The run is fixed, so something has to absorb the remainder
Every frame-and-panel layout is the same equation. The wall run equals two end stiles, plus the panels, plus the stiles between them. Three of those four are choices and the fourth is whatever is left. The question is only which one you let float.
This page floats the panel width and keeps every panel identical. That is the version most people want, because unequal panels on a single wall are the thing the eye finds first. The cost is that the end stiles carry the remainder, so they come out at whatever width makes the arithmetic close. With the defaults — a 144 in run, four panels, 3 in intermediates and 3-1/2 in ends — the panels land at 32 in each.
What actually changes the proportion
Panel count moves the width far more than stile width does. On the default run, going from four panels to five takes the panel from 32 in to 25 in, a seven inch move. Widening every intermediate stile by half an inch instead takes it from 32 to 31-5/8, three eighths of an inch. If the panels are the wrong shape, change the count first.
The alternatives block on the page prints two panels either side of your choice with the ratio for each, so the comparison is in front of you rather than in your head.
The vertical stack
The height divides top down: cap, top rail, panel, bottom rail, floor. The bottom rail is normally the deepest of the three, for two reasons that have nothing to do with taste. It has to be deep enough to swallow the baseboard if the baseboard is running through, and a bottom rail the same depth as the top one looks skinny from standing height because you are seeing it foreshortened. With the defaults — 36 in overall, 1-1/2 cap, 3 top rail, 7 bottom rail — the panel is 24-1/2 in tall.
If the floor is not level, decide early whether the cap runs level or follows the floor. A cap that follows the floor keeps the bottom rail a constant depth and puts the error at the top, where it shows against a level thing like a window stool. A level cap puts the error in the bottom rail, where the baseboard hides some of it. There is no right answer, only which error is less visible in that particular room.
Panels from sheet stock
The page reports how many panels come out of one sheet at your panel size, trying the sheet both ways round and taking the better of the two. It makes no allowance for grain direction, which matters on veneered panel and does not on primed MDF. If the panels are veneered and the grain has to run vertically, take the smaller of the two numbers rather than the better one.
What this page will not tell you
It supplies no proportion, no rule and no height. Wainscot has been built at every height from 30 in to the top of a door for four centuries and the choice belongs to the room. The numbers in the fields are placeholders. It also has no view on the fixings: what holds panelling to a wall depends on what the wall is made of, and a nail gun does not know what is behind the plaster. Chase the services before the pins go in.
Board wainscot works completely differently — it counts courses off the exposed reveal and is on the wall panel calculator. Boxes of applied moulding stuck to flat drywall are on the picture frame molding wall calculator. Room-wide footage for the cap, base and casing sits on the trim and molding calculator.
Questions people ask
How wide should wainscot panels be?
There is no rule and this page states none. What it does is make every panel identical and tell you what identical comes out at. With the defaults — a 12 ft run, four panels, 3 in stiles — that is 32 in wide against 24-1/2 in tall. Whether that reads well is a judgement about the room, the ceiling height and what else is on the wall.
Should the end stiles be wider than the intermediates?
It is common, and there is a practical reason as well as a visual one. The end stile is what gets scribed into a corner that is never plumb, so extra width there is material you can lose. Making them wider also gives the remainder somewhere to hide, which is exactly what this page does with it. The alternative, where every stile matches, is printed too so you can see what it costs.
Why is the bottom rail taller than the top rail?
Partly so it can swallow the baseboard, and partly because you look at a wainscot from above rather than square on. A bottom rail cut the same depth as the top one foreshortens and reads thinner than it is. The defaults here are 7 in bottom against 3 in top, which is one common ratio out of many, not a specification.
How do I get equal panels when the wall is not a round number?
By letting the end stiles absorb the odd inches, which is what the calculator does. Fix the panel count and the intermediate stiles, and the two ends take whatever is left over, half each. The remainder ends up in the corners where a scribe was going to happen anyway. The other route — equal stiles everywhere and one narrow panel — is shown as a comparison line, and that narrow panel is always noticed.
Does changing the stile width fix a panel that is the wrong shape?
Hardly at all. On the default 144 in run, half an inch on every intermediate stile moves the panel width by about three eighths of an inch. Adding one panel moves it by seven. If the proportion is wrong, the count is the lever; the stiles are trim on the margin.