practical note on buying: check the pieces for bow before you load them. Baseboard is thin and long, and a stick with a curve in it will fight you the whole way along a wall. Sight down each one on the rack.