Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Minimum tank size (gal)
1,500
Years between pump-outs
6.9
Months between pump-outs
83
Daily wastewater (gal)
240
Retention time (days)
6.3
Amortized pumping cost per year
$61
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter bedrooms.
- 2Enter people in household.
- 3Enter water use per person (gal/day).
- 4Enter pump-out cost ($).
- 5Read your minimum tank size (gal) on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Most jurisdictions require at least 1,000 gallons for a three-bedroom home and add 250 gallons per extra bedroom. Pump-out interval falls fast as household size grows.
Worked example
Using the values the calculator loads with:
Inputs
- Bedrooms: 3
- People in household: 4
- Water use per person: 60 gal/day
- Pump-out cost: 425 $
Results
- Minimum tank size (gal): 1,500
- Years between pump-outs: 6.9
- Months between pump-outs: 83
- Daily wastewater (gal): 240
- Retention time (days): 6.3
- Amortized pumping cost per year: $61
What each field means
Inputs
- Bedrooms
- The bedrooms used in the calculation. Starts at 3 so you have a working example on load. Accepted range: 1–10.
- People in household
- The people in household used in the calculation. Starts at 4 so you have a working example on load. Accepted range: 1–15.
- Water use per person (gal/day)
- The water use per person used in the calculation, measured in gal/day. Starts at 60 gal/day so you have a working example on load. Accepted range: 20–150 gal/day.
- Pump-out cost ($)
- The pump-out cost used in the calculation, measured in $. Starts at 425 $ so you have a working example on load.
Results
- Minimum tank size (gal)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Years between pump-outs
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Months between pump-outs
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Daily wastewater (gal)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Retention time (days)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Amortized pumping cost per year
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the septic tank sizing & pump-out calculator work?
Most jurisdictions require at least 1,000 gallons for a three-bedroom home and add 250 gallons per extra bedroom. Pump-out interval falls fast as household size grows. The underlying maths is: Gallons = base (bedrooms) ; interval ≈ tank ÷ (people × daily solids accumulation).
What do I need to enter?
4 values: bedrooms, people in household, water use per person, and pump-out cost. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the minimum tank size (gal) result mean?
It is returned as a whole number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Septic Tank Size Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/septic-tank-size
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/septic-tank-size" target="_blank" rel="noopener">Septic Tank Size Calculator — RevenueLab</a> (2026).</p>
Source: [Septic Tank Size Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/septic-tank-size) (2026).