Septic Tank Size Calculator

Find the minimum septic tank size for your home by bedroom count and occupancy.

septic tank
MIN TANK SIZE
— gal
minimum

Usage Tip

Codes size tanks by bedrooms, not bathrooms. Bigger means longer between pump-outs.

THE MATH
size = max(bedroom, occupancy)
Where: bedroom = max(1000, 250 + 250 × beds)  •  occupancy = 250 + 150 × people
• Result is the larger of the two methods
• Local code sets the legal minimum
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.