Not sure what size septic tank you need? Use the free septic tank size calculator below. Enter your number of bedrooms and household size and it will calculate the recommended minimum tank capacity in gallons — plus how often that septic tank will need pumping. Correct tank sizing protects your whole septic system, and once you have your number you can find a licensed installer near you to confirm it and quote the job.

Septic Tank Size Calculator

Estimate your minimum tank size and pumping frequency in seconds.

Number of bedrooms People living in the home Do you have a garbage disposal?

What size septic tank do I need?

For most homes, septic tank size is set by the number of bedrooms, not by how many people currently live there — because bedrooms are what the house could hold over its lifetime. As a rule of thumb, a typical 3-bedroom home needs a 1,000-gallon septic tank, and each additional bedroom adds roughly 250 gallons. A garbage disposal or heavy water usage pushes you up to the next size. The right way to size a septic tank is to base the required capacity on the number of bedrooms in your home; the table below shows the minimum tank capacity most U.S. states require by bedroom count, which is exactly what the calculator above uses.

Septic tank size by number of bedrooms

Bedrooms	Minimum tank size	Typical use
1–2 bedrooms	1,000 gallons	Small home / cabin
3 bedrooms	1,000 gallons	Most common family home
4 bedrooms	1,250 gallons	Larger family home
5 bedrooms	1,500 gallons	Large household
6 bedrooms	1,750 gallons	Very large home
7+ bedrooms	2,000+ gallons	Estate / multi-family

Some counties still permit 750–900 gallon tanks for a one or two-bedroom house, but 1,000 gallons is the most widely required minimum tank and gives a homeowner a longer gap between pump-outs. This is the recommended septic tank size for the majority of family homes.

How is septic tank size calculated?

To calculate septic tank size, the tank has to hold each day's sewage long enough for solids to settle out before the liquid effluent flows on to the drain field. Tank sizing is based on the average daily wastewater flow — roughly 100–150 gallons of water use per person, or about 150 gallons per bedroom per day, driven by the number of bathrooms and water fixtures in the home. The tank is sized to hold about two days of that daily flow plus a reserve for accumulated solids, so the required capacity rises with each bedroom. If you install a garbage disposal, add roughly 250 gallons (or move up one size), because it sends far more suspended solids into the septic tank system.

Tank size, soil type and your drain field

Getting the correct size septic tank is only half of the job — the drain field (also called a drainfield, leach field or soakaway) has to be sized to match. The liquid effluent leaving the tank drains into the soil, so soil type matters: sandy soil accepts drainage quickly, while heavy clay soils drain slowly and usually need a larger soil absorption area. That is why a percolation test is part of the system design for any new septic tank — it measures how fast your ground absorbs water. When you size a septic tank for a new system, the installer sizes the tank and the drain field together as one onsite wastewater treatment system, based on your soil classification, daily flow, and local regulations. Minimum size is set by state codes, so the required capacity is always based on your state's code as well as your household. Concrete septic tanks are the most common, with fiberglass and plastic as lighter alternatives.

What happens if your septic tank is too small?

When a septic tank is too small for the household, wastewater moves through it too fast. Solids don't have time to settle, so they carry over into the drain field, clog the soil and cause slow drainage, odors and even sewage backups indoors. A septic tank too small for the home also needs pumping far more often. Getting the tank size for your home right from the start — or fitting a new septic tank and a larger tank when you remodel or add a bedroom — protects the drain field, the most expensive part of the wastewater system to replace. For a big property the same rules scale up: a 2,000-gallon septic tank or, on a commercial and industrial site, a small sewage treatment plant.

How often should you pump a septic tank of this size?

A bigger tank with fewer people means longer between pump-outs; a smaller tank with a full house needs service more often. The chart below shows the estimated years between pumping, based on data from Penn State Extension.

Tank size	2 people	4 people	6 people
1,000 gallons	~5.9 yrs	~2.6 yrs	~1.5 yrs
1,250 gallons	~7.5 yrs	~3.4 yrs	~2.0 yrs
1,500 gallons	~9.1 yrs	~4.2 yrs	~2.6 yrs

Most households land in the standard every 3–5 years range. See our full guide to septic tank pumping cost and how pumping works.

Getting the right septic tank size

Good tank sizing comes down to matching the size of your septic tank to your household and your soil. Use the calculator to find the right septic tank size, then confirm it with a licensed installer, since the best tank for your property depends on tank sizing, your drain field and local code together. Knowing the size of your septic tank also makes it easy to plan pumping and budget ahead.

Does tank size affect the cost of a new system?

Yes. A larger tank costs more to buy and install, so tank size is one of the main drivers of the price of a new septic system. A standard 1,000-gallon concrete septic tank is the most economical, while a 1,250 or 1,500-gallon tank, a fiberglass unit, or a difficult installation raises the cost. Because sizing, the drain field and a septic inspection all feed into the final quote, the best way to budget is to get the recommended size from the calculator and then compare local installers.

Frequently asked questions

What is the most common septic tank size?
The 1,000-gallon tank is the most common septic tank size in the U.S., used for most 2–3 bedroom homes.

Does a garbage disposal need a bigger septic tank?
Yes. A garbage disposal adds solids, so add about 250 gallons or move up one size, and expect to pump a little more often.

Does water usage change the tank size I need?
Higher water usage — extra bathrooms, large families, high-flow fixtures — increases daily flow, so a home with heavy water use may need a larger tank than bedroom count alone suggests.

Can a septic tank be too big?
An oversized tank is not harmful, but you pay more upfront. Match the size to your bedrooms and your state's code requirements.

How do I find the size of my existing septic tank?
Your county health department usually keeps the permit on file, or a septic pro can measure it during a pump-out or inspection.

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