How to Calculate Septic System Sizing
What is Septic System Sizing?
Septic System Sizing Calculator helps contractors and tradespeople accurately size, spec, and estimate materials and systems.
Formula
Tank Size = Daily Flow × Retention Time (min 2 days)
Step-by-Step Guide
- 1Estimate daily wastewater flow per bedroom, size tank accordingly
- 2Apply the formula using your project measurements
- 3Add required safety factors or waste allowances per code
Worked Examples
Input
3-bedroom home, 150 GPD/bedroom
Result
Tank = 3 × 150 × 2 = 900 gallons → use 1000 gallon
Common Mistakes to Avoid
- ✕Not applying code-required safety factors or derating
- ✕Forgetting to account for waste and cut losses in material estimates
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