Septic Tank Size: How Many Gallons You Need
Short answer: for a home, the septic tank is sized from the number of bedrooms. Many state codes set a minimum of 1,000 gallons for a typical house and add capacity for each bedroom above a set number. Washington, for example, requires at least 1,000 gallons for up to four bedrooms and 250 gallons more for each additional bedroom. Your state's or county's table is the one that counts, so check it before you buy a tank.
This guide explains how tank sizing works, why it uses bedrooms instead of people, what changes the size, and how to find the number that applies to your property.
How septic tank size is worked out
A septic tank has one job: hold the wastewater long enough for solids to settle to the bottom and grease to float to the top, so that only the clearer liquid goes on to the drain field. The bigger the tank compared with the daily flow, the longer the wastewater stays, and the more room there is for sludge between pump-outs.
So the size starts from the daily design flow, the volume of wastewater the building is assumed to make on a busy day:
- Bedrooms set the flow. Most rules assign a flow per bedroom per day. The EPA's mound fact sheet describes typical design flows for homes of up to 150 gallons per day per bedroom; some states use less. landplan.io uses 120 gallons per day per bedroom unless your area sets another value.
- The flow sets the tank. The code turns the flow, or the bedroom count directly, into a minimum liquid volume, usually through a table.
- Extras add to it. Some codes add capacity for a garbage disposal, a large tub or a commercial use.
Examples from two state rules
The two tables below are real, and they show how close yet different the rules can be. They are examples, not advice for your state.
Washington (WAC 246-272A-0232, Table VII):
| Bedrooms | Minimum liquid volume |
|---|---|
| Up to 4 | 1,000 gallons |
| Each additional bedroom | add 250 gallons |
Nebraska (as summarized by University of Nebraska–Lincoln Extension):
| House | Minimum tank |
|---|---|
| Any house | 1,000 gallons |
| 3 bedrooms, no whirlpool | 1,000 gallons |
| 4 bedrooms, no whirlpool | 1,250 gallons |
| With a whirlpool bath | add 250 gallons |
So a four-bedroom house needs at least 1,000 gallons in Washington but 1,250 gallons in Nebraska, and a five-bedroom house needs at least 1,250 gallons in Washington. Other states use other steps. To see the rules for your state, start at septic rules by state.
Why bedrooms and not people?
People move in and out; bedrooms stay. A house sold to a family of six has to work as well as it did for a couple. Counting bedrooms gives the health department a number it can check on the house plans, and it sizes the system for the house's capacity rather than its current residents.
That's why it matters what counts as a bedroom. Many health departments count any room that could be used as one, such as an office or a den with a closet. Ask before you label rooms on the plans.
What else changes the size
Commercial and other uses
A business is sized by its use: a flow per seat, per employee, per meal or per site. A restaurant's grease and a salon's chemicals may call for a separate grease interceptor or pretreatment. These flows come from your state's tables.
Compartments and tanks in series
Many codes require or prefer a two-compartment tank, or two tanks in series, so that solids that escape the first chamber settle in the second. The UNL Extension page notes that tanks may have one, two or three compartments, and that multiple tanks can be connected in series. Where two tanks are allowed, their volumes together usually have to meet the minimum.
Pumped systems and treatment units
A pump does not live in the septic tank. A pressure system, a mound or a drip system adds a dose tank (pump tank) after the septic tank, sized for the dose and a reserve. An aerobic treatment unit has its own chambers, set by its manufacturer and approval. These are separate from the septic tank size. See Dose Tank and Floats.
Effluent filters and risers
An effluent filter at the outlet doesn't change the size, but many codes now require one, along with risers that bring the lids to the surface for pumping. Plan for them in the budget.
Nominal size vs working capacity
Tanks are sold by a round number, such as 1,000 gallons, but codes usually talk about the liquid volume: the volume below the outlet. A manufacturer's spec sheet gives both, and a tank's actual working capacity can differ from its name. When a code sets a minimum, compare it with the liquid capacity on the spec sheet, and check that the model is on your state's approved list if it keeps one.
Tank sizes on offer also depend on who makes them near you. A precast plant may make 1,000 and 1,500 gallon tanks but not 1,250; a plastic tank maker may offer different steps. Designers often pick the next size up that's available locally. See Septic Tank Suppliers and the delivered prices by size on the price index.
Concrete, plastic or fiberglass?
The EPA notes that septic tanks are usually made of concrete, fiberglass, or polyethylene. Size rules are the same for each, but the material changes delivery, installation and sometimes the approved models. Light plastic and fiberglass tanks can go where a truck can't, but they need care against floating out of the ground where the water table is high. See Common Components of Septic Systems.
Bigger tank, less pumping?
A larger tank holds more sludge before it needs pumping, so it can stretch the time between pump-outs. The EPA says household septic tanks are typically pumped every three to five years, depending on household size, the wastewater it makes and the tank size. A bigger tank is never a reason to skip inspections: sludge and scum still build up.
How landplan.io sizes the tank
In landplan.io, you draw the house with the Building with Flow tool and set its bedrooms (or a fixture count or a commercial flow where your area allows it). The design flow follows from your state's or county's rule. When you add a tank, or let Smart design add one, landplan.io lists tanks that distributors near the site actually stock, with their capacity, material and price, and checks the size against your area's rules.
- Building with Flow: bedrooms, fixture count and commercial flow.
- Septic Tank Tools: adding tanks, dose tanks and treatment trains.
- Details and the Report PDF: the Tanks card in step 1.
The tank size is only half the design. The drain field is sized from the same flow and the soil: see Drain Field Size.
Frequently asked questions
What size septic tank do I need for a 3-bedroom house?
In many states, 1,000 gallons is the minimum for a three-bedroom house; Washington and Nebraska both set 1,000 gallons for three bedrooms. Some states or counties require more, for example with a garbage disposal. Check your state's table on the septic pages by state.
What size septic tank do I need for a 4-bedroom house?
It depends on the state. Washington's table still allows 1,000 gallons for four bedrooms, while Nebraska requires at least 1,250 gallons. Use your own state's table.
Can I install a bigger tank than required?
Usually yes. A larger tank is rarely a problem for the health department, and it can lengthen the time between pump-outs. It costs more and needs a bigger hole, so weigh it against the drain field, which is where most systems fail.
Does a garbage disposal need a bigger tank?
Some codes add capacity or require a second compartment for a garbage disposal, because it adds solids. Ask your health department.
Is the dose tank part of the septic tank size?
No. A dose (pump) tank comes after the septic tank and is sized for the pump's doses. Only the septic tank's liquid volume counts toward the septic tank minimum.
Design it from your address
Create a free account, type the address and the number of bedrooms, and landplan.io sizes the tank and the drain field to your county's rules, with tanks stocked by distributors near the site.