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The septic tank size is a critical parameter to how effective and efficient an onsite wastewater management system functions. As a homeowner looking to install an onsite wastewater management system, you will need to figure out what septic tank size you will need.

Septic tank size must be designed to allow the wastewater effluent to remain in the tank for at least 1 day to undergo this sedimentation process before being pumped out of the tank. If undersized, the wastewater effluent will pass through the septic tank chambers too quickly and unsettled solids can significantly reduce the lifespan of the disposal fields.

So before it is too late, find out the size of your septic tank.


How Septic Tanks Work

Before you get started, it is worth understanding how septic tanks work. When you send water down the drain—whether by running the sink and shower, doing the dishes or flushing the toilet—the wastewater travels through your plumbing and ends up in your septic tank. Any solid waste sinks to the bottom, while grease, fat and oil rise to the top. The wastewater (also known as effluent) drains out through a series of perforated pipes. It ends up in the drain field, where helpful bacteria in the soil treat the water and send it into the groundwater supply.

The solids, fats and oils stay in the tank, however. Some organic matter may be “digested” by helpful bacteria, but for the most part, the solid waste sticks around until it’s time to pump your septic tank. (This usually needs to happen every three to five years.) That is why it is important to get a large enough tank for your family—if the septic tank fills up, no one will be able to use the indoor plumbing until it’s pumped.


Types of Septic Tanks

There are three common types of septic tanks used in residential construction.

  • Concrete septic tanks
  • Polyethylene/plastic septic tanks
  • Fiberglass septic tanks

Concrete septic tanks are the most common, but because of their weight require heavy machinery to install. Polyethylene and fiberglass are much lighter, one-piece units. This makes them ideal for remote, hard to reach places. Before purchasing a septic tank system, check with your local building department for codes and regulations regarding onsite wastewater treatment.


Why Choosing the Right Septic Tank Size Matters

When wastewater enters the reservoir, it needs time to separate. Buoyant materials collect at the top, in a layer called “scum.” This will include things like grease, oil, and toilet paper. Dense materials will fall to the bottom in a layer referred to as “sludge.” The partially clarified liquid in the center is called “effluent,” and, once the solids separate from it, it can move onto the next phase, which is typically a leach field. If a unit is too small, it can be easily overwhelmed by several people taking showers or by doing too many loads of laundry. The liquid will try to go out to the leach field before it has had a chance to separate and this can clog the system, as well as pose health risks. The bacteria in the tank also break down the sludge, and as much as 50% will be turned into liquid and gasses if the reservoir is effective.


The Risks of Getting a Septic Tank that is Too Big or Too Small

Septic tanks need the proper volume of wastewater running through them to function correctly. If your tank is too big, it may not be able to collect the amount of liquid needed to produce bacteria and break down waste.

On the other hand, if your tank is too small, it may not have the capacity to treat the water effectively. Excessive water may cause the tank to overflow, pushing the solids into the drain field before it has a chance to filter them out. The tank may also clog, causing a septic system backup in your home.


What to Consider When Choosing a Septic Tank Size

  1. The number of bedrooms in your home: The number of bedrooms in your home will help you determine the size of the septic tank you need. For one to two-bedroom homes, a 300-gallon tank should be sufficient. If your home has three or more bedrooms, then you will need a 500-gallon tank. Keep in mind, these are just guidelines. Your specific needs may vary depending on the size of your home and the amount of wastewater it produces.
  2. The number of people in your home: The higher the number of individuals in your house, the larger your septic tank has to be. For instance, if your home has two occupants, you will require a small septic tank. On the other hand, if your property has more than five occupants, you will need a bigger septic tank to manage your wastewater more effectively and hygienically. When considering what size septic tank you need, you should know that septic sizes determine your septic system’s overall efficiency. When your home’s septic tank surpasses its holding capacity, wastewater can go back up your house causing blockages, flooding, and putting you and your family at risk. Therefore, to prevent these inconveniences, it is crucial you evaluate septic tank sizes to select the right option for your home.
  3. Your soil type: The type of soil on your property will play a role in the size of your septic tank. If you have sandy soil, you may need a smaller tank, as it has a high rate of percolation. If you have clay soil, you will need a larger tank, as it has a low rate of percolation.
  4. The amount of wastewater your home produces: This is an important factor to consider because it will impact how often you need to pump your septic tank. If your home produces a lot of wastewater, you will need to pump your septic tank more often.
  5. Your budget: This is an important factor to consider because the size of your septic tank will impact the overall cost of installation and maintenance. Larger septic tanks will cost more to install and maintain than smaller septic tanks.
  6. Your long-term needs: This is an important factor to consider because the size of your septic tank will impact your long-term needs. If you plan on expanding your home in the future, you will need a larger septic tank. If you have no plans to expand your home, you can choose a smaller septic tank.
  7. The climate: This is an important factor to consider because the climate can impact the overall efficiency of your septic system. If you live in an area with a lot of rainfall, you will need a larger septic tank because the extra water will increase the amount of wastewater your home produces.
  8. Your property size: We can calculate typical daily water usage based on the number of people living in your home. We can also consider your overall property size to determine a septic tank that would fit well in your available space. If you have a large property, you will need a larger leach field. If you have a small property, you will need a smaller leach field.
  9. The type of septic system you want: This is an important factor to consider because the type of septic system you choose will impact the size of your septic tank. If you want a standard septic system, you will need a larger septic tank. If you want a low-profile septic system, you can choose a smaller septic tank.
  10. Your local regulations: This is an important factor to consider because your local regulations may dictate the size of septic tank you are allowed to install. Be sure to check with your local authorities to see what size septic tank is allowed in your area.


How Much Water Do You Use?

There are a number of methods of calculations to determine the septic tank size that is needed for your home. The most accurate and reliable way is with water usage. The size of the septic tank required is based on the amount of water it will handle and in turn will be dispersed into the field lines. It should be noted that in many parts of the country, the minimum size tank allowed is 1,000 gallons.

Regular Use: 50-100 gallons per person per day

Bathing: 25-60 gallons per bath/ shower

Dishwasher: 7-50 gallons per load (check your manual)

Garbage Disposal: 4-6 gallons per day

There are other things, such as pools, water softeners, and lawn irrigation that are not included here. Try to keep these in mind as you work out your estimate. In homes with low water usage, your reservoir will need to hold roughly twice the number of gallons you use. As your water usage goes up, the gap narrows some.


Calculations by Water Usage

The most reliable and effective way of determining the septic tank size suitable for your property is by calculating the amount of water you use. The septic tank size needed is dependent on the volume of water it can hold, which will be drained into the soil absorption field. In many parts of the United States, the minimum septic tank size permitted is 1,000 gallons. Based on the overall water consumption of your household, the recommended septic tank sizes are as follows.

  • Less than 1,240 gallons a day: a septic tank of 1,900 gallons
  • Less than 900 gallons a day: a septic tank of 1,500 gallons
  • Less than 700 gallons a day: a septic tank of 1,200 gallons
  • Less than 500 gallons a day: a septic tank of 900 gallons


Calculations by House Size

A less accurate guide to calculating your tank size is the number of bedrooms in your home or the square footage of the home. For a 1,000 gallon septic tank, how many bedrooms can you have? It is difficult to say because of varying water usage based on your situation. These calculations assume all bedrooms will be occupied and base the estimated water usage on this data. If you live alone in a three bedroom house, these calculations will be off. The reasoning for using these calculations is that a new owner may occupy all the bedrooms and the tank must be of an adequate size to handle the load.

Listed here are the recommended tank sizes based on number of bedrooms.

  • One or two bedrooms under 1,500 square feet: 750 gallon tank
  • Three bedrooms under 2,500 square feet: 1,000 gallon tank
  • Four bedrooms under 3,500 square feet: 1,200 gallon tank
  • Five or six bedrooms under 5,500 square feet: 1,500 gallon tank


Septic Tank Size Affects Pumping Schedule

The size of your septic tank matters because that affects how often it needs to be pumped to keep operating at its peak. Generally, we recommend pumping your septic tank every 3-5 years — the smaller the tank, the more often it must be pumped. Tanks that go too long without this maintenance are likely to become backed up or fail, requiring expensive repairs or replacement.


Call West Coast Sanitation Today!

The fact is that many homes and septic systems were not designed to handle the load put on them from modern appliances and modern lifestyles. But, with a little care and awareness, you can manage this impact and help to avoid increased maintenance problems and the need for costly system repairs and replacement.

At West Coast Sanitation, we know that you don’t have time to deal with septic problems. If you think that your system has reached capacity, please give us a call right away. We have professionals ready to answer your questions and get your system working properly again.

Please give us a call at (951) 780-5922 right away. We have professionals ready to answer your questions and get your system working properly again.