10/30/2026
Concrete weighs about 150 lb/ft³. Water weighs 62.4. So a concrete septic tank can’t float, right?
It can. And when it does, it takes your inlet and outlet lines with it.
A tank floats for the same reason a steel ship does. What matters is the weight of the whole tank compared to the water it pushes aside, not how heavy concrete is.
Here’s the math on our standard 1,000-gallon tank:
🔹 Outside size: 98" × 68" × 64", which pushes aside 247 ft³ of water
🔹 Uplift with groundwater at the surface: 247 × 62.4 = 15,400 lb
🔹 Weight of the empty tank: 10,100 lb
🔹 Net result: about 5,300 lb pushing it up out of the ground
Averaged over its whole size, an empty tank weighs 41 lb/ft³. That’s lighter than water.
The key word is empty. A full tank holds about 8,300 lb of liquid, and that keeps it in the ground. Pump it out and that weight is gone.
Around here, it usually happens in spring. Snowmelt brings the water table up, the tank gets pumped, and it pops.
We can run buoyancy calcs for your site to keep your structures from floating in high groundwater. We check the groundwater level and how much soil will sit on top. For this tank, about 3.5 ft of cover holds it down even with water right at the surface. With less cover, we add an extended base. The wider footprint carries extra soil that holds the structure in place.
If your site has high groundwater, bring us in before the set, not after the tank surfaces.
Contact us:
📞 (406) 752-7163
📧 [email protected]
🌐 glacierprecast.com
