Seam position is the decision, panel count is the consequence
Ask somebody what they notice about a mirrored wall and they will describe the seam. They will not describe how many panels it took. This matters because the two are set by different things: the panel count comes from the largest sheet you can get, which is fixed by the fabricator and the stairwell, while the seam position comes from how you choose to distribute the leftover, which is entirely yours.
Set out from one end and the seam falls at exactly the maximum panel width from that end. On a twelve foot vanity wall with a five foot maximum panel, that puts a seam sixty inches from the left, which on many bathroom layouts is close enough to the middle of the mirror to run down the middle of a face. Centre the layout instead and the two seams move symmetrically outwards, with a full panel between them. Same glass, same number of pieces, entirely different wall.
The three set-outs, and when each is right
Centred puts full-width panels in the middle and splits the remainder equally between the two ends. It is the default here because symmetry hides a seam better than anything else, and because two matching narrow panels at the ends read as a deliberate border rather than as a mistake.
All equal divides the run into identical panels. It is the right answer when the wall is a long run in a gym or a studio, where the seams become a rhythm and an odd one at the end would break it. It costs you nothing except that every panel is a custom width instead of some being the standard maximum.
From one end is what happens by default if you do not think about it, and it is genuinely the right choice when one end of the run is hidden — behind a door swing, above a cabinet, into a corner nobody stands in. Then you want the offcut there and the seams as far away as possible.
The end gaps are not optional
Mirror cannot be scribed. There is no way to shave a sixteenth off the edge on site, and there is no way to hide a bow in the wall behind a mitre. So the layout has to start by subtracting a clearance at each end, and whatever the wall does gets absorbed there and covered by a filler, a J-channel or a piece of trim.
How much to leave is a judgement about the wall in front of you. A newly framed and skimmed wall between two square returns might take an eighth each side. An older house where the two returns are out of parallel by half an inch over the run needs more than that, and needs the measurement taken at the height the mirror will actually sit rather than at the floor. The calculator subtracts whatever you put in before it does anything else, so it is worth measuring first and setting the field second.
Weight, and what holds it up
Quarter inch mirror runs about 3.25 pounds a square foot, so a five by three and a half foot panel is around 57 pounds and a twelve foot wall of it comes to about 136 pounds of glass. That is the number to have before deciding how the panels are being carried in and who is carrying them, and the page reports the heaviest single panel separately from the total for that reason.
What holds it up is not a question this page answers. Mirror mastic makers publish a coverage rate and a fixing pattern for the panel sizes they support, clip makers publish their own, and the two together decide the job. What is worth stating flatly is what does not work: ordinary construction adhesive. The solvents in it attack the silvering from the back, and the failure is slow enough that the wall looks fine when you leave and shows dark blooms creeping in from the adhesive lines a few months later.
What the calculator does not decide
It does not tell you whether a mirror in a given location needs to be safety glazing, whether it needs a backing film, or how it should be fixed. Those come from the code your jurisdiction has adopted, from the inspector, and from the manufacturers of the mastic and the clips. A large mirror that comes off a wall is a serious event, and a broken one is a floor covered in edges, so the fixing specification belongs to somebody who is prepared to stand behind it.
Questions people ask
How wide can a single mirror panel be?
Whatever your fabricator will cut, the truck will carry and the route into the room will take, and it is usually the last of those three that decides. A panel that a fabricator will happily make at seven feet is no use if it cannot turn the corner at the top of the stairs. That is why the maximum panel width is a field rather than a fixed number here: ask before you design around a size, because the answer is specific to your job.
Should the seam go in the middle or should a panel?
A panel, almost always. Two seams placed symmetrically read as a deliberate division; one seam down the centre reads as a mirror that was too small. The centred set-out on this page does exactly that — full panels in the middle, the leftover split equally at the two ends — and the page reports how far the nearest seam lands from the true centre of the wall so you can see what you are getting before ordering.
Can I butt mirror panels with no gap?
You can set the gap field to zero and the layout will assume it, but glass edges that touch are glass edges that chip, and a wall that is not perfectly flat will bring two panels into contact at one point before the rest. A sixteenth is a common working figure. What matters more is that it is consistent, because an uneven gap between two panels is more visible than a wide even one.
How much mirror mastic will I need?
That comes off the cartridge, not from here, because coverage varies a great deal between products and depends on the bead pattern the maker specifies. Put their square-feet-per-tube figure in and the tube count appears. The important part is using a mastic sold for mirror at all: ordinary construction adhesive attacks the silvering from behind, and the dark blooms that follow show up months after the job is finished.
Does a mirrored wall need to be safety glazing?
That depends on where it is and on the code your jurisdiction has adopted, and this page states no requirement either way. Mirror in some locations is treated differently from mirror in others, backing films exist for exactly this reason, and the person who can answer for your building is your glazier or your building department. Ask before ordering — a panel cannot be changed after it is cut, and tempered glass cannot be cut at all.