Crank Up Your Understanding of Septic Tank Volume

D.P. Taylor
Written by D.P. Taylor
Updated December 28, 2021
close up shot of bathroom toilet
Photo: ArLawKa AungTun/ iStock / Getty Images Plus/ Getty Images


  • An undersized septic system causes wastewater back up.

  • Your tank should handle 95 liters of wastewater per person, per day.

  • Warning signs are strong odor, water backing up, and an increase in water consumption.

  • A concrete septic tank is often the best option.

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Septic systems make modern life possible for homes in rural areas. They process all that unpleasant wastewater from our toilets, sinks, and laundry machines and safely release it into the surrounding environment. But you need to choose a septic tank with the proper volume for your home. 

This guide will help you make an informed choice to keep your home’s wastewater where it should be—pleasantly out of sight… and smell.

How Septic Tank Volume Works

A septic system is a structure built under the ground that treats residential wastewater. Rural homes that don't have easy access to a centralized sewer system tend to use them.

The septic system includes a septic tank and a drainfield. The tank separates matter like oil, grease, and solids from the wastewater. The system slowly releases the treated sewage—also called “effluent"—into the surrounding environment.

Choosing the right volume for your septic tank is very important. If you choose a tank that is too small, it won't be able to handle the volume of wastewater your home produces, and it'll start backing up into your home or into your yard. Needless to say, that’s not a good scenario.

How to Calculate Septic Tank Size

So what size septic tank do you need? To determine the septic tank daily liter volume you need, multiply the total number of people in your household by 95. Here’s another way of looking at it—let's say you're living in a home with three other people. You'd need to calculate the daily wastewater production for each individual and multiply it by four to determine what kind of capacity you'd need from a septic system.

Use this list of daily average wastewater production to come up with a rough calculation:

  • Bathroom: 85 liters per person

  • Laundry and dishwasher: 6 liters per person

  • Cooking: 1 liter per person

  • Other: 3 liters per person

  • Total: 95 liters per person

So for a four-person household, you'd need a septic system that can support 380 liters per day of wastewater production (4 x 95 = 380).

Signs You Need to Replace Your Septic System

septic tanks in backyard lawn
Photo: senssnow/ Adobe Stock

Is it time to replace your septic system? Since the cost of a new septic system can be as much as $20,000, obviously you want to avoid replacement if you can. But there are four main things to look out for that suggest you should replace it.

Your Water Consumption Has Increased

If your water consumption has dramatically increased due to new members in your household, that's a good sign it's time for a septic system replacement. It's possible your current septic system has the capacity to handle the additional water flow, so check its volume first.

Water Is Backing up in Your Yard or Home

If you're seeing standing water in your yard or water is backing up in your toilets and sinks, that's an indication that your septic system is overloaded and needs replacing. However, first check to see if perhaps a clog is the cause rather than not enough volume. Also, maintenance is important; you can avoid septic backups with preventative maintenance.

Tubs and Sinks Take a Long Time to Drain

Even if water isn't backing up, if you've noticed that it is taking forever for a sink or a tub to drain no matter how much drain cleaner you pour down there, it may be an issue with the septic system rather than a clog in your pipes.

You Notice a Strong Odor

Wastewater is unpleasant to say the least, so before you notice any of the other signs mentioned, a powerful odor may knock you off your feet first. If this odor is particularly strong around the area of the septic tank and drainfield, that's an indication that wastewater is leaking out of your system and into your yard. And that's a problem you need to take care of right away before it gets worse.

Concrete Septic Tanks Are Probably the Best Option

Septic tanks can be built from a variety of materials, including steel, plastic, and fiberglass. However, old-fashioned concrete is probably your best bet because of its durability.

Concrete is a much stronger material that will retain its shape even after many years of usage. They can also do a better job of retaining heat, which helps with the growth of the bacteria that breaks down the waste entering the tank and results in a cleaner effluent that drains into your field.

How to Find Your Septic System

To ensure your system is working properly, it’s important to inspect your septic tank often. But what if you don’t know where your septic system is? That's fine, as there are a few ways to find it. You could check your home's "as built" drawing, which should indicate the location of the septic system. Or, you could do a visual inspection of your yard to see if you can find any lids or manhole covers. As a last resort, contact a septic system service provider near you to help you find it.

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