Septic Tank Size Calculator

Who it's for: Plumbing contractors, septic installers, estimators, and property owners in early planning.

Calculator Inputs

Built for: Plumbing contractors, septic installers, estimators, and property owners in early planning.
Key outputs: Estimated daily flow, Base and reserve-applied tank volume, and more.
Use this page for early planning estimates before final code and manufacturer verification.

Calculates estimated daily flow from occupants times gallons-per-person assumptions.
Applies a two-day retention heuristic and then reserve factor for conservative planning volume.
Optionally enforces a bedroom-based minimum and maps to the nearest standard tank size.

Cross-check assumptions with Pipe Volume Calculator for a more complete planning workflow.

Local septic regulations vary widely, so always verify minimum size and design criteria locally.
Bedroom-based minimums can dominate the recommendation in low-occupancy scenarios.
Use conservative daily flow assumptions when occupancy patterns are uncertain.
Treat this output as early planning only, not permit-ready design.

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No. This is a planning estimate and does not replace local septic regulations, design review, or permitting requirements.

Occupants estimate daily use while bedroom-based minimums provide a common conservative baseline in many jurisdictions.

Reserve factor increases the base estimate to provide additional capacity margin for variability.

Use the largest listed size as a placeholder and confirm final design with a qualified local septic professional.

Disclaimer: Use results for planning only. Final design and installation decisions must follow local code and manufacturer documentation.