How to use the septic tank calculator
Prepare the values for this calculation
Calculate a planning tank volume from entered design flow, retention time, and reserve multiplier without selecting jurisdictional sizing criteria. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Design wastewater flow (gal/day): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Entered retention time (days): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Reserve/sizing multiplier: enter a value at least 1e-06. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Calculated planning volume first. With the unchanged example inputs, it is 900 gal. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Design wastewater flow (gal/day) from 450 to 495. The resulting Calculated planning volume is 990 gal. This illustrates the equation’s response to that input, rather than a forecast of real events.
Read the chart’s axis labels and data table. A scenario curve changes one stated input while holding the others fixed; it does not add external observations. Use Download graph to save an available chart as a PNG.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Septic Tank Calculator formula
Planning volume = design flow × entered retention time × reserve multiplier.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Design wastewater flow (gal/day) | minimum 0 | 450 |
| Entered retention time (days) | minimum 0 | 2 |
| Reserve/sizing multiplier | minimum 1e-06 | 1 |
Worked example
Using the example inputs above, the calculated planning volume is 900 gal.
- Calculated planning volume
- 900 gal
- Design flow
- 450 gal/day
- Entered retention time
- 2 days
- Reserve multiplier
- 1
Results and assumptions
EPA notes that individual onsite systems are generally regulated by state, tribal, and local authorities. This planning arithmetic does not select a legal tank size or replace permitting/design requirements.
Sources and method references
Fix an input or unexpected result
Design wastewater flow (gal/day) is required. Enter design wastewater flow (gal/day) using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.