Two directions, one of them lumpy
Horizontal packing is smooth. Shave the gap between pieces and you gain a mug here and there, in ones. Vertical packing is not smooth at all: a level either fits under the lid or it does not, and when it does you gain a whole shelf of ware at once. That is why the page prints the leftover headroom, and why that line is the one to read first.
With the defaults — a 23.5 in round chamber 27 in tall, 21 in shelves, 4 in mugs 4.5 in tall with a half inch between them and an inch of clearance over the top — the shelf takes 12 pieces and the chamber gives 4 levels, so 48 pieces a firing. Each level wants 6.125 in: 4.5 of mug, 1 of clearance, 0.625 of shelf, and 1 in of bottom post plus four of those is exactly the 25.5 in available under the lid. Zero headroom to spare.
Now look at what the clearance is worth. Tighten it to half an inch and you still get 4 levels: nothing gained. Loosen it to an inch and a half and you drop to 3 levels and 36 pieces: twelve mugs gone for half an inch of air. The penalty is entirely one-sided at these numbers, and you would not know that without printing all three.
When it does flip
Change the piece to something 6 in tall and the pitch becomes 7.625, which gives 3 levels with an inch and five eighths of headroom sitting there doing nothing. That leftover is what the headroom line is for: it is not quite a level, so the only ways to use it are a shorter piece, a taller piece filling the top level, or a mixed load where one level takes the tall work and the others take mugs.
Mixed loads are how experienced people actually fill a kiln, and they are exactly what a single-piece-size calculator cannot model. Run the page once per piece size and treat the levels as a budget to divide up.
The round shelf
A circle is a bad shape to lay a grid on. The usable circle here is 21 in, set by the shelf rather than by the chamber — 23.5 in of brick less an inch of clearance each side would allow 21.5, so the shelf is the tighter of the two. Twelve four-inch mugs go on it in rows of 2, 4, 4 and 2, covering 55 percent of the shelf and leaving the edges empty. Nobody who fires regularly accepts that. They stagger the rows, they turn handles inward, they slot small pieces into the edge gaps, and they beat the grid by several pieces. Take the count here as a floor.
What is not in it
There is no view here on how close ware should sit to the elements, on whether two glazed pots at a half inch apart will end up stuck together, on what a shelf will carry, or on how a tightly packed load changes the way heat gets around the chamber. Those are questions your own kiln and your own glazes answer over a few firings, and a page that pretended otherwise would be lying. The wall clearance is a field for that reason rather than a constant.
Shelves and posts add up
Four levels on posts means four shelves and, for a top loader, eight half shelves, plus twelve posts at 5.5 in. Shelves are the expensive part of kiln furniture and the part that warps and gets ground down, and a chamber packed to the last level is one that has enough shelves to do it. If the count says you are two shelves short, that is a purchase before it is a packing problem.
Questions people ask
How many mugs fit in a kiln?
For a 23.5 in round chamber 27 in tall with 21 in shelves, 4 in mugs 4.5 in tall at half an inch apart with an inch of clearance over the top: 12 per shelf and 4 levels, so 48. The number is set by two things — how the footprint tiles onto the shelf, and whether the level height divides into the chamber. Change the piece height by an inch and the second one can drop you a whole level.
What is the right gap between pieces in a kiln?
That depends on what is on them and this page has no view on it. Bisque ware is often stacked touching or even nested; glazed ware needs whatever separation stops two pieces fusing, and that is a matter of your glaze, how thickly it went on and how it runs at your cone. The gap is a field on the form so you can put your own figure in it.
Does a smaller gap between shelves get me more in a load?
Only when it buys a whole level, and with the defaults it does not — half an inch of clearance and a full inch both give 4 levels. The penalty runs the other way, though: an inch and a half drops you to 3 levels and costs twelve mugs. The page prints the count at three clearances side by side so you can see which side of a step you are sitting on.
How many kiln shelves do I need?
One per level if the bottom row sits on posts, or one fewer if the lowest ware sits straight on the kiln floor. Four levels on posts is four shelves, which for a top loader means eight half shelves. The page counts posts too, at whatever number you use per level.
Why is my shelf only half covered?
Because a square grid on a round shelf leaves the edges empty and cannot help it. Twelve 4 in mugs on a 21 in circle, in rows of 2, 4, 4 and 2, cover about 55 percent of it. Staggering the rows and slotting small pieces into the edges beats that, which is why the count here is a floor rather than a target — it is what a grid gives, not what a careful hand gives.