A pack is a depth problem, not a volume problem
The volume going onto a bedded pack is fixed by the animals. Twenty head at 0.6 cubic feet of bedding and 1.1 of manure each is 34 cubic feet a day loose, and after 40 percent settling the pack gains 20.4 cubic feet a day. Nothing about the building changes that number.
What the building changes is depth. Spread 20.4 cubic feet over a 40 by 30 foot pen — 1,200 square feet — and the pack rises 0.204 inches a day. A fifth of an inch. It looks like nothing and it is 1.43 inches a week and 6.1 a month, and from a bare floor to a 24 inch limit it takes 118 days. Just under four months.
The part that catches people out
Double the pen to 2,400 square feet and the interval doubles to 235 days — but the cleanout at the end is twice as big, 177.8 cubic yards instead of 88.9. Halve the pen and you get 59 days and 44.4 cubic yards. Three different pens, three different intervals, three different load-out sizes, and exactly the same annual figure: 276 cubic yards a year in every case.
That is not a coincidence and it is not approximate. Annual volume is head times daily volume times 365. Pen area appears in the depth calculation and cancels straight back out of the yearly total. A bigger pen buys you a longer run between cleanouts and a longer day when the cleanout comes. It does not reduce by one bucket what has to be hauled off the place over a year.
The limit is a measurement, not a rule
The depth field that everything hangs on is the one nobody can supply for you. It is whatever the pack fouls first in your particular building: the swing clearance under a gate, the bottom rail of a feeder, a door sill, the top of a wall panel, the point where a scraper or a bucket stops fitting. Go and put a tape on it. On the defaults it is set at 24 inches, and if the real answer in your shed is 16 the interval drops from 118 days to 78 and everything downstream moves with it.
Bedding is the input you control
Of the 34 cubic feet a day going down at the defaults, 12 is bedding and 22 is the animals. The bedding is the half of the equation that is a purchasing decision, and over the 118 day interval it comes to 1,412 cubic feet — 71 bales at 20 cubic feet each. Over a year it is 219 bales. Cutting bedding lengthens the interval and reduces the bales, and it also changes what the pack is, how it behaves and what it is like to stand on, which is a management question rather than an arithmetic one and is not answered here.
Where the arithmetic stops
A settling percentage is one number standing in for a process that is not uniform: the middle of a pack, where the animals lie, compacts far harder than the corners nobody uses. Real packs are deeper in some places by a foot. The page gives an average depth and an average date, and the real cleanout gets triggered by the deepest part rather than by the mean. Take the interval as a planning figure and keep a stick in the pack.
Questions people ask
How fast does a bedded pack build up?
At the defaults on the form — 20 head, 0.6 cubic feet of bedding and 1.1 of manure each per day, 40 percent settling, a 40 by 30 ft pen — the pack gains 20.4 cubic feet a day, which spread over 1,200 square feet is 0.204 inches. That is 1.43 inches a week. All three of those daily figures are yours to supply; the page does not hold any of them.
How long before a pack needs cleaning out?
It depends entirely on the depth at which it fouls something in your building, which is a measurement you take with a tape rather than a number this page has. At the defaults, going from a bare floor to 24 inches at 0.204 inches a day takes 118 days. Set the limit to 16 inches instead and it is 78 days.
Does a bigger pen mean less manure to handle?
No. It means fewer, larger cleanouts. The annual volume is the head count times the daily volume times 365, and pen area cancels out of it completely. On the defaults, a 1,200 square foot pen gives 118 day intervals of 88.9 cubic yards and a 2,400 square foot pen gives 235 day intervals of 177.8 cubic yards — and both come to 276 cubic yards a year.
How many loader buckets is a pack cleanout?
At the defaults, 88.9 cubic yards at a 1 cubic yard bucket filled to 80 percent is 112 loads. The fill percentage matters more than people expect on a pack, because material that has been walked on for four months does not scoop like loose bedding.
Why does the page ask for a settling percentage?
Because loose material going down and packed material sitting there are not the same volume, and the difference is large. Forty percent means 34 cubic feet loose becomes 20.4 in the pack. It is your own observation — a stick in the pack and a note of what went down will give you a better figure than any default, and packs settle differently by material, by species and by how wet the season is.