Septic System 101: How To Calculate Proper Septic Tank Sizing

Hello friends, and welcome! Today, I want to talk with you about a topic that often seems mystical: how to properly size a septic tank. Why are tanks sized the way they are, and why does it matter? Let's dive in.
Septic systems handle all the wastewater from your home, functioning as a large concrete or plastic box in your yard where waste is processed. Solids settle, and liquids flow into drain fields or absorption systems. But there’s actually math and science behind sizing these tanks.
In Maryland, for instance, modern septic systems typically use 1,500-gallon tanks for homes up to five bedrooms, sometimes larger for bigger homes. Tank size is based on occupancy, calculated by multiplying the number of bedrooms by two (assuming two people per bedroom) and then by 75 gallons of daily water use per person. For example, a four-bedroom home would need a tank sized for at least 600 gallons of daily flow, but a 2X multiplier means a 1,200-gallon minimum tank size is required.
Proper sizing ensures enough retention time for solids to settle and scum to float, allowing clearer effluent to enter the absorption system, and preventing premature failure due to Biomat buildup. Manufacturer specifics may vary, but the principle remains the same.
If you’re planning an addition to your home, you might need to upgrade your septic tank to accommodate increased water flow, particularly if adding significant living space.
I hope this short video helps you understand the importance of proper septic tank sizing. How does your county handle tank sizing? Have you ever dealt with permit issues related to septic upgrades? Share your experiences below, and if you found this useful, please hit the like button and subscribe. Until next time!

[@DawnSchwartz-fk5jc]

Thanks for this!! I was wondering about factors of safety built in. I knew about the 2 people per bedroom assumption in Virginia, as an engineer I wonder if that 2x holding time is a factor of safety or actually needed. Bridge design multiplies tons of things by 2 as well ;)

[@MDwellandsepticguy]

The 2x rule for the septic sizing is mostly to prevent damage or premature failure to the absorption system. No matter how well you take care of a tank, a little bit will work it’s way out of the tank. That’s what we refer to as bio material. The larger the tank or more chambers a tank has, you decrease the volume of escaping solids. This will help the absorption system last the full design spec life time and/or out perform the life span