The trim calculator turns your room's perimeter into a board count, and that math is solid. What it can't tell you is how you'll actually join the corners — and that choice affects both how much you waste and how good the finished room looks.
Miter Cuts
A mitered joint uses matching angled cuts (usually 45°) on both boards meeting at a corner. It's the easier technique to learn and works fine on outside corners every time. The catch: inside miter joints can open up into a visible gap if the wall corner isn't a perfect 90° — which, in older homes especially, it often isn't.
Coping
For inside corners, professional trim carpenters usually cope instead of miter: one board runs straight into the corner, and the second board is cut to match the exact profile of the first board's face, then fitted against it. It takes more practice with a coping saw, but the joint stays tight even when the wall behind it isn't perfectly square — which is most walls.
Outside Corners Always Cost You a Bit
There's no coping equivalent for outside corners — they're mitered by necessity. Precise angle cuts here mean a small amount of guaranteed waste at every outside corner in the room. If your room has several outside corners (an alcove, a jog in the wall), nudge your waste percentage up slightly to cover it.
Run your own numbers with the baseboard & trim calculator, and bump the waste field up a bit if your room has multiple outside corners.