Installing a new septic system in your home is a big job. Selecting the correct size is important in having an efficient system. An undersized system can cause backups. The question is then: “What size septic do I need?” When we speak of size, we are referring to total volume handling capacity.
Wastewater production and the number of people in your home are the biggest factors in calculating tank size. Soil conditions will also play a role in the effectiveness of its size. Local government regulations will provide guidelines for you to follow and will determine the minimum size for your septic, usually set at 1000 gallons.
There are other important things to consider other than the size of the tank. The type of system you are installing may change your volume requirements. The size of the septic system will also affect your drain field size. If this seems like a lot to take in, don’t worry. Our comprehensive guide will help you determine the best size septic system for your needs.
The Main Types of Septic Tanks & Their Sizes
Septic Tanks can come in a number of configurations. Here are the most common.
- Conventional, Gravity-Fed, Anaerobic System – These make up the majority of septic systems. These are common in family homes.
- Above-Ground Septic System – Above-ground systems are set up where system failure is a concern. Failure can be due to soil conditions or a high water table. It offers the advantage of requiring little excavation.
- Pressure System – A pressure system septic tank uses pressure to divide the wastewater evenly into the individual trenches of the drain field. It is used when the drain field is on an upslope or far from the septic tank.
- Aerobic System – Aerobic Systems are essentially mini sewage plants. They are used in homes where lot size, soil conditions, or other factors do not allow for conventional systems.
- Mound System – Mound systems can be used with shallow soil or high groundwater. They are bigger than conventional systems and require maintenance.
- Recirculating Sand or Gravel Filter System – These systems are found near bodies of water or high water tables. They are more expensive than conventional systems.
- Bottomless Sand Filter System – A bottomless sand filter lacks a drain field. It is installed when a drain field is impossible or impractical.
Why Septic Tank Sizes Are Important
Your septic tank’s size is important for its ability to handle waste and wastewater from your home. You need a properly sized septic tank to fit your needs.
Having a septic tank that is too small is a serious issue. It will result in backups, foul odors, and contamination. The septic tank needs to be able to handle your wastewater usage.
The most common recommendation for tank size is 1000 gallons for home use. This size can accommodate a three-bedroom, two-bathroom home.
But it is important to remember that one size does not fit all. Two houses of the same size can have different needs depending on their occupants and individual use.
When installing a new septic system, give careful consideration to what your needs actually are. The best thing to do is consult with a professional. They can give you an accurate recommendation based on your household’s needs.
How Septic Tank Size Is Determined
The septic tank size is determined based on water usage, number of bedrooms and number of occupants. Let’s review each of these factors in more detail.
Based on Water Usage
Wastewater volume is the most important factor for the septic tank size. All wastewater that is flushed down your toilet or runs down the drains in your sink and bath will be processed through your septic.
Your septic needs this water to function correctly. The presence of water and human waste will help bacteria to grow, which will break down the waste in your septic.
The pressure from the water will also push out the water waste to the drain field, where it will be treated by the soil there. If your tank is too small, it will push the water out too quickly and will not give the soil enough time to treat the water.
Another factor related to the size of your tank is the efficiency of your toilets, sinks, and laundry system. If they are pushing more water through your septic than it can handle, it will create backups or overflows.
Size of Your Property
A septic system requires room not only for the tank but also for the drain field, which takes up a sizable amount of property. If a drain field or tank cannot be installed, this will change the type of system you require. This in turn will change the size of the tank that your family home will require.
Typical alternatives to conventional tanks include aerobic systems and bottomless sand filter systems. Calculating these size needs is a bit more technical. If your property does not support a conventional system, you should consult with a professional to get an accurate size that meets your home’s needs.
The Number of Bedrooms Your Property Has
One method to calculate a home’s needs is to consider the number of bedrooms your property has. This gives a rough idea of what size tank is needed.
According to Michigan State University, the average person uses 75 gallons of water a day. They recommend considering each bedroom as being able to hold two occupants (150 gallons) and then multiplying this number by the number of bedrooms in the house. Finally, each septic tank should be able to hold two days’ worth of water.
For a single-family home with three bedrooms, this method will require a 900-gallon tank. Local regulations may require a size slightly bigger than this.
The Number of Occupants
Using the bedroom count method may not be an accurate reflection of your needs. If you have a lot of people in your house, your estimates should be based on this.
Using the 75-gallon figure given by Michigan State University is a good place to start. Suppose your household has three children, both your parents, your spouse, and an additional guest that stays with you. In reference to this recommendation, your septic needs will be 1,200 gallons.
If you are calculating based on occupants, you may also want to consider the personal habits of each individual. Having someone who uses more water than average can dramatically change your septic needs.
Also, consider that when installing a new septic tank, you are basing the size on how many occupants your home can reasonably hold. This number is not based on how many people currently occupy the house. If you live in a three-bedroom home by yourself, you may not need a big tank, but future occupants will.
Minimum Septic Tank Capacity
| Number of Bedrooms | Size of Tank Required | Size of Tank Recommended |
| 1 | 300 Gallons | 1000 Gallons |
| 2 | 600 Gallons | 1000 Gallons |
| 3 | 900 Gallons | 1000 Gallons |
| 4 | 1200 Gallons | 1250 Gallons |
| For each additional bedroom | +300 gallons | Use size required |
* Required sizes calculated based on Michigan State University’s recommendations.
** Recommended sizes calculated using typical state legal requirements (here is Georgia’s septic laws, for example).
How Many Bedrooms Does a 500-Gallon Septic Tank Support?
A 500-gallon septic tank can support one bedroom. This is an estimate, however.
Michigan State University indicates that the amount of water used by a person on average a day is 75 gallons.
They also indicate that a septic tank should be able to process two days’ worth of water. Finally, they calculate a two-person occupancy per bedroom.
Using these figures, every bedroom in the house should correspond to 300 gallons of septic tank volume. These figures are all estimates, however. How many bedrooms a 500-gallon septic tank can support can also change based on the type of system that it is.
What Determines Septic Sizes?
Aside from the calculations that you make based on occupancy, local government regulations, suitability of the ground geology, and the expected volume of wastewater are key. These factors will determine not only the final size of the tank in your home but also the type of tank you may have to install.
Local Government Regulations
Local government regulations will give you guidelines as to the minimum size that you can install on your property. In addition, they may mandate material type or system type.
These regulations make for a good baseline to judge your needs. Much research and study have likely been done in your area on the soil and groundwater, and the local regulations are based on this data.
Remember that these are the minimum requirements. However, always base your decisions on your home’s actual needs.
Suitability of the Ground Geology
The suitability of the ground geology refers to the soil on your property and the depth of the groundwater. Based on these factors, the type of system that you can install on your property can be radically different.
The primary concern with ground geology is the drain field. The drain field processes wastewater from the septic tank and cleans it of contaminants before the water drains to groundwater. Therefore, drainage cannot be too fast or too slow.
Some soils will allow the wastewater to drain too quickly. In this case, the wastewater can contaminate local groundwater and pose a health risk.
Other soils will not process wastewater fast enough. In this case, the system can back up and seepage to the surface can occur.
The soil can also cause problems with the installation of the tank itself. If the soil is too rocky, excavation for the installation of a conventional system may be impractical or impossible. This will necessitate the installation of an above-ground system.
Most of the recommendations provided in this article are for conventional systems. If you are using another type of system, the calculations of what you need in terms of tank size can change.
How Deep in the Ground is a Septic Tank?
The depth of a septic tank in the ground will depend upon the system, local conditions, and local government regulations. Typically, it can be anywhere between four inches to four feet.
Soil conditions can also play a role in septic tank depth. Check with local government regulations and a professional for your home’s individual needs.
The Expected Volume of Wastewater
The expected volume of wastewater is the most important factor that you need to consider when determining what size septic tank is needed. Recommendations for septic tank size are largely reflective of the average amount of wastewater that will be pumped into the system from drains leading out of your house.
When you have a septic system, all wastewater from your toilets, sinks, and laundry machine will go through the system when leaving your house. This is the focus point for research and government recommendations.
Local governments will typically calculate the smallest septic tank allowed in a home based on the number of bedrooms in the house. The starting point is typically 1000 gallons, which is calculated based on a three-bedroom home. Legal requirements will typically add 250 gallons of septic tank per bedroom for every bedroom over this 1000-gallon figure.
You should also consider upping your calculated needs based on additional fixtures in your home that may increase your use of wastewater. Large-volume bathtubs, hot tubs, and laundry sinks are typical examples.
Also consider that these calculations are based on the needs of the home, rather than your family’s individual needs. If there is the potential for higher use of wastewater from other appliances or additional occupants, this must be added to your calculations.
Can a Septic Tank Be Too Big?
It may be tempting to get a septic size larger than what your needs are. However, a septic tank that is too big cannot run properly.
Part of the septic tank’s proper functioning requires a certain amount of wastewater to flow through it. If the tank is too big, it will not allow for the conditions needed for bacteria production.
Without this bacteria portion, your system cannot break down the waste properly. There is nothing wrong with overestimating your needs for the size of the septic tank.
Having a septic tank that is too small is a bigger issue. But installing a septic tank that many times the needs of a single household is not cost-effective or efficient.
How Do I Determine the Size of My Septic Tank?
There is no one-size-fits-all way to determine the size of a septic system. The size of your recommended septic can change based on individual use, septic system type, and local government regulations.
As a rule of thumb, calculate 300 gallons per bedroom in your house to obtain a recommended size for your septic system. An alternative method is to multiply the number of occupants your house can hold by 150 gallons. (These figures are taken from Michigan State University’s recommendations for calculating water use.)
When determining size requirements, you should consult with a professional. They can assist you in adjusting your calculations for local regulations and your home’s needs.
How To Determine Septic Tank Costs
Once you figure out the size septic tank you will need, the question will become how much it will cost you. Costs typically range from $4,500 to $9,000, and the national average for a new septic system is $6,100 for a conventional 1000-gallon septic system.
Note that these are estimated prices, and do not necessarily match up to how much a septic system will cost in your area. However, there are several factors that will move that price tag up or down.
Based on Septic Tank Type
The type of system that you are installing will change the cost dramatically. A mound or Aerobic system can double the cost of septic installation.
Capacity
In general, the higher the capacity of your system, the more it will cost. On average, the difference of 1000 gallons will mean $2000 in cost difference.
Material
Your septic tank will come in one of three materials: concrete, fiberglass, or plastic. Concrete is the most common and cheapest option, whereas fiberglass will be the most expensive option.
Installation Costs
In addition to the septic tank, you will have to pay for the labor to install the system. The type of septic tank, its capacity, and its material will affect the cost of installation.
A fiberglass tank, for example, is lighter than a concrete septic system. These are much easier to install, thus reducing your costs.
Land Preparation
While not necessary in all cases, the land where you are installing the septic system may need to be surveyed and studied for ground geology. This will require additional labor that you will have to pay for. It will be required in order to know what type of septic system your land can take.
Permits
You may be required to have a permit in order to install the system on your land. Check your local government regulations before installing a new system.
Septic Tank Costs
| Capacity | Cost of Installation* |
| 750 Gallons | $1,500 – $3,000 |
| 1000 Gallons | $3,000 – $7,000 |
| 1250 Gallons | $7,000 – $12,000 |
| 1500 Gallons | $12,000 – $20,000 |
*These are estimated, national average price ranges, and may be different from actual costs in your area.
What Are The Ways To Lower Your Septic Tank Costs?
A new septic system is a significant investment of your finances. Knowing this, you may be looking for ways to lower your costs.
Hire Knowledgeable Professional
The most important thing you can do is hire a highly experienced and knowledgeable professional. A professional will be the best source of advice for a system that is cost-effective and takes care of your home’s needs.
He/she is familiar with all the local regulations and ground geology. A professional will ensure the proper installation of your system. He/she can also give you guidance on the specific maintenance of your system and how to keep it running efficiently.
After installation, a professional can assist you with any maintenance needs that your system requires, including inspecting and pumping your system.
In addition, your septic system will need to comply with local ordinances. Mistakes can cause contamination in the local water supply.
Research Government Programs To Save Even More Money
Also, it is a good idea to research whether government assistance is available to help subsidize the cost of a new system. You may be able to get a grant or a loan to help you cover the cost of installation.
Proper Septic Maintenance
Once your septic system is installed, it is important to properly maintain it. Not only will this keep the system running efficiently, but it will also prevent expensive breakdowns in the future.
You will not have to do anything to start your system running. The act of using it will provide the system with the components it needs to get going.
Set a schedule for an inspection of your septic system. This should be done every one to three years.
Your septic system will also need to be pumped roughly every three to five years. Each system is different, and your use of it can change this figure. A septic system inspector can tell you how to find your septic tank and recommend when to pump your system.
Make sure that you are not overstressing your system. Do not flush garbage down your toilet – as a rule, only flush human waste and toilet paper. Also, be careful about toxins or chemicals entering your drainage system, as this can destroy the bacteria in your system.
Be aware of the signs of backup or overstressed systems. In general, it will be easy to detect – foul odors, toilet or sink backups, and water seepage above ground are all signs that your system may need maintenance.
Final Considerations
Installing a new septic system involves a lot of factors. Size (or rather, the septic system’s total volume) can be determined by the type of system used and ground geology.
Conventional system considerations rest heavily on wastewater needs as measured by the number of occupants or bedrooms in your home. Additional fixtures in your home that use a lot of water should also be considered.
Fortunately, you don’t have to figure it out on your own. Local government regulations are a good starting point for determining the size and type of system you should install. You should also consult a professional to get your home’s exact needs correct.
Ruben has a diverse background in the home services industry, with experience running a construction company, a kitchen and bath showroom, and a moving and relocation company. This breadth of experience has provided him with a wealth of knowledge and expertise in various areas of home improvement in general and specifically in the heating and plumbing niche.