Published
Drainfield size is a division. Your soil is the denominator.
The arithmetic, in one sentence
A drainfield is not sized by guesswork or by what the neighbor got. Idaho sizes it by division, and the rule says so in almost that many words:
“Total required absorption area is obtained by dividing the estimated maximum daily flow by a value below or as specified in the TGM.”
— IDAPA 58.01.03, Individual/Subsurface Sewage Disposal Rules, section 008.02.b. Idaho Department of Environmental Quality. Rule text as amended 1 July 2025. Retrieved 7 September 2026
Two numbers go in: how much wastewater the place generates on its biggest day, and how much wastewater a square foot of your ground can accept per day. Divide the first by the second and you have the absorption area, in square feet. Everything about drainfield size — why one cabin needs a pocket of ground and an identical cabin nearby needs a hayfield — falls out of that one division.
The rule's own rate table was re-read from the state's current PDF on 7 September 2026, in a rule text carrying 1 July 2025 amendment dates throughout. The word “TGM” is the Technical Guidance Manual, the state's design companion document — a designer can depart from the tabulated rates through it, but the table is the baseline every permit starts from.
The rate your soil sets
The second number in the division comes from the soil. The rules sort drainfield soils into three design groups plus an unsuitable pile, and assign each group one application rate:
| Design Soil Group | Application rate (gallons per sq ft per day) | Textures in the group |
|---|---|---|
| A | 1.0 | Coarse sand, medium sand, fine sand, loamy sand |
| B | 0.5 | Very fine sand, sandy loam, very fine loamy sand, loam, silt loam |
| C | 0.2 | Silt, clay loam, sandy clay loam, silty clay loam |
| Unsuitable | — | Gravel, sandy clay, silty clay, clay, shrink-swell clays, organic mucks, claypan, duripan, hardpan |
The pattern is intuitive once seen: water moves fast through sand and slowly through loam-and-finer ground, and the rate follows the percolation. A group A soil accepts five times what a group C soil accepts from the same square footage. The unsuitable row is not a slow rate but a refusal — gravel lets effluent through untreated, tight clays and hardpans barely let it through at all, so neither is a standard drainfield soil no matter how much area you offer.
The group call is made from the soil profile, which is one of the things the site evaluation exists to read. It is not a judgment an owner can eyeball from the lawn.
The same cabin, three sizes
Watch what the soil does to a perfectly ordinary dwelling. The rule's flow table puts a three-bedroom single-family dwelling at 250 gallons per unit, with an add-or-subtract of 50 gallons per day per bedroom — so a four-bedroom is 300, a two-bedroom is 200.
| Your ground | Rate | Area for 250 gallons a day | versus sand |
|---|---|---|---|
| Sand country (group A) | 1.0 gal/sq ft/day | 250 square feet | — |
| Loam country (group B) | 0.5 gal/sq ft/day | 500 square feet | twice the ground |
| Clay loam country (group C) | 0.2 gal/sq ft/day | 1,250 square feet | five times the ground |
Identical cabin, identical flow, five times the ground. That is the whole secret of why septic quotes on similar-looking lots differ so sharply: the lots are not similar underneath. It is also why a flow estimate worth arguing about matters at permit time — the same arithmetic that grows the field in better soil grows it again for every bedroom of optimistic sizing, a point that bites hardest on rental cabins permitted by bedroom count.
Where the flow number comes from
For a dwelling, the flow follows the bedroom count as described above. For everything else — lodges, bunkhouses, camp spaces, RV sites — the rule's flow table prices each unit of occupancy directly. The rows that matter around here, all from section 007.09 of the same rule:
| Unit of occupancy | Gallons per day | Reading it |
|---|---|---|
| Single-family dwelling, apartment or mobile home, 3 bedrooms | 250 per unit | Add or subtract 50 per bedroom — a four-bedroom is 300 |
| Travel-trailer space, water and sewer hookup | 125 per space | Twenty spaces is the flow of ten three-bedroom houses |
| Designated camp space, with showers | 90 per space | Showers are the difference between this and the next row |
| Designated camp space, toilets only | 65 per space | The cheaper utility set, priced accordingly |
| Hotel or motel bedspace, with private bath | 60 per bedspace | Private plumbing carries the higher figure |
| Hotel or motel bedspace, without private bath | 40 per bedspace | Central facilities share the load |
Run the same division on those and the scale of a commercial project becomes concrete: twenty RV spaces at 125 gallons a day is 2,500 gallons of design flow — the flow of ten three-bedroom houses, on ground that has to absorb every gallon of it. And because the application rate divides whatever the flow is, a project on group C ground needs five times the absorption area of the same project on sand. Flow and soil multiply; neither forgives the other.
Projects at that scale cross into the state's large-system rules, where sizing is only the beginning — that threshold and its consequences are covered on their own page, and the groundwater findings underneath the soil call decide the depth side of the same site.
What the division does not include
Two duties sit outside the arithmetic, and both surprise people who first meet them in a permit letter.
- The absorption area the division produces is one field's worth. The rules also require the site to hold a full replacement field for the day the first one reaches the end of its life — a site-size requirement with real consequences on small parcels, set out with the alternative-system discussion on the sand mounds page.
- The area has a shape, not just a size: the state's standard specifications cap how long a single distribution lateral may run and set minimums and maximums on trench dimensions, which live in the tables on the separation distances page. A 1,250-square-foot field is not one long slit across the lot; it is laid out as trenches within those caps.
- The soil group can vary across a single parcel, and the permit rests on the profile actually read at the test holes — which is why the evaluation location matters as much as the evaluation verdict.
Reading this against a lot you are considering
- Get the design soil group from the site evaluation before assuming any number — it is the single input that most changes the answer, and it is established by reading the soil profile, not by looking at the lot.
- Write down the honest daily flow: bedrooms for a dwelling, the per-space or per-bedspace figures from the rule's table for anything commercial, and the add-or-subtract of 50 gallons per day per bedroom in between.
- Divide flow by the group's rate to get the absorption area, then double the ground in your head for the replacement field the site must also hold.
- Compare that footprint against the buildable ground, keeping the trench shape rules in mind rather than picturing one rectangle carved out of the lawn.
- If the group comes back unsuitable, or the area will not fit the parcel, the conversation moves to alternative systems — not to a bigger version of the same field, which the rules do not allow.
- Check any designer's arithmetic the same way: their permit application states a design flow and a soil group, and the division above should reproduce their square footage from those two numbers.
- Treat any quote that predates the soil call as exactly what it is: a number without its denominator.
None of this is arithmetic an owner must do — the designer of record does it for the permit. But it is arithmetic an owner can do, which makes it the fastest sanity check on any bid or any lot. When someone tells you what a system will cost or whether a lot works, you now know which two numbers they divided to get there.
Common questions
How is drainfield size determined in Idaho?
By division: the estimated maximum daily wastewater flow divided by an application rate the soil sets. The rate is 1.0 gallons per square foot per day in design soil group A, 0.5 in group B, and 0.2 in group C.
What are the application rates by soil group?
Group A soils absorb 1.0 gallons per square foot per day, group B 0.5, and group C 0.2. The rates come from IDAPA 58.01.03 section 008.02, and the Technical Guidance Manual can specify other values.
Which soils are in each design soil group?
Group A is coarse, medium and fine sand plus loamy sand. Group B is very fine sand, sandy loam, very fine loamy sand, loam and silt loam. Group C is silt, clay loam, sandy clay loam and silty clay loam.
What soils are unsuitable for a standard drainfield?
Gravel, sandy clay, silty clay, clay, clays with high shrink-swell potential, organic mucks, and claypan, duripan or hardpan. These are refusals rather than slow rates: gravel passes effluent untreated and tight layers barely pass it at all.
How much drainfield does a three-bedroom cabin need?
A three-bedroom dwelling is 250 gallons per day, so it needs 250 square feet of absorption area on group A sand, 500 on group B loam, and 1,250 on group C clay loam. The same cabin, three sizes, depending entirely on the soil.
How does the flow estimate work for a lodge or RV sites?
The rule's flow table prices non-dwelling units directly: a travel-trailer space with water and sewer hookup is 125 gallons per day, a camp space with showers is 90, and a hotel or motel bedspace is 60 with private baths or 40 without.
Does the calculated area include the replacement drainfield?
No. The division yields one field's worth of absorption area. The rules separately require the site to hold a complete replacement field sized for the same flow, which is a site-size requirement on top of the sizing arithmetic.
Can a designer use different rates than the table?
The rule says the division uses the table value or a value as specified in the Technical Guidance Manual, the state's design companion. The table is the baseline permits start from, and departures run through the TGM rather than around it.
Why do septic quotes differ so much between similar lots?
Because the denominator differs. Two lots that look alike at the surface can sit in different design soil groups, and moving from group A ground to group C ground multiplies the required absorption area five times for the identical wastewater flow.