What Size Septic Tank Does My House Need?

Residential septic tanks aren't like stretchy knit hats — one size doesn't fit all. If you choose a unit that's too small, it can result in blockages or overflows, but if you choose a model that's too large, the flow of wastewater may not be sufficient for the system to run properly.

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That's why it's important to select a vessel that can adequately accommodate your household's water usage.

What Does Septic Tank Size Mean?

Before you can understand what septic tank size means, you need to know how a septic system works. These systems separate solid waste and floatable materials, such as grease, from liquid wastewater. The remaining liquid is then slowly discharged into nearby drain fields, where it can be filtered naturally through the soil.

Septic tank size typically refers to the capacity of the septic tank, which is measured in gallons. It represents how much wastewater the tank can accommodate. When installing a septic tank, you should consider a tank that can hold approximately two days' worth of wastewater.

What's the Most Accurate Way to Measure For a Septic Tank?

The most precise way to determine the size of the tank you'll need is to measure your household's daily water usage using a water meter. If you have a well, check the meter there at about the same time each day for at least a week, preferably longer. Take an average of the readings to determine your annual daily water usage.

Because a septic tank should ideally hold at least two days' worth of wastewater, double your daily average water usage to determine the proper tank size. For example, if your household uses about 750 gallons of water each day, consider a tank with a 1,500-gallon capacity.

How Big of a Septic System Do I Need for My House?

You can also estimate the proper size tank for your household based on the number of bedrooms in your home:

1- or 2-Bedroom House

1,000- to 1,500-gallon tank

3-Bedroom House

1,000- to 2,000-gallon tank

4-Bedroom House

1,250- to 2,500-gallon tank

5- or 6-Bedroom House

1,500- to 3,500-gallon tank

Septic Tank Size by Number of Bedrooms

For larger houses, increase the tank size by about 250 gallons for each additional bedroom.

However, judging tank requirements based on the number of bedrooms in a house can be misleading. A more accurate reflection of the ideal tank size may be how many bedrooms are being used and by how many people. For example, a single person who occupies a two-bedroom house may only need a 750-gallon tank, while a family of four occupying a similar-sized home will need a substantially larger unit.

You may also want to factor in the number of bathrooms your home has. A one-bath house will create less wastewater at one time than a three-bath house.

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What Factors Can Change the Size of the Tank My House Needs?

The number of people in the household isn't the only factor that contributes to the size of the septic tank needed. A few other things to consider before choosing a septic tank include:

Type of Septic System

When people think about septic systems, they typically envision a conventional underground anaerobic system. However, other types of systems are available, and each has different guidelines for sizing.

Type of Soil

The composition and quality of your soil absorption field can affect how quickly and effectively liquids drain away. If you have poorly draining soil, you may need a tank that holds more than 2 days' worth of liquids.

Climate

If you live somewhere cold, you may need a larger tank than you would if you lived in a warmer climate because waste breaks down more slowly in colder weather.

Appliances

Garbage disposals send more waste through your septic system, so you'll need a larger-capacity tank.

Capacity of Sinks and Tubs

If you have a large-volume bathtub or regularly use a laundry sink, you may need a larger septic tank to accommodate the excess water that drains from it.

Regulations

Your state or local government may have specific regulations concerning the size and type of septic tank you can legally install. For example, many areas require a 1,000-gallon vessel at minimum.

Err on the Larger Side

Additionally, if you've calculated your household's needs and the ideal capacity falls between two septic system tank sizes, you should always choose the bigger tank size to ensure your home's needs are adequately addressed. If you are installing a brand-new tank, an expert can help you determine the septic tank size requirement for your home.