What a mound is doing
A mound exists because the ground will not do the job on its own. Shallow soil over rock, a seasonal water table close to the surface, soil that accepts water too slowly or too fast to treat it — in each case the natural profile does not give the vertical separation and the residence time that treatment needs. So the design builds the missing profile above grade out of specified sand, puts a pressurised bed on top of it, and lets the effluent work its way down through the sand and into the original soil across the whole footprint.
Two consequences follow and both matter for the arithmetic. The sand is a treatment medium, so its gradation is specified and substituting it is not a saving, it is a change to how the system works. And the effluent has to arrive evenly across the bed, which is why every mound has a pump, a dose and pressure laterals rather than a pipe from a tank.
The geometry, and why the shoulders dominate
The mound is a truncated pyramid: a flat top the size of the bed, sides falling away at a stated slope, sitting on grade. Its cross-sectional area at any height is
A(h) = (L + 2z(H − h)) × (W + 2z(H − h))
where H is the total height and z is the horizontal run per foot of rise. That is a quadratic in h, which means the prismoidal formula — end areas plus four times the middle, over six — gives the volume of any slice exactly rather than approximately. This page uses it, and the sand, aggregate and cap volumes it produces add back to the whole mound exactly, which is a useful thing to be able to check.
The reason the shoulders dominate is that the run per side is z times the whole height, and it applies on all four sides. The section this page opens with — a 50 by 10 foot bed, 12 inches of sand, 9 of aggregate and a 12 inch cap, so 2.75 feet of total height, on 3 to 1 slopes — gains 8.25 feet on every side. The footprint becomes 66.5 by 26.5, which is 1,762 square feet against a bed of 500. Over seventy percent of the ground the mound occupies is slope, and most of the sand is under it rather than under the bed.
Flattening the slope makes this worse fast. Going from 3 to 1 to 4 to 1 on that same mound adds another 2.75 feet to every side and takes the footprint from 1,762 square feet to 2,304, with a corresponding pile of sand, and the bed has not changed at all.
Ordering
Sand is sold loose and placed compacted, so the order is larger than the in-place volume by a compaction allowance. The right figure comes from the supplier and the placing method rather than from a general assumption, which is why it is a field here. Sand is also normally sold by weight, and the weight per cubic yard depends on the gradation and on how wet it is when it is loaded.
Cap soil is the other bulk item and it is easy to underestimate because it covers the sloped sides as well as the top. The cap volume this page reports is the whole shell above the bed, slopes included, not a slab over the top.
For a plain dig rather than a built-up mound, the excavation volume calculator handles bank yards, swell and truck loads.
On sloping ground
Most mounds are on a slope, because most sites that need a mound are not flat. The sand depth under the bed is then different on the upslope and downslope edges and the design will give both. Averaging them, as this page does, is exact for the bed itself.
It is not exact for the downslope shoulder, because the ground under it keeps falling away and the toe runs out further than a level-site calculation predicts. On any appreciable slope the toe position and the downslope material come from the design and the survey, not from a rectangular approximation. The figures here will be low on the downslope side.
What this page will not do
Not one design figure appears on this page, and that is deliberate. Dose volumes, orifice sizes, loading rates, sand depths, trench widths, separation distances, reserve areas and pumping triggers are set by the county or state health department and worked out for your particular ground by a licensed onsite designer, engineer or installer. Every figure the page uses is one you typed in off your own approved design or your own measurement. The page arranges arithmetic; it does not decide anything, it does not check anything against a rule, and it will never tell you whether what you have is right.
Nothing here is a verdict. The page does not say a system is compliant, adequate, undersized, failing or working, because it cannot see your site, your soil, your permit or your local rules, and because that judgement belongs to people who are licensed to make it and who carry the responsibility for it.
Call 811 and have the utilities located before any digging, and allow days rather than an afternoon for it. Trench walls collapse without warning and buried people are recovered, not rescued. Once anything is in the ground, keep vehicles, trailers, stock and stored material off the field and off the reserve area: the soil structure is the working part of the system and compaction from one truck on wet ground is not repairable.
Septic and pump tanks are confined spaces and they kill people. The atmosphere above the liquid has no oxygen worth breathing and the hydrogen sulphide in it destroys the sense of smell in seconds, so there is no warning once you are over the opening. The recurring pattern is somebody reaching in for a dropped phone or float and a second person dying trying to pull them out. Nobody leans over an open tank and nobody enters one, ever, for any reason. An open or cracked lid is separately a fall-in hazard that has killed children, so a lid comes off only when a crew is standing over it and it goes back secured. Raw sewage is a disease exposure and effluent on the skin or in a cut is treated as one.
The rest of a mound system: the dose volume and cycle calculator for what the pump sends and how often, the total dynamic head calculator for what the pump has to produce, and the orifice flow calculator for what the laterals in the bed will pass. For a conventional gravel trench field instead, the material take-off is at the drainfield aggregate and pipe calculator and the area arithmetic at the drainfield area organizer.
Questions people ask
How deep should the sand be?
That comes from the soil evaluation and the design for your site, and it is one of the figures that varies most between jurisdictions and between sites. The sand is providing treatment that the natural profile cannot, so its depth is tied to what the evaluation found — the depth to a restrictive layer or seasonal water table, and how the receiving soil behaves. There is no general answer, no way to infer it from a soil name, and no version of it that survives being copied from a neighbouring property. This page takes whatever depth is on your drawing and works out the material.
Why is the footprint so much bigger than the bed?
Because the side slopes run out by the slope ratio times the full height, on all four sides. A mound 2.75 feet high on 3 to 1 slopes adds 8.25 feet to each side, so a bed 50 by 10 sits inside a footprint 66.5 by 26.5. That is over three times the area of the bed, and the extra is entirely slope. The practical consequence is that a mound commits a great deal more ground than its bed dimensions suggest, and setbacks and the reserve area are applied on top of that. How much ground your site actually needs is a question for the designer, and on a tight lot it is the first question.
Can I use ordinary fill sand?
No. Mound sand is a specified gradation and it is doing the treatment, so substituting it changes what the system does rather than just what it costs. Too fine and it will not accept the loading; too coarse and the effluent passes through faster than it is treated. Designs state the specification, suppliers can normally provide material with documentation against it, and inspectors commonly want to see that documentation before the mound is built over. This is one of the places where the cheaper load is not a saving, because the failure it produces surfaces years later and the remedy is rebuilding the mound.
Does the compaction allowance go on the aggregate and the cap too?
The page applies it to the sand only, because that is where it is normally significant and where suppliers quote against it. Aggregate placed in a bed is not compacted in the same sense, and cap soil placed over a mound is placed and settled rather than compacted to a specification, with the settlement usually managed by placing a little proud and coming back. If your supplier or your installer works to different allowances, the honest thing is to use theirs — every figure on this page is one you supply for that reason.
How much of the mound can I mow or plant?
Mounds are normally grassed and mown, and that is part of why the slope ratio is chosen — a steep mound is a hazard to mow and a difficult one to keep vegetated. Beyond that, what may be planted, driven on, built near or stored on is a matter for the design and the health department, and the general rule on any onsite system applies here with more force because the mound is the treatment: nothing with aggressive roots, nothing with wheels, and nothing stored. Compaction of the sand or the receiving soil under the toe is not repairable, and the mound is on the surface where people forget it is a system at all.