What the aggregate is actually for
In a conventional gravel trench the stone is doing three jobs at once, and none of them is drainage in the ordinary sense.
It stores. The voids between the stones hold a dose of effluent while the soil takes its time accepting it, which is what stops a surge from simply running to the low end of the trench.
It spreads. The pipe has holes at intervals; the aggregate turns those discrete discharge points into a wetted line along the whole trench bottom.
It holds the trench open. Without it the walls and the soil above would close on the pipe, and the sidewall of the trench is part of the absorbing surface, not just the bottom.
The pipe, by contrast, is a distributor and nothing else. It is the cheapest component in the trench and the one people picture when they think of a drainfield, which is a fair summary of how backwards the popular mental model is.
The take-off
Volume is length times width times depth, and the depth is the sum of three numbers off the section: aggregate below the pipe, the pipe itself, and aggregate over it. Then subtract what the pipe displaces, because it is sitting in the middle of the stone and it is not stone.
That displacement is small but not nothing. Two hundred and forty feet of 4.5 inch outside diameter pipe occupies about 26.5 cubic feet, which is close to a cubic yard. On a short run it disappears into the waste allowance; on a long field with two pipes per trench it is worth carrying.
Converting to tons needs a weight per cubic yard from whoever is selling it. Washed drainfield stone, crushed base and river gravel are not the same weight, and the same material weighs meaningfully more after a wet week. The pit will tell you what they are loading; a number from a web page is a guess with a delivery charge attached.
The bit people forget: getting the spoil out of the way
A trench 3 feet wide, 24.5 inches deep and 240 feet long — the section this page opens with — is about 54 cubic yards in place, and soil swells when it is dug, commonly by a quarter or more depending on what it is, so what comes out is nearer 68 loose yards sitting beside the trench. Some of it goes back as cover. The rest has to go somewhere, and on a site where the whole point is that particular ground must stay undisturbed, "somewhere" needs thinking about before the machine arrives rather than after.
The general version of that arithmetic, for any dig rather than a trench section, is at the excavation volume calculator.
What is not in this take-off
Chamber systems, drip dispersal, mounds and sand filters do not have this section at all and this page does not describe them. A mound has its own volumes, which are at the sand mound volume calculator.
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.
To get from a flow and a loading rate to the trench length in the first place, use the drainfield area organizer. For the aggregate arithmetic in a completely different setting, the gravel driveway calculator handles compacted layers by the ton and the interior drain tile calculator covers a channel cut into a basement slab.
Questions people ask
How deep should the aggregate be under the pipe?
That is a design figure and it is on your drawing. It varies between jurisdictions and between systems, and it interacts with the trench width, the pipe, the depth to a limiting layer and the way the design intends the trench to store and spread a dose. This page takes whatever depth you enter and works out the material it comes to. If you are trying to work out what the number should be rather than what it costs, that is a conversation with your designer and your health department, and it is one that starts with a soil evaluation on your own ground.
Cubic yards or tons — which do I order in?
Ask the supplier which they sell in, because the conversion between them is their number and not yours. Aggregate is sold by weight in most places and by volume in some, and the weight per cubic yard depends on the specific material, its gradation, and how wet it is on the day. This page gives you volume first, because volume is what geometry produces, then converts using a density you supply. If you have to guess the density, expect the tonnage to be the least reliable line on the page and order in a way that lets you top up.
Does the geotextile go over the stone or around it?
That depends on the design, and both arrangements are used. Its purpose is to keep the cover soil from washing down into the voids between the stones and filling them, because a stone bed clogged with silt from above has lost the storage that was the reason for putting it there. What material, where it goes and whether it is used at all is specified rather than optional, and there are materials that must not be substituted for it. The overlap allowance on this page is a waste factor for ordering, not a specification.
Can I put the spoil back over the trench?
Sometimes, and it is a design question rather than a matter of convenience. What sits above a drainfield is part of how the system works: it is the layer through which the field breathes, it is what protects the aggregate, and it is the thing that gets compacted when anything drives over it. Some designs specify the cover material or how it is placed, and a heavy clay spoil going back over a field on lighter soil is not necessarily the same as what came out. Ask before assuming it is free fill.
Why does my calculated tonnage differ from the quote?
Several ordinary reasons before anything is wrong. The supplier may be quoting a different density or a different material. They may be adding their own waste allowance on top of yours. They may be sizing to full loads rather than to your exact figure, and a truck does not deliver two thirds of a load. And the section they were given may not be the section you typed — the commonest single cause is aggregate depth being measured differently, with or without the pipe included. Reconcile the section first and the rest usually falls out.