SizingKitThree minimums, one of them governs
Septic Tank Size Calculator
Minimum capacity for a residential septic tank, free and with no signup — and it is almost never the number the arithmetic produces. A three-bedroom house at 150 gallons per bedroom per day generates 450 gal/day, and one day of detention is 450 gallons — while the code table asks for 1000 and a great many county health departments ask for 1000 or 1250 whatever the bedroom count. So this page runs all three candidates side by side and names which one decides. Every figure it uses is editable, with its source printed beside it.
- 100% free
- No signup
- 1 to 6 bedrooms
- Names what governs
- Drain field area
Bedrooms, not occupants — a bedroom is what the house is permitted and sold with, and the next family to live in it is a different size from this one.
Default 150 — The design daily flow per bedroom used by most state onsite-wastewater codes for a single-family dwelling, on the convention that a bedroom represents two occupants at roughly 75 gal per person per day. States use figures from 100 to 150 gal per bedroom per day, and some size on occupancy or on fixture count instead. Actual measured indoor use is far lower — around 50–60 gal per person per day, and lower again in a house with modern fixtures — so the design figure carries a deliberate safety factor. The local health department's number is the one that governs the permit.
Default 24 — The conventional minimum hydraulic detention time for a septic tank at design daily flow: one day of flow must be held in the tank for solids to settle and scum to rise before the effluent leaves for the drain field. Some codes require 48 hours, and a two-compartment tank effectively lengthens it. Detention is only part of the sizing — the rest of the tank's volume is storage for the sludge and scum that accumulate between pump-outs, which is why the tank is two to three times the daily flow rather than one times it.
Onsite wastewater is permitted by the county or state health department, not by the plumbing code. Enter their floor and it joins the race below — it usually wins.
Deliberately blank. There is no default here and there should not be one: this rate is assigned to your lot by the agency that reads your percolation or soil-morphology test, and it ranges across ordinary soils by more than a factor of eight.
Only the bottom of the trench counts as absorption area, so the width you dig is what turns square feet into linear feet of laterals.
MINIMUM TANK CAPACITY
1,000 gal
3,785 L · set by the plumbing-code table
DESIGN DAILY FLOW
450 gal/day
1,703 L/day · 3 × 150
| Candidate | Gallons | On what basis |
|---|---|---|
| Detention arithmetic | 450 | 450 gal/day held for 24 hours |
| Plumbing-code table | 1,000 | 3 bedrooms, UPC private sewage disposal |
| Local health department | — | not entered — and it is the one that permits |
DRAIN FIELD, ABSORPTION AREA
Not computed. Enter the soil acceptance rate your permit assigns and this becomes design flow divided by that rate — the whole of the arithmetic, and the reason the rate is the only number that matters.
The table governs, and by a wide margin: 1,000 gallons against 450 from the detention arithmetic. The gap is not slack — the extra volume is storage for the sludge on the floor and the scum on the surface, which accumulate between pump-outs and which no daily-flow calculation sees.
These are floors, and in most of the United States they are not the operative floor. Onsite wastewater is regulated by the state or county health department rather than by the plumbing code, and a great many jurisdictions set a flat minimum of 1000 or 1250 gallons regardless of bedroom count, require two compartments, or add capacity for a garbage disposer, a whirlpool bath or a water softener discharge. The local health department's table governs and will usually ask for more than this one. Beyond six bedrooms, and for anything other than a single-family dwelling, the tank is sized from design flow directly and this table does not apply.
Absorption area is design flow divided by the soil's long-term acceptance rate: a 450 gal/day three-bedroom house on soil rated at 0.5 gal/day per square foot needs 900 square feet of trench bottom. This site does not supply the acceptance rate and no calculator should. It comes from a percolation test or a soil-morphology evaluation on the specific lot, it is assigned by the health department that reviews that test, and it varies by more than a factor of eight across ordinary soils — coarse sand accepts several times what a clay loam does, and dense clay is often rated unsuitable at any area. Treat the rate as an input the permit provides, and treat any figure typed into it before the perc test as an illustration.
A percolation test result is written in minutes per inch and is not the acceptance rate. The conversion from one to the other is a table the reviewing agency owns, it differs between states, and several states have replaced the perc test entirely with a soil-morphology evaluation that assigns the rate from texture and structure instead. This site does not reproduce any of those tables, because publishing one of them as though it were general is how a field gets built to the wrong state's arithmetic.
UPC (Uniform Plumbing Code) private sewage disposal provisions, Chapter 17 / Appendix K, capacity of septic tanks for single-family dwellings: 1 or 2 bedrooms, 750 gal; 3 bedrooms, 1000 gal; 4 bedrooms, 1200 gal; 5 or 6 bedrooms, 1500 gal.
How to find the tank a lot actually needs
Count rooms, check the design flow your state uses, then find out whose number wins.
Count bedrooms, not people
Onsite wastewater codes size on bedrooms because a bedroom is a permanent feature of a house and an occupancy is not — the family that buys it next may be twice the size. A room with a closet and an egress window will usually be counted as a bedroom by a reviewer whatever the plan calls it, and an unfinished basement that could become one is often counted too. If your local rule adds capacity for a garbage disposer, a whirlpool tub or a water softener discharge, add it to the health-department field rather than inflating the flow.
Set the design flow your state actually uses
150 gal per bedroom per day is the common figure and it is prefilled, but states use anything from 100 to 150 and some size on fixture count or occupancy instead. It is a design number carrying a deliberate safety factor — measured indoor use runs nearer 50 to 60 gallons per person per day, and lower again in a house with modern fixtures — so do not replace it with what your household really uses.
Read which candidate won, then add the drain field
The table shows the detention arithmetic, the plumbing-code table and your county's floor with the governing figure marked. For the absorption field, enter the soil acceptance rate from your percolation or soil-morphology report; there is no default and there should not be one, because that rate is assigned to your specific lot and varies eightfold across ordinary soils.
The figures behind the answer
| Code table | 750 gal for 1 or 2 bedrooms, 1000 for 3, 1200 for 4, 1500 for 5 or 6 — the UPC private sewage disposal provisions. The table stops at six bedrooms and this page stops with it. |
|---|---|
| Design flow default | 150 gal/day per bedroom, on the convention that a bedroom stands for two occupants at roughly 75 gal each. Editable, with the 100–150 span states actually use printed under the field. |
| Detention default | 24 hours at design flow — a full day held for solids to settle and scum to rise. Some codes require 48, and a two-compartment tank effectively lengthens it. Editable. |
| Why the table beats the arithmetic | Detention is only part of the volume. The rest is storage for sludge on the floor and scum on the surface that accumulate between pump-outs, which is why a code tank is two to three times the daily flow rather than one times it. |
| Drain field relation | Absorption area = design flow ÷ long-term acceptance rate. 450 gal/day on soil rated 0.5 gal/day per ft² needs 900 ft² of trench bottom. |
| Acceptance rate | No default is supplied, deliberately. It comes from a perc test or a soil evaluation on the lot, it is assigned by the reviewing health department, and it ranges by more than a factor of eight across ordinary soils — dense clay is often rated unsuitable at any area. |
| Trench arithmetic | Only the trench bottom counts as absorption area, so linear feet of lateral is the area divided by the trench width. A 900 ft² field in 3 ft trenches is 300 linear feet. |
| Not a permit | Reference tables and arithmetic. An onsite system is designed, permitted and inspected by a health department or a licensed designer, and nothing here substitutes for that review. |
Frequently asked questions
What size septic tank does a 3-bedroom house need?
1000 gallons under the code table, and possibly more under your county. The arithmetic asks for far less — 450 gal/day of design flow held for a day is 450 gallons — and the gap between the two is the point rather than a rounding: the extra volume is sludge and scum storage for the years between pump-outs, which no daily-flow calculation can see. Then check the health department, because a flat local minimum of 1000 or 1250 gallons regardless of bedroom count is common and it overrides both.
Why does the code count bedrooms instead of people?
Because a tank outlives an occupancy and a bedroom does not move out. Sizing on the current household would produce a system that fails the first time the house is sold to a larger family, and there is no mechanism for a health department to re-inspect on a change of occupant. A bedroom is also something a reviewer can count off a plan and an inspector can verify on site, which an occupancy is not — which is why a den with a closet and an egress window is usually counted as one however it is labeled.
Is the perc test result the same as the acceptance rate?
No, and conflating them is the most common error in drain field sizing. A percolation test returns a rate in minutes per inch of drop; the long-term acceptance rate is in gallons per day per square foot, and the conversion between them is a table the reviewing agency owns. Those tables differ between states, several states have replaced the perc test outright with a soil-morphology evaluation that assigns the rate from texture and structure, and publishing one state's table as though it were general is how a field gets built to the wrong arithmetic. Enter the rate your report or permit assigns.
Does a garbage disposer change the tank size?
In many jurisdictions yes, and the requirement is local rather than general. Common local amendments include a percentage uplift in capacity, a mandatory second compartment, an effluent filter, or a shorter pump-out interval; some codes ignore a disposer entirely. Because the size of that uplift is a local rule and not a national figure, this page does not apply one — put the resulting figure into the health-department field and it enters the race with the others.
What happens above six bedrooms?
The table stops, and so does this page. Beyond six bedrooms, and for anything that is not a single-family dwelling, capacity is worked from design flow directly against a commercial schedule that asks for seats, fixtures, employees or occupants rather than rooms — a restaurant and an office of the same square footage are not the same load, and neither is a bedroom count. The detention figure the page still shows is checkable arithmetic; the code column goes blank because the code has no row to read.
How big does the drain field have to be?
Design flow divided by the acceptance rate — 450 gal/day on soil rated at 0.5 gal/day per square foot gives 900 square feet of trench bottom. The whole difficulty lives in that divisor: coarse sand accepts several times what a clay loam does, so the same house needs a field several times larger on one lot than on another two hundred meters away. That is why no calculator should offer a default for it, and why the field on this page is blank until you fill it from your soil report.
Can this replace a designer or a permit?
No. It reproduces the tables and does the arithmetic so you can check a proposal or budget a job, but an onsite wastewater system is permitted on the strength of a soil evaluation, a site plan and a design that a health department or a licensed designer signs. Setbacks from wells, property lines, surface water and foundations, the reserve area a repair will need, and whether the lot needs a pressure-dosed or a mound system rather than a conventional trench are all decisions this page does not touch.
Why the calculation almost never decides
Septic sizing looks like arithmetic and is mostly jurisdiction. The arithmetic is a detention-time calculation: work out the design daily flow, hold it in the tank long enough for the heavy solids to settle to the bottom and the grease to rise to the top, and let the clarified middle leave for the absorption field. At 150 gallons per bedroom per day and a day of detention, a three-bedroom house needs 450 gallons of tank. Nobody builds one. The code table calls for 1000, and the reason is that detention is only one of the two jobs the tank does — the other is storing the sludge and scum that accumulate between pump-outs, which is a multi-year accumulation that no daily-flow figure sees. That is the whole explanation for a tank sized two to three times the daily flow rather than once.
Above that sits a second layer, and it is the one that actually issues the permit. Onsite wastewater is regulated in most of the United States by state or county health departments rather than by the plumbing code, and their rules routinely exceed the code table: a flat 1000 or 1250 gallon minimum whatever the bedroom count, a mandatory second compartment, an effluent filter, extra capacity for a garbage disposer or a water softener discharge. So there are three candidate numbers for any given house and the largest of them is the answer — which is why this page shows all three and marks the winner rather than printing one figure and hiding the decision. The one that governs is usually the one only you can supply.
The absorption field is where a calculator has to stop. Its area is design flow divided by the soil's long-term acceptance rate, and that rate is not a property this site can look up: it is assigned to your particular lot by whoever reads your percolation test or soil-morphology evaluation, it spans more than a factor of eight across ordinary soils, and dense clay is frequently rated unsuitable at any area at all. A calculator that offers a default for it is guessing at the only number that matters. Where the system needs a pump rather than gravity — a tank below the field, or a mound — the head it has to make is a pump head calculation rather than a septic one, and the gravity line feeding the tank is sized by the drain pipe slope calculator.
What this page knows about your property
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Nothing identifying the lot is asked for and nothing is stored — no address, no parcel, no soil report. The bedroom count and the rate from your permit stay in the tab.