The arena is the drainage structure. The footing is a wear layer.
Almost every argument about a riding arena is about the top four inches, and almost every arena that fails does so below them. The footing is loose material that gets dragged around and topped up; it holds no shape and carries no load once it is wet. What decides whether the surface is rideable in March is the compacted base underneath it and the fall built into that base, both of which are finished and buried before the first load of sand arrives.
That ordering is why this page exists separately from the arena footing calculator. That page sizes the wear layer at a depth you choose. This one sizes the structure it sits on, and the grade that gets the water off it.
Why a crown costs half of what people expect
A crown is a triangular prism, not a slab, and this is the single most common arithmetic error on an arena job. Take a 200 by 100 ft arena at a half percent fall, crowned down the centreline. The half width is 50 ft, so the rise at the centre is 0.5 percent of 50 ft, which is 0.25 ft or three inches. The cross-section of the crown is a triangle 100 ft wide and 0.25 ft tall, so its area is half of 100 times 0.25, which is 12.5 sq ft. Multiply by the 200 ft length and the crown is 2,500 cubic feet, or 92.6 cubic yards.
Treat it as three inches of extra base everywhere and you get 5,000 cubic feet — exactly double. At 105 lb per cubic foot and a 20 percent loose allowance, that is the difference between about 158 tons and about 315 tons, which is seven truck loads of material bought for nothing.
| Profile at 0.5% on a 200 x 100 ft arena | Rise | Wedge volume |
|---|---|---|
| Crown at the centreline, falling both ways | 3 in at the centre | 92.6 cu yd |
| Single cross-fall, high on one long side | 6 in across the full width | 185.2 cu yd |
| Flat | None | None |
A single cross-fall costs exactly twice a centre crown at the same percentage, because the span the fall works over is twice as long and the rise at the high side is twice as high. It also puts all the water on one long side, where a centre crown splits it between two. Neither is automatically right; the site decides, and where the water goes after it leaves the arena decides more than either.
Build the crown in the subgrade or in the base
The switch on the form is a real decision with a real number attached. Shape the crown into the subgrade first and the base course goes on at an even thickness over a surface that is already the right shape: you buy no extra base, you move the same wedge of earth in cut and fill, and the layer above stays uniform, which is what a compaction spec wants. Build the crown by varying the base thickness and you buy the wedge, and the base is three inches thicker at the middle than at the edges.
The earthwork side of the subgrade option has its own arithmetic in the cut and fill calculator and the excavation volume calculator, and the offsets in the output above are the numbers a grade stake needs. For the stakes themselves, the grade stake cut and fill calculator converts an instrument reading into a cut or fill at the stake, and the site layout squaring calculator gets the rectangle square before anything is shot.
Loose, compacted and the number the quarry quotes
The two volumes on this page are not interchangeable. Compacted volume is what the finished layer occupies: length times width times the compacted thickness. Loose volume is what you have to buy, and it is larger, because material placed and rolled loses air. Twenty percent is a common working allowance and it is not a constant — a well graded crushed base with fines loses more than clean screenings, and the supplier or the contractor placing it is who knows.
Density is where the tonnage is decided. It is a loose delivered figure in pounds per cubic foot, it varies with gradation, source and how wet the material is when it is weighed, and asking the quarry for theirs is a two-minute phone call that moves the invoice by a third. The page prints tons per cubic yard for whatever density you enter so a quote by the yard and a quote by the ton can be put side by side, which is the only way to compare them.
What is not on this page
The perimeter drainage, the outfall, the subgrade preparation and whether the base needs a separation fabric are all outside it. A trench and a perforated pipe around the arena is the french drain calculator. Fall along a run with the string-line offsets at each station is the hardscape slope drainage calculator. What thickness of base a given subgrade needs, what gradation, and how it is compacted are engineering questions for the people designing the arena, and this page has no view on any of them.
Questions people ask
How much extra base does a crown add?
Half of what a slab of the same rise would add, because the cross-section is a triangle. The volume is length times width times rise, divided by two. On a 200 by 100 ft arena at a half percent centre crown, the rise is 0.5 percent of the 50 ft half width, which is three inches, so the wedge is 200 times 100 times 0.25 ft divided by two, or 2,500 cubic feet. That is 92.6 cubic yards, about 158 tons at 105 lb per cubic foot with a 20 percent loose allowance. The common mistake is multiplying the rise by the whole area, which doubles the order.
Should the crown be shaped in the subgrade or built up in the base?
Both are done, and the arithmetic differs enough to be worth deciding on purpose. Shaping the subgrade means the same wedge of earth is moved as cut and fill and the base course goes on at an even thickness, which is easier to compact to a consistent result and buys no extra imported material. Building it in the base means the base varies in thickness across the width and you buy and haul the wedge. Which one suits depends on the subgrade material, the equipment on site and what the arena designer specifies. The switch on the form prints the tonnage for one and the cubic yards moved for the other.
What is the difference between compacted and loose volume?
Compacted volume is the space the finished layer occupies once it has been rolled: area times the compacted thickness. Loose volume is what has to be delivered, and it is larger because placed material loses air under compaction. The allowance depends on the gradation and the effort applied and it is commonly in the region of fifteen to twenty five percent, but it is a property of the specific material, not a universal number. The contractor placing it and the supplier producing it both have a view, and both are better sources than any published figure.
Does a cross-fall really cost twice a centre crown?
At the same percentage, yes, and the reason is geometric rather than accidental. A centre crown works over half the width in each direction, so its rise is the fall times half the width. A single cross-fall works over the full width, so its rise is twice as high, and the wedge is a triangle over a span twice as long with a peak twice as high. Both effects point the same way. The trade is that a cross-fall sheds all the water to one side, which may be exactly what a site with a fall in one direction needs, and it avoids a ridge down the middle of a working surface.
How do I use the grade offsets in the output?
They are the drop from the high point to each station across the fall, printed in feet and inches so a string line or a stake can be set to them. Set the high point first — the centreline for a crown, the high long side for a cross-fall — then measure out to each station and set the grade that far below it. With a crown the offsets mirror to the other half, so one table does both sides. They describe the finished surface shape, not the depth of any layer, so the same offsets apply to the subgrade, the top of the base and the top of the footing if the whole section is built parallel, which is the usual intent.