Storage period is the whole calculation
Daily volume is small and boring; the storage period is what turns it into a pad. Four animals at 2.5 cu ft a day between manure and bedding is 10 cu ft a day, which fits in three wheelbarrows. Hold it 30 days and it is 300 cu ft. Hold it 180 days and it is 1,800 cu ft, and after 20 percent settling that is 1,440 cu ft, or 53 cu yd — a pile you need a machine for.
The reason the period is a field rather than a constant is that almost nobody chooses it freely. It is set by when a spreader is available, when the ground is fit to drive on, when the neighbour who takes it has room, and in a lot of places by a season when nothing can be spread at all. Size the pad for the longest gap you actually expect, not the average one, because the pad that overflows does it in the worst week of the year.
Bedding is usually the bigger half
The default fields put manure at 1.5 cu ft a head a day and bedding at 1.0, and for a lot of bedded systems that ratio is the wrong way round. Shavings and straw go in dry and fluffy and come out only slightly compressed, and a deep-bedded stall stripped weekly can produce several times the volume of the manure in it. Both are fields because both vary by more than any single number could cover, and the only reliable way to get yours is to measure a real cleanout — count the barrows for a week and divide.
A free pile is a cone with a windrow in the middle
Material with a side slope will not stand vertically, so a free-standing pile is a windrow: a triangular ridge with a half-cone at each end. The two half-cones together make one full cone, which makes the volume easy to write down. With a side slope of s feet out per foot of height and a working height h, the ridge cross-section is s times h squared, and the two ends together hold pi times s squared times h cubed, divided by three.
On the defaults — 1,440 cu ft to store, 6 ft high, 1.5 to 1 side slope — the ends hold pi x 2.25 x 216 / 3 = 509 cu ft, leaving 931 cu ft for the ridge. The cross-section is 1.5 x 36 = 54 sq ft, so the straight section is 17.2 ft long. The toe is 9 ft out each side, so the footprint is 18 ft wide, 35.2 ft overall, and comes to 565 sq ft.
Working height matters more than it looks, because volume goes with the cube of it in the ends and with the square in the ridge. Stacking 8 ft instead of 6 on these numbers takes the footprint from 565 sq ft to 511, and it does it by turning a long low ridge into something close to a single cone — the straight section drops from 17.2 ft to 2.4 ft. Push the storage period out and the height matters far more, because the ridge is what grows. Whether you can stack 8 ft depends entirely on the machine, which is why the height is a field and not an assumption.
Push walls trade concrete for ground
Between vertical walls the same 1,440 cu ft at 6 ft high needs only 240 sq ft of floor rather than 565, because nothing is wasted on the slope. In a 20 ft wide bunker that is a 12 ft long pile with 44 ft of wall around it at 6 ft high — 264 sq ft of wall face to build. Whether that is worth it against 325 sq ft of extra pad is a straight cost comparison, and it usually turns on whether the walls have to resist a loader pushing into them, which is a structural question for whoever designs them.
Every drop that lands on the pad leaves the pad
A pad is impervious by design, so its whole area becomes runoff. The 706 sq ft pad on the defaults sheds 440 gallons in a one-inch rain, and 440 gallons of water that has run through a manure pile is not the same as 440 gallons of rain. Where that goes is a question for the extension service and the local authority, not for this page, but the number is worth having before the pad is laid out — it is often what decides where the pad goes.
The compost amendment calculator covers what happens at the other end, when the material is spread on ground. The stall bedding calculator works out the bedding volume going in, which is the input this page needs. If the pad is concrete, the concrete slab calculator handles the pour in more detail than the single thickness figure here.
Questions people ask
How big a manure pad do I need for four horses?
It depends far more on the storage period than on the horses. At 2.5 cu ft per head per day of manure and bedding, four animals make 10 cu ft a day. Thirty days is 300 cu ft and fits on a small pad; 180 days is 1,800 cu ft, 1,440 after settling, and needs about 565 sq ft of pile footprint plus a working apron. The per-head figure is yours to supply — measure a week of real cleanouts rather than taking a number off any page, this one included.
Should I use push walls or just pile it?
Walls store the same volume on less ground because nothing is lost to the side slope. On the default numbers a free pile needs 565 sq ft of footprint and a 20 ft bunker needs 240, so the walls save about 325 sq ft. Against that they are a structure that has to take a loader bucket pushing into them, which is an engineering question and a real cost. If ground is cheap and flat, the free pile is usually the cheaper answer.
How much does the pile shrink?
Enough to matter and not enough to rely on. The settling field defaults to 20 percent, which is a moderate figure for material that sits and compacts, but it depends on what the material is, how wet it is, how long it sits and whether anything is turning it. Setting it to zero sizes the pad for the loose volume, which is the conservative end and the one to use if you are unsure. The output shows both the loose and the settled figure so you can see how much of the answer rests on that assumption.
Does stacking higher really help?
Yes, and more so the bigger the pile. The end cones hold volume proportional to the cube of the height and the ridge to the square, so on the default numbers going from 6 ft to 8 ft takes the footprint from 565 sq ft to 511, and on a longer storage period the saving is much larger because the ridge is the part that grows. The limit is not the material, it is the reach of whatever stacks it — a wheelbarrow tops out around 4 ft and a compact tractor bucket around 7. Enter the height you can actually build, not the one that gives the nicest answer.
What about the runoff and where the pad can go?
The volume is straightforward: a pad is impervious, so an inch of rain on 706 sq ft is 440 gallons, all of it leaving the pad. What has to happen to that water, and how far the pad has to be from a well, a watercourse, a property line or a house, is decided by your local authority and your extension service and varies enormously between places. Get the footprint from here, then have that conversation before anything is built. Nothing on this page is a statement about what is permitted anywhere.