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How to Size a Septic Tank


STEP 1: Calculate Minimum Property Population
The volume of effluent coming from your house is determined by the number of people that could utilise the system. This is calculated based on the number of bedrooms in your home.
*Please note that the abbreviation for Population is “P”
Single House
Up to and including three bedrooms: P = 5, add 1 P for each bedroom thereafter.
  • House with three bedrooms = minimum 5P system
  • House with four bedrooms = minimum 6P system (5+1)
  • House with five bedrooms = minimum 7P system (5+2)
  • House with six bedrooms = minimum 8P system (5+3)
Multiple properties are sized individually and then added together

STEP 2: Calculate Wastewater Production (Litres)
British Water estimates the that one person creates 150L per day. To calculate the number of litres per day for your property, just multiply the number of people x 150L.
Example: three bedrooms house = 5P (see step 1)
5 P x 150L = 750L per day

STEP 3: Add 2,000L 
Carrying on with the example above: 750L (step 2) + 2,000L = 2,750L
Therefore, the minimum size septic tank required for this example is 2,750L