Septic Tank and Drainfield Calculator

A septic system is not sized from the number of people in the house, or from its floor area, or from its fixtures. It is sized from the bedroom count, and the state document says exactly why:

Design flow is estimated by multiplying the number of household bedrooms by 150 gallons per day (gpd). This is based on 75 gallons per person per day for two people in each bedroom. This accounts for the number of people that can occupy the home for extended periods rather than how many actually live there when the system is installed.

75 gallons a person, 2 people a bedroom, 150 gallons per bedroom per day. And the reason it is bedrooms rather than occupants is in the last sentence: the system has to serve whoever can live there over its life, not whoever is standing in the kitchen on the day it is designed.

Then the tank table is that number and two rules:

The minimum septic tank capacity is two times the daily wastewater flow using 150 gallons per bedroom or 1,000 gallons, whichever is larger. Tanks sized at three times daily flow are recommended and shall be required when garbage disposals are used.

Every printed cell reproduces. Four bedrooms is 600 gallons a day, so twice that is 1,200 and three times is 1,800, which is what the table says. The 1,000 gallon figure in the top row is the floor rather than the arithmetic: three bedrooms would only need 900.

And read the last clause of the quote, because it is a decision somebody makes in a kitchen showroom: three times daily flow is required when garbage disposals are used. A waste disposal unit moves you out of the minimum column and into the recommended one, which on a four bedroom house is 600 gallons of extra tank.

The drainfield is sized from a percolation test, which measures how fast water moves into the soil:

The perc test really measures an infiltration rate for water into a wet but unsaturated soil at the depth of expected system placement.

The published table gives an area per bedroom and a loading rate for each band of perc rate. Multiply the two:

Every row comes out at 150, which is the design flow for one bedroom. So the area column is not independent data: it is the flow divided by the loading rate, and the loading rate is the only real measurement on the table.

Which means the whole sizing of a septic system, tank and field together, comes out of one assumption about how many people a bedroom holds and how much water each of them uses.

The perc rate is the part that is genuinely about your ground, and it can disqualify it. Faster than five minutes an inch and the soil will not treat the effluent before it reaches groundwater. Slower than sixty and it will not accept it at all. Both ends are marked not recommended for a conventional system, which means an engineered alternative and a different order of cost.

The perc rate is the only genuinely site-specific number in this calculation, and both ends of its range are refusals rather than sizes.

Faster than five minutes an inch and the standard does not recommend a conventional absorption system. Water moves through that soil too quickly to be treated before it reaches groundwater, so the problem is not capacity, it is that the field would not do its job.

Slower than sixty minutes an inch and it is the opposite: the soil will not accept the effluent at any reasonable area, so a conventional field simply backs up.

Either answer means an engineered alternative, a designer, and a different order of cost. Mounds, sand filters, aerobic treatment units and drip dispersal all exist for exactly these soils and none of them are sized from a table on a web page.

Which is why the perc test comes first, before the house is designed and ideally before the lot is bought. The document is explicit that the wastewater system area should be chosen prior to any construction on a site.

The published separations turn a large lot into a much smaller buildable one, and the well distances do most of the damage.

Fifty feet from a private water supply well is required and a hundred is recommended. If the well serves a public supply it is a hundred required and two hundred recommended. On a property with its own well that is a circle you cannot put any part of the system inside, and it is often in the middle of the best ground.

Twenty feet from the absorption system to the dwelling foundation is required and fifty is recommended, which is a large gap on a small lot, and ten feet from the tank to any building.

And nothing permanent may go over any of it. No patio, no driveway, no building, over the tank or the laterals or anything else. That is worth knowing before the field is put where the garage will eventually be.

The figure on this page draws the well setback and the field to the same scale, which is usually more persuasive than the numbers.

A septic system needs land, and not just its own footprint. The published separations are what turn a large lot into a small buildable one:

The well distances are the ones that shape a site. Fifty feet from a private water supply well is required and a hundred is recommended, and if the well serves a public supply it is a hundred required and two hundred recommended. On a lot with its own well that is a circle you cannot build the field inside.

And the field needs a replacement area. The document is explicit that the wastewater system area should be chosen before any construction and that space should be available for the absorption field and its replacement. A field is not permanent, so a lot that fits exactly one is a lot with a problem in twenty years.

One more that catches people after the house is built:

There shall be no permanent structure (patio, building, driveway, etc.) over the tank, lateral or other part of an onsite wastewater system.

No patio, no building, no driveway over any part of it. Which is worth knowing before the field is put where the garage will eventually go.

Every figure on this page is one state's. Septic is regulated state by state and frequently county by county, the tables differ, and a page that presented these as national numbers would be wrong in most of the country.

What generalises is the structure rather than the values: a design flow per bedroom, a tank sized as a multiple of it, a field sized from a perc test, and a set of separation distances. Every state's standard has those four parts and fills them in differently.

So use this to understand the shape of the answer and to check a designer's arithmetic, not to size a system for a permit. The county health department or sanitarian has the numbers that apply to your ground, and on most sites a perc test and a soil profile are required before anything can be designed at all.

The calculator lets you override the design flow per bedroom and the loading rate for exactly this reason. Put your own jurisdiction's figures in and the arithmetic still works.

One small thing the document mentions that nobody counts:

When calculating wastewater flow, note that a water softener may increase water use by as much as 10 gallons per capita per day.

Read 2026-09-10. Bulletin 4-2, Minimum Standards for Design and Construction of Onsite Wastewater Systems, Kansas Department of Health and Environment, Bureau of Water, Nonpoint Source Section, in cooperation with K-State Research and Extension. Publishes the design flow assumption, the septic tank capacity table and the absorption field loading rates by percolation rate, plus the separation distances. Used because it is a state regulator's own document and is free to read, which most septic standards are not.

State onsite wastewater standards are mostly published, which makes this a rare area where a regulator's own document is free to read. That is why Kansas is here: not because Kansas is typical, but because Kansas publishes.

The tank detention requirement behind all of it is worth quoting, because it explains why the multiples are what they are:

The septic tank is sized so that wastewater flow through the tank takes at least 24 hours even with sludge and scum accumulation.

Twenty four hours of settling, maintained even as sludge and scum accumulate. That is what the two and three times daily flow are buying, and it is why a tank that is technically large enough on the day it is installed stops being large enough as it fills.