Dimensions of a septic tank: How to determine what size should you get?

Septic tanks are often required when you live in a rural area. This makes sure that your wastewater is taken care of. These systems can be quite expensive and finding out what system you should get can be quite complex. Some companies might want to sell you a bigger version while you don’t need this. Therefore it is good to get an idea of what size you should get. Let’s dive in.


Septic tank sizing. How big are septic tanks? 750 gallons, 1000 gallons, 1250 gallons and more

Septic tanks are made out of concrete, plastic, or fiberglass. Each of these has unique advantages and disadvantages. They often are pre-made so you don’t have to assemble anything. The shape is generally square, round tanks are not used that often. The material of the tank doesn’t affect the size that you need.

The size is mainly determined by the amount of sewage that needs to be processed. The number of people that live in a house and the size of the home is used to determine the size of the tank. We can simplify this by just looking at the number of bedrooms as bigger houses generally have more rooms.

Number of bedroomsSize of tank required
1-2750 gallons
31000 gallons
4-51250 gallons
61315 gallons
6+Talk to your plumber

The size is measured in gallons. It is often best to take a tank that is slightly too big as your family might grow over time. If you often have guests over or hold parties, you might want to get a bigger tank as well. It is not possible to make a tank bigger so you have to replace it (or add one). The tank shouldn’t be too big though as this can limit how well it works.

This makes it necessary to pump it more often. Tanks generally get pumped empty once they are 25-33% full. This should happen every 2-5 years. Pumping a tank can be annoying as it is quite expensive and it can mean that your toilets don’t work for a while as you need to wait until the service team gets to your home.

Tanks generally have 2 parts. The first room filters the sludge and this results in a cleaner liquid that goes in the second tank. The size of these compartments is often determined by the manufacturer of the tank and there is no need to adjust this.

Image by Aunt_Spray via Canva.com

How to determine what size septic tank you need?

Based on the amount of sewage and how well this processes, we can determine how many gallons of water remain in the tank. This allows us to get an idea of what size of tank is needed. Some local communities have specific requirements so check the local code before you order your septic tank.

Keep in mind that a sewage tank brings limitations to how often you can use a washing machine. The loads should be spread out over the day as the water can dilute the sewage too much. If this is not an option, you might consider a bigger sewage tank. You will have to discuss this with your plumber to see what is possible. In some instances, a part of the water can be diverted to other areas.

Rainwater generally doesn’t go in a sewage tank. This is also the reason why the tanks have to be sealed properly. Rain or groundwater that gets in the tank can fill it up quite fast. Inspectors spend a lot of time making sure that the tank is sealed properly to avoid this.

The previous discussion is only for traditional systems and not for systems with a pump or more advanced systems. Often engineers are needed to determine the size of these tanks as there are more variables at play. The climate and local rules can also affect the size of the tank.

Some people upscale their drain field as they don’t have enough space for a tank (or vice versa). Make sure to check this with your plumber and inspector before you do this. If you have a small garden, it can be a challenge to install a system within the boundary lines.

It is often necessary to use an excavator as tanks can be quite big and heavy. If you make the tank bigger, you will also need a bigger distribution box and drain field. A septic system can therefore scale up quite a lot once you start adding more bedrooms.

If you already have a system, it is possible that clogs are making it work less well and therefore you might have to clean it more often than you want. You can get the tank inspected with a camera to see what is going on. This is not cheap as the tank needs to get pumped and then inspected.

This costs more than 1000 USD but can fix a lot of problems in the long run. It is important that you use your tank correctly. For example, you shouldn’t flush wet wipes or harsh chemicals in it. If you have guests over, you might have to explain how this system works. This makes sure that they don’t fill the tank with items that can damage it.

Some companies are experimenting with ways to reduce the water in a sewage tanks. For example, they try to break the excrement down in the toilet already. The goal is to make it possible to work with smaller sewage tanks. These efforts are still being worked on so it is still early days for this type of system.

A properly sized septic tank can save you a lot of money in the long run. Not only does a small septic tank cost more to maintain, but it can also help increase your risk of water damage and Cross-Contamination. For these reasons, it is essential that you find the right size for your needs. We have outlined some reasons before how this can be done.

If you already have an old concrete septic tank, it is quite easy to determine how big the new one should be. If it is your first septic tank, it can take more work and calculations. You also have to think about how many people will live in the house in the coming years.

It can be quite challenging to find the right size for a septic system. If it is too small, you have to pump it out often. If the tank is too big, you have to spend more on installing it. Your plumber can help you to find the right size for your system.