Septic Tank Size Calculator
Last updated: June 2026 · Free · No sign-up required
Enter values above and click Calculate to see your result instantly.
Quick reference
| Bedrooms | Minimum tank size |
|---|---|
| 1–2 | 750 gallons |
| 3 | 1,000 gallons |
| 4 | 1,250 gallons |
| 5–6 | 1,500 gallons |
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Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.
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Interpretation guide
| If… | Then… |
|---|---|
| Flow-based size exceeds the bedroom size | Use the larger figure — heavy water users need more capacity |
| Garbage disposal or jetted tub installed | Many codes add 250 gal to the minimum |
| Local code differs | The county health department always wins |
Formula & methodology
Formula: Tank size = max(bedroom-table minimum, 2 × estimated daily flow), rounded up to the next 250 gal
- Pick the number of bedrooms — US codes size primarily by bedrooms, a proxy for potential occupancy.
- Optionally enter occupants and per-person water use (default 70 gal/day).
- The recommendation is the larger of the bedroom-table minimum and twice the estimated daily flow, rounded up to a standard size.
Worked example: 3-bedroom home → table minimum 1,000 gallons. With 8 occupants at 100 gal/day: flow 800 gal/day → 2× = 1,600 → round up = 1,750 gal governs.
Frequently asked questions
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