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Septic tank systems can seem pretty complicated if you are new to living in a home that requires one, especially if you are used to living in an area previously serviced by public sewer. In today’s article, we will be answering that age-old question-just how big of a septic tank do I need?

How Big of a Septic Tank Do I Need? The size of your septic tank or just how big your septic tank needs to be will depend on four factors:

  1. The Size of the Home (Square Footage)
  2. Municipality Requirements
  3. # of People currently in the house
  4. The Number of Bedrooms (# of People home is designed for)

In the remainder of this article, we will explain this factors further so you can make the best decision when it comes to design and selecting a septic tank system for your home.

Why is Selecting the Right Sized Septic Tank So Important?

Let’s take a closer look at why people ask this question in the first place. Again, most people who are new to septic tanks or have never installed one themselves may never have considered the proper size of a septic tank before.  So let’s quickly reiterate why selecting the proper size for a septic tank is so important.

Remember Septic Tank Systems are charged with the proper sewage disposal for your entire home or commercial site. Septic Systems are one of the most common forms of private sewage disposal in America for several reasons.

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A septic system is comprised of two main components: 1) a Septic Tank and 2) a Drainage field.  This design allows the wastewater from your home to flow into holding tank, where it is treated by bacteria before the effluent or wastewater leaves the tank and flows into the drainage field.

When water leaves your home it flows into the Septic tank. The septic tank can also be considered a holding tank. In the septic tank, the solid waste falls to the bottom of the tank, (this is called the sludge layer). Oil, Soap, and Grease float to the top of the tank creating the Scum Layer. The remaining wastewater or effluent sits in the middle layer and as water fills the tank it exits the tank into the drainage area or leach field.

As you can imagine if your house has an undersized septic tank it will not be able to keep up with the flow of sewage and water that enters the tank from your home, this could cause sludge and solid waste to exit the tank and potentially clog the drainage field causing big problems.

Essentially an undersized septic tank means that the bacteria in your tank that is charged with breaking down the solid waste in your tank may not have enough time to do their job before the water starts flowing out of the tank.

The EPA estimates that the average water usage is about 70 Gallons per day per individual in the home. You can see those numbers here.

How Long Should Your Tank Hold Water Before It Flows into the Drainage Area?

Like we mentioned above the right-sized tank is crucial if you want to make sure the bacteria in your tank have enough time to do their job. To ensure you have proper sludge and scum layer formation a tank should hold about 1 and a half days worth of water usage from the building it services.

This should essentially be the minimum size of septic tank you install at your home.  In order to estimate this correctly we will look closer at the factors mentioned above:

  • Size of the home
  • Number of people residing in the home
  • Usage
  • Number of bedrooms

How does the Size of My Home (Square Footage) Impact the Size of Septic Tank?

Remember when we are talking about the proper size of a septic tank for your home, all we are trying to account for is the usage load that can be expected from the home.  So here are some general industry standards to consider.

  • 1-2 bedroom homes, less than 1,500 sq. ft. – need a 750 Gallon tank.
  • 3 bedroom homes, less than 2,500 sq. ft. – need a 1,000 Gallon tank.
  • 4 bedroom homes, less than 3,500 sq. ft. – need a 1,250 Gallon tank.
  • 5 bedroom homes, less than 4,500 sq. ft. – need a 1,250 Gallon tank.
  • 6 bedrooms homes, less than 5,500 sq. ft. – need a 1,315 Gallon tank.

***Remember that each municpality will have thier own standards that must be followed in your town or city. Make sure you check with the munciple requirements in your area.

You should always have your septic tank installed by a licensed professional to ensure they are up to the building codes required by your town and to make sure the job is done right. If you are in need of a local septic tank professional click here.

How Do You Calculate Septic Tank Capacity in Gallons?

How to Calculate the Septic Tank Capacity in Gallons

Round Septic Tanks3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity.
Rectangular Septic TanksLength x Width x Depth in feet x 7.5 = gallons
Rectangular Septic Tanks
(alternative method 1)
Length x width in inches / 231 = gallons per inch of septic tank depth. Multiply this number by septic tank depth in inches to get gallons
Rectangular Septic Tanks
(alternative method 2)
Length x Width x Depth in feet / .1337 = gallons

Why do the Number of Bedrooms Affect the Size of my Septic Tank?

When installing a new septic tank you need to make sure it is the right size by municipal requirements for the size of home and number of bedrooms you have in your home.

The reason this is taken into account when installing a new septic tank and making sure it is sized properly is that septic tanks will usually last 40-50 years if properly maintained, and thus there may be various owners or inhabitants in the home over the life of the septic tank.

So when your home is having a septic tank installed it is very important you have it done by a professional and follow all the municipal guidelines to make sure you don’t have a problem when it comes to selling your home in the future. Make sure you take that into account when having a septic tank system installed or replaced.

You may have a huge 12 bedroom house and live in it alone or with one other person at the moment. But from a government perspective, the home needs to have a sewage system that can accommodate the number of occupants the home is designed for.  Usually, regulations will estimate usage based on 2 people per bedroom.

This becomes a bit tricky in huge homes or older homes that are being upgraded, but your licensed septic professional and civil engineer will be able to help you design a system that is right-sized for your home.

The conclusion of How Big Should Your Septic Tank Be?

In the end, a few things are very important when figuring out the right size of your septic tank.

Make sure you have a correct estimate of your daily usage and more importantly the expected daily usage of the home or building you are sizing for the septic tank. This will help you avoid big issues in the future.

Consider the amount of usage you will have, that means anything that puts water into the system should be considered when coming up with the right size of your septic tank and system.  Include Toilets, Showers, Dishwashers, Washers, Sinks, and anything else you might have in your home that will put a demand on your septic tank.

Remember you want the waste in your septic tank to have enough time to have the solid waste sink to the bottom and the grease and oil and soap residue to float to the top in scum layer.  You want to keep solid waste and scum out of the drainage field to keep it from clogging and extend its lifespan.

When in doubt make sure you empty your tank at the appropriate intervals to ensure it keeps working properly. If you think you septic tank might be full read this to find out for sure.

If you need to get your septic system serviced find a local professional in our state by state directory. Simply click your state below.

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