Why this page refuses to size your field
There is a version of this calculator on the internet that asks for your number of bedrooms and your soil type from a dropdown and returns a figure in square feet. It is worth understanding exactly what is wrong with that, because the error is not small and it is not conservative in a helpful direction.
A drainfield is sized from how fast the soil under your particular piece of ground will accept water and keep accepting it for decades. That is established by a test or a soil profile evaluation carried out on the site by someone licensed to do it, looking at a pit or a bore, reading the horizons, finding the seasonal high water table and any restrictive layer, and reporting a rate. It cannot be inferred from a soil name, from a county map, from what the neighbour got, or from what was there before. Soil varies across a single lot. The sizing factor that turns that rate into an area is set by the jurisdiction and differs between them. And the flow used is a permitted design flow set by the health department, not the flow your household happens to produce.
So this page takes the two numbers only you can supply, multiplies them, and arranges the result into trenches. That is genuinely useful — it lets you check a proposal, understand a quote, and see how much ground is going to be committed — and it is the whole of what can be done honestly without standing on the site.
The arithmetic, such as it is
Absorption area = daily flow ÷ loading rate
150 gallons a day at a loading rate of 0.6 gallons per square foot per day is 250 square feet. In three foot wide trenches that is 83 linear feet of trench, which at a 100 foot maximum is one trench, and at a 40 foot maximum is three of about 28 feet each.
The footprint is larger than the absorption area because trenches are separated. Three trenches at 9 foot centres occupy 21 feet across regardless of how narrow the trenches themselves are. Add a reserve area held clear for a replacement system and the ground committed can be several times the absorption figure. That gap between area and footprint is what catches people out when they look at a lot and assume it will fit.
Loading rate: the term that swamps everything else
Because area is flow divided by rate, the rate is in the denominator and small differences in it are large differences in ground. Halve the rate and the area doubles. The spread between the fastest soils a designer will accept and the slowest is not a few percent, it is several fold, and it is not a smooth line across soil classes — very coarse material can be rejected for accepting water too fast to treat it, and very fine material for not accepting it at all.
The sensitivity table this page prints is deliberately expressed as multiples of whatever rate you typed rather than as a list of soils with numbers against them. That is not coyness. Publishing a table of soil types against loading rates would be publishing a design factor, those factors are jurisdictional, and someone would use it.
What a failing field looks like, and what it is not
The common signatures are wet or unusually green ground over the field, sewage odour outdoors, slow drains throughout the house rather than at one fixture, and backup at the lowest drain. Any of those is a call to the health department and to a licensed contractor, not a project.
Some of what gets diagnosed as a failed field is not one. A single slow fixture is a fixture problem. A backup that clears and returns can be a blocked line, an effluent filter that has never been cleaned, or a tank that is overdue and passing solids downstream — which is the case where a pump-out fixes something. Water reaching the field it should not be getting is another: a leaking toilet, a sump discharging into the sanitary line, or roof water routed the wrong way can drown a perfectly good field. Check the actual household flow and the tank interval before concluding the field is finished, because those two are cheap and the field is not.
Surface water routing around the field is a real and separate matter. A french drain intercepting hillside water above a field is a normal thing to have, but where it is placed relative to the system is a design question, not a weekend decision.
Living on top of one
| Do | Do not |
|---|---|
| Keep grass over it and mow it | Plant trees or anything with aggressive roots on or near it |
| Route roof and surface water away from it | Send gutter downspouts, sump discharge or a pool drain anywhere near it |
| Know where it is, and where the reserve area is | Build a shed, deck, patio or driveway over either of them |
| Keep vehicles and livestock off | Park, turn a trailer, or store material on it — the compaction is permanent |
| Have it inspected on whatever schedule your health department sets | Assume that no symptoms means no problem |
Before any digging near a tank, a field or a supply line, call the national dig line and have the utilities located. Free, required in practice everywhere, and it takes days rather than minutes, so make the call before you rent anything. Once a drainfield is in the ground, keep vehicles, trailers, livestock and stored material off it. The gravel and the soil structure under it are what make it work, and compaction from a single truck parked on a wet field can end its life.
A septic tank is a confined space and it is lethal. The gases above the liquid displace oxygen and hydrogen sulphide paralyses the sense of smell before it kills, so there is no warning from inside. People die every year going in after a dropped tool, a phone, or to look at a baffle, and rescuers die going in after them. Nobody opens a tank and leans in, and nobody enters one. If something is in the tank, it stays there until a pumper with confined-space equipment and a trained standby crew deals with it.
Onsite wastewater systems are designed by a licensed designer or engineer from a soil evaluation carried out on your own ground, permitted by the county or state health department, installed to that permit and inspected before anything is covered. Nothing on this page is a design, a code figure, or a substitute for that process. A system installed without a permit is not only a health problem: it is an unpermitted structure that will surface at sale, in an insurance claim, and in any future permit application on the property, and the usual remedy is to dig it up.
Questions people ask
Can I work out my own perc rate?
You can dig a hole and time water going down it, and people do, and it will give you a number. What it will not give you is a number anybody will accept or that means anything for a design. A recognised percolation test has a specified hole geometry, a presoak period, a specified measurement interval and multiple test locations, and in many places it must be witnessed or performed by a licensed evaluator. Increasingly jurisdictions have moved away from percolation testing entirely towards soil profile evaluation — reading the horizons, texture and structure in a pit, and identifying the seasonal high water table and any restrictive layer — because the profile predicts long-term behaviour better than a timed hole does. Either way it is a site visit by a specific licensed person, and a hole in your back garden on a dry week in August is not it.
How much land does a septic system need?
More than the absorption area, and the gap is the part people miss. The absorption area is trench bottom. The footprint adds the space between trenches. A reserve or replacement area, where one is required, can add as much again. Then setbacks from wells, property boundaries, buildings, surface water, drainage ways and slopes remove ground from consideration entirely, and those distances are set locally and vary a great deal. The result is that a system whose absorption area is a few hundred square feet can commit several thousand square feet of a lot once everything is applied, and which particular ground it can commit is not a matter of preference. On a tight or awkward lot this question is the first one to put to a designer, before you buy anything or plan anything else.
Does a chamber, drip or mound system need less area?
They compute area differently, and that is not the same as needing less. Chamber systems are commonly credited with some reduction relative to gravel trenches, drip dispersal spreads water over a shallower and wider area on a different basis entirely, and mound systems exist precisely for sites where the natural soil cannot take a conventional field and they bring their own sizing rules along with a pump, a control panel and ongoing maintenance. The credits, the multipliers and whether a given technology is even permitted where you are all come from the jurisdiction. This page does one calculation, area equals flow over rate, and if your design is not of that shape then the page is the wrong tool and the designer is the right one.
What does the reserve area actually have to be kept free of?
Whatever your jurisdiction says, and the answer usually amounts to anything that would prevent a second system being built there: structures, paving, driveways, decks, pools, and often trees. The point is that drainfields have finite lives, and a lot developed with no room left for a replacement becomes a lot with no legal way to dispose of wastewater the day the first field stops working. That is why the requirement exists and why it is enforced at the permit stage rather than left to good sense. If you are planning an addition, a shop or a pool on a lot with a septic system, find out where the reserve area is before you draw anything.
The system was here when I bought the house and there is no permit on file. What now?
Talk to the health department, and do it before something forces the conversation. An unpermitted or undocumented system is a problem in three directions at once: nobody knows what is in the ground or where, which makes any future work on the property a guessing game; it will surface at sale, because a buyer's lender or the transfer inspection many jurisdictions require will ask; and it can complicate an insurance claim or any permit application on the property. Health departments deal with this situation constantly and generally have a process for evaluating and recording an existing system. It is far better handled on your own timing than during a sale.