Concrete Cistern & Tank Wall Calculator

Frequently Asked Questions

• How thick should the walls of a concrete cistern be? +
  For a residential cistern up to 6 feet deep, 8 inches is the standard minimum for reinforced poured concrete. Shallower tanks (under 4 feet) can use 6-inch walls. Tanks deeper than 6 feet or larger than 10,000 gallons should be engineered — 10 to 12 inches is common in those cases. The wall must also provide adequate cover over the reinforcing steel: ACI 318 requires 2 inches of cover in water-retaining structures, which effectively rules out 6-inch walls if you're running two-way rebar.
• What is the difference between a cistern and a water storage tank? +
  Functionally, both store water — but the term "cistern" typically refers to a below-grade or partially below-grade structure built to collect and store rainwater or harvested water, often for potable or irrigation use. "Tank" is a broader term that can describe above-grade containers (steel, plastic, or concrete) or in-ground structures for any liquid. For concrete construction purposes, the engineering principles are identical: hydraulic mix design, waterproofing, and reinforcement sized for hydrostatic pressure.
• Do I need to use interior or exterior dimensions in this calculator? +
  Always use interior dimensions — the usable inside space of the tank. The calculator adds the wall thickness on all sides to derive the total concrete volume. Using exterior dimensions would cause the tool to add wall thickness again, double-counting the corners and significantly overstating how much concrete you need. For a rectangular tank, measure from inside face to inside face on both length and width. For a cylindrical tank, measure inside diameter face to face across the center.
• What PSI concrete do I need for a cistern? +
  Minimum 4,000 PSI for any water-retaining structure. Specify a maximum water-to-cement ratio of 0.50 to limit permeability. For potable water cisterns, add a crystalline waterproofing admixture (products like Xypex or Kryton) directly to the mix — this fills capillary pores and is NSF 61 certified for contact with drinking water. Do not use a plain 3,000 PSI mix designed for flatwork; it is not dense enough to resist water infiltration under sustained head pressure.
• Should I pour the base slab and walls in one pour or separately? +
  Most residential cisterns are poured in two stages: the base slab first, then the walls after the slab has cured (minimum 3–5 days). The wall rebar is set in the base slab before the pour so the two elements tie together structurally. The joint between the slab and wall base is a construction joint — it must be treated with a waterstop strip (swelling bentonite or PVC) to prevent leakage at that plane. Attempting a single monolithic pour is possible but requires skilled formwork and is rarely done outside precast or specialized projects.
• Can a concrete cistern float out of the ground when empty? +
  Yes — this is called hydrostatic uplift, and it's a real failure mode. If the water table rises around an empty underground tank, the buoyant force can exceed the weight of the concrete structure and lift it out of the ground. Mitigation strategies include designing for minimum concrete weight that resists uplift, anchoring the base slab with deadman anchors or helical piers, adding ballast around the exterior, or ensuring the tank is never fully emptied when the water table is seasonally high. This is another reason to involve a structural engineer for buried tanks.
• How long does a concrete cistern last? +
  A properly designed and waterproofed concrete cistern can last 50 to 100 years. The key variables are mix design quality, cover over rebar, waterproofing treatment, and whether the lid/access is properly sealed to prevent freeze-thaw damage. Concrete cisterns that leak tend to fail within the first 5–10 years — usually at the wall-to-slab joint or through cracks caused by inadequate wall thickness and reinforcement. Inspect annually for cracks, efflorescence, or discoloration indicating seepage.
• How many gallons does a concrete cistern hold per cubic foot? +
  One cubic foot holds approximately 7.48 US gallons. To estimate capacity, multiply the interior length × width × height (all in feet) to get cubic feet, then multiply by 7.48. For a cylindrical tank, use π/4 × diameter² × height × 7.48. In practice, design for 85–90% usable capacity to allow for sediment accumulation and to avoid drawing the pump inlet dry at the bottom of the tank.
• Is a permit required to build a concrete cistern? +
  In most US jurisdictions, an in-ground concrete cistern requires a building permit and often a plumbing permit if connected to a water supply or distribution system. Some states (particularly in the Southwest and Pacific Northwest) have specific rainwater harvesting regulations — some require a permit, others actively encourage harvesting with rebates. Cisterns for potable use may also require health department approval and inspection. Always check local codes before excavating; installing a cistern without permits can result in mandatory removal and fines.
• What is the most common cause of concrete cistern leaks? +
  The wall-to-floor construction joint is the most common leak point — water finds the path of least resistance and that joint is almost always it. The fix is to install a waterstop (PVC or swelling bentonite strip) in the joint before pouring the walls. The second most common leak cause is using a non-hydraulic concrete mix (plain 3,000 PSI) without waterproofing admixture or membrane. Shrinkage cracks from inadequate curing are the third — concrete must be kept moist for at least 7 days after placement.