Charge size is a packing problem
Three independent divisions decide it: how many stacks go across the width once you allow clearance between and beside them, how many go end to end down the length, and how many go one on top of another before you run out of height. Multiply those and you have the stack count; multiply by the board feet in a stack and you have the charge. All three round down, and a chamber that is an inch short in any direction loses a whole row.
The clearance figure is where the width goes. A 12 foot chamber is 144 inches, which takes exactly three 48 inch stacks side by side with nothing between them. Ask for six inches of clearance and it takes two, with 42 inches spare, and the charge drops from 5,400 board feet to 3,600. What clearance your air path and your forklift need is a kiln design question the manufacturer answers, not this page. The page shows what your number costs you in rows.
Turnaround, not drying time
The number that governs how much a kiln produces is not the drying hours. It is the total hours the chamber is occupied, and that includes warming up, equalising, conditioning, cooling and the day spent loading and unloading. On a 240 hour drying schedule with 68 hours of everything else, the chamber is tied up for nearly 13 days for 10 days of drying, and the difference is 22 percent of the chamber time.
Those extra hours are charged per charge, not per board foot, which is why they punish fast schedules hardest. If you halve the drying time but keep the same 68 hours of overhead, you do not double the output. The page shows the drying share of the chamber time for exactly this reason.
What this page will not tell you
It will not tell you a schedule. Schedules are specific to species, thickness, kiln type and starting moisture content, and they come from the kiln manufacturer and from the drying literature. It will not tell you whether the lumber is dry, whether it is dry enough, or whether it is fit for any use. It will not say what a charge should cost. All of those are inputs, and it says so on every field that carries one.
The board feet per stack come from the stacking and sticker calculator. What the drying actually removes, in pounds and gallons, is on the drying water weight and shrinkage calculator. If you want the energy in a firing broken down segment by segment with a duty factor, that arithmetic is on the kiln firing cost calculator, which was written for ceramics but does the same job. And the sawing cost that sits upstream of all of this is the cost per board foot calculator.
Questions people ask
Why does the page round every direction down?
Because a stack that does not fit does not go in. If the width allows 2.8 stacks, you load two, and the remaining space is aisle whether you wanted it to be or not. Rounding down in all three directions is also why small changes matter so much: a stack an inch narrower can turn 2.8 into 3.0 and add a full row, which on a three row load is a third more lumber for no change in the schedule at all.
Should loading and unloading be in the schedule hours?
If the chamber is occupied during them, yes, because throughput is about chamber time rather than drying time. A kiln that is being emptied is not drying anything and it is also not available for the next charge. Some operations overlap this by having a second set of stacks ready to go straight in, which cuts the hours down but does not remove them. Put in whatever the chamber actually loses.
What moisture content should I dry to?
That is set by what the lumber is for and by whatever specification governs it, and this page has no view. It is not even a single number for a single use, since it depends on where the wood will end up living. Your schedule, from the kiln manufacturer or from a drying reference, will name a target and will tell you how to get there. This page only counts the hours you tell it the schedule takes.
The whole charges figure dropped by one when I changed the schedule slightly. Why?
Because whole charges is a floor division and it steps rather than slides. At 12.83 days a charge, 340 available days gives 26 charges with 6.4 days spare. Add half a day to the schedule and it is still 26, but add a full day and it becomes 24. That is why the leftover days figure is shown: it tells you how close you are to losing or gaining one, which is worth more than the charge count on its own.
Can I use this for air drying instead of a kiln?
Partly. The stack packing arithmetic works for a shed or a yard, and so does the board feet per charge. The turnaround does not, because air drying does not have a schedule with hours in it and its pace is set by weather over months. If you are air drying, the useful pages are the stacking calculator for the pile and the drying water weight calculator for what has to leave the wood.