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Calcimator

Septic Tank Sizing Calculator

Size septic tanks and drain fields based on bedrooms, occupants, soil percolation rate, and daily wastewater flow.

Inputs

EPA/state codes: 1–2 BR 750 gal min; 3 BR 1,000 gal; 4 BR 1,200 gal; 5–6 BR 1,500 gal min

min/inch

EPA 625/R-00/008: sandy 1–5 min/in; loam 5–30 min/in; clay loam 30–60 min/in; >120 min/in unsuitable

GPD

Results

Minimum Tank Size

1,000 gallons

≈ 25 bathtubs

Drain Field Area

500 sq ft

≈ 12 king-size beds

Trench Length (3ft wide)167 ft
Pumping Frequency1.4 years
How to Use This Calculator
  1. Enter the number of bedrooms — most codes size septic systems by bedroom count regardless of actual occupancy.
  2. Enter the actual number of occupants for a more accurate daily flow estimate.
  3. Input the soil percolation rate in minutes per inch from a perc test (slower soil = larger drain field).
  4. Review Minimum Tank Size in gallons and Drain Field Area in square feet from the results.
  5. Use Pumping Frequency to schedule routine service — most residential tanks need pumping every 3-5 years.

How the result changes with Number of Bedrooms

Number of BedroomsMinimum Tank SizeDrain Field Area
2.9975 gallons500 sq ft
7.652,163 gallons500 sq ft
133,500 gallons500 sq ft
184,750 gallons500 sq ft

What each input means

Number of Bedrooms
Number of bedrooms in the home. Primary sizing factor per most state codes and EPA guidance. EPA/state typical minimum tank sizes: 1–2 bedrooms: 750 gal; 3 bedrooms: 1,000 gal; 4 bedrooms: 1,200 gal; 5–6 bedrooms: 1,500 gal.
Number of Occupants
Number of people living in the home. Affects pumping frequency.
Soil Percolation Rate
Percolation test result per EPA 625/R-00/008 and state health codes. Sandy soil: 1–5 min/in (fast); loam: 5–30 min/in (suitable); clay loam: 30–60 min/in (marginal); >120 min/in typically unsuitable for conventional drain fields.
Daily Wastewater Flow
Estimated daily wastewater volume. Typically 75 GPD per person or 150 GPD per bedroom.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Number of Bedrooms = 3, Number of Occupants = 4, Soil Percolation Rate = 15, Daily Wastewater Flow = 300 = 4 input(s) provided
  2. Calculate Minimum Tank Size
    Minimum Tank Size
    1000 = 1000
  3. Calculate Drain Field Area
    Drain Field Area
    500 = 500
  4. Calculate Trench Length
    Trench Length
    167 = 167
  5. Calculate Pumping Frequency
    Pumping Frequency
    1.4 = 1.4

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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