How to Calculate The Size Of An Off-Grid Septic System?

Determining System Capacity

To determine the size of the septic system, you’ll need to calculate the daily water usage in gallons. A general rule of thumb is to allow 1 gallon per person per day for washing, bathing, and sanitation. For example, a family of four would need a system capable of handling at least 16 gallons per day. You’ll also need to consider greywater (wastewater from sinks, showers, and washing machines) and blackwater (toilet waste) separately.

System Design Requirements

Septic systems come in various designs, including drainfields, mound systems, and seepage pits. For a typical drainfield system, you’ll need to calculate the surface area required to accommodate the wastewater. A common formula is to use 10-20 square feet of surface area per 1,000 gallons of system capacity. For example, a 20,000-gallon system would require a drainfield of 200-400 square feet.

Local Regulations and Soil Conditions

Local regulations may dictate the minimum septic system size and design requirements. You’ll also need to consider soil conditions, including the soil’s permeability, depth of the water table, and any nearby water sources. A percolation test (perc test) can help determine the soil’s ability to absorb wastewater. A slow-percolating soil may require a larger system or a different design, such as a mound system.