Last updated: 2026-06-30

What Sets Septic Tank Size

A septic tank has two jobs at once, and it must be big enough for both. It has to hold wastewater long enough for solids to settle and bacteria to start breaking them down, and it has to store the sludge and scum that accumulate between pump-outs. Size only for the daily flow and the tank clogs with solids years too soon; ignore the flow and partly-treated effluent escapes to the drain field.

A four-person household using about 150 L each per day produces 600 L of wastewater daily. Held for two days that is 1,200 L of liquid; add a few years of sludge and the tank needs to be around 2,000 L.

A four-person household using about 40 gallons each per day produces 160 gallons of wastewater daily. Held for two days that is 320 gallons of liquid; add a few years of sludge and the tank needs to be around 540 gallons.

The Formula

The required volume is the sum of a liquid part and a sludge part:

where $N$ is people, $q$ is wastewater per person per day, $t$ is the liquid retention in days, $s$ is sludge per person per year, and $T$ is the years between pump-outs.

Worked Example

Round up to the next standard tank your supplier stocks.

People vs. Bedrooms

This calculator sizes by occupants and flow — the international engineering approach. Many building codes instead size by number of bedrooms, a deliberately conservative proxy for the home's maximum occupancy that does not shrink when fewer people live there. Where a code table applies, treat its figure as a minimum and use the larger of it and the flow-based estimate.

Practical Notes

• Only wastewater belongs in the tank. Toilets, sinks, showers, dishwasher, and laundry — never roof or surface drainage, which would flood the system.
• Water use is a good cross-check. Because wastewater closely tracks the water you draw, your metered consumption is a quick reality check on the per-person figure.
• A bigger tank buys time. More capacity (or fewer people) lengthens the interval between pump-outs, but the tank still needs periodic sludge-depth checks.

Septic design is regulated almost everywhere: tank size, drain-field area, and soil percolation must meet local code and usually require a permit and a licensed designer. Use this estimate to plan and to sanity-check a quote — not as a substitute for an engineered design.