Septic Tank Size for a House in Thailand: What Does the Law Require?

Why Septic Tank Size Matters When Building a House

A septic tank is a system many house owners let the contractor size on its own, without checking whether it actually matches the household’s real needs. A tank too small for the number of people in the house cannot treat wastewater fast enough, leading to odor and drains that clog more often than they should.

Across Isaan, in places like Korat and Roi Et, many houses are not connected to a municipal sewage treatment system, which makes the household’s own septic tank the main system it relies on for the entire life of the house, not just a backup option. Houses in Khao Yai and Pak Chong outside municipal boundaries are in the same situation.

Choosing the tank size is therefore not just a technical decision left to the contractor. It comes with a legal requirement attached, so a house owner should know the basic criteria before signing off on the sanitary system drawings, in order to ask the designer the right questions from the start.

What Thai Law Requires for a House’s Septic Tank

Ministerial Regulation No. 71 (B.E. 2566) requires that construction or modification of residential buildings, including detached houses, twin houses, row houses, and shophouses, include a septic and treatment section that meets criteria set by ministerial announcement (according to Ministerial Regulation No. 71). This applies to both new construction and modifications that affect the existing wastewater system.

In addition, an announcement from the Ministry of Natural Resources and Environment, on discharge standards for certain building types and sizes, sets the quality standard for treated water released from a residential wastewater system, an area overseen by the Pollution Control Department, PCD (according to PCD).

In practice, the municipality or subdistrict administration reviewing a building permit application usually asks to see the wastewater system details in the construction drawings as part of the approval, not just the structural drawings. If that detail is missing, officials may require a revision before issuing the permit.

How to Calculate the Right Septic Tank Size

The basic formula for sizing a septic tank is: average water use per person per day, multiplied by the number of people in the house, multiplied by the retention time, plus a reserve volume for sludge, giving the tank size in liters, approximately.

The retention time commonly used for a septic tank combined with an anaerobic filter tank is roughly 1.5 to 2 days, approximately, and the average water use figure commonly used in the calculation is roughly 200 liters per person per day, approximately.

The reserve volume commonly used for sludge accumulation is roughly 100 liters, approximately, to hold sediment until the next pump-out. This figure can be increased if the goal is to stretch out the interval between pump-outs.

A Worked Example: Sizing a Septic Tank for a 5-Person House

Suppose a house has 5 residents using roughly 200 liters of water per person per day, approximately, multiplied by a 1.5-day retention time, giving 1,500 liters. Add the 100-liter reserve volume for sludge, and the total tank size comes to roughly 1,600 liters, approximately.

If the house has more residents, say 7 to 8, the calculation needs to be redone with the real number of people, since a tank sized for 5 people would be too small for a larger family and could fail to keep up when many people use water at the same time.

This formula gives a starting point. The most accurate figure should be checked against the manufacturer’s specifications for the actual pre-fabricated tank chosen, since treatment efficiency inside the tank varies by brand, and some brands already provide a sizing table based on the number of users.

When to Install the Septic Tank During Construction

Most septic tanks are buried underground, so the position and size need to be set during the sanitary system design stage, with pipe routing laid out before the ground-floor slab is poured, not decided close to handover.

The tank’s location should also account for how easily a pump-out truck can reach it later, especially for houses on the large plots common across Isaan, where some spots can be surprisingly hard for a truck to reach if the access route was not planned in advance. Fixing an access problem afterward often means digging up landscaping or paving already completed.

Before requesting a final inspection or a house registration number, check that the wastewater system actually installed matches the approved drawings and meets the discharge standard under the Ministry of Natural Resources and Environment’s announcement, since fixing an underground system after the site is closed up costs far more than checking during construction.

Before Choosing a Septic Tank Size for Your House

The right septic tank size has to be calculated from the actual number of people in the household, not just picked from whatever size a shop recommends as a one-size-fits-all standard, since every household has a different number of residents and water-use pattern.

The OBILIS team calculates the septic tank size to fit each household’s actual number of members during the sanitary system design stage, so there is no need to fix or replace the tank after the house is already finished.

If you want a figure for the house you actually have in mind, try the budget planner first, or contact us for an estimate based on your real plan and land.

Whoever you build with, ask the designer to specify the tank size along with the calculation used in the construction drawings clearly, and confirm it meets Ministerial Regulation No. 71 before the sanitary work begins.

Frequently Asked Questions

What is the difference between a pre-fabricated septic tank and one cast on site?

A pre-fabricated tank is factory-made to a standard and installs faster and more easily. A tank cast on site is built with reinforced concrete on the ground, taking longer but allowing the size to be adjusted to the actual site conditions. Both types still need to be sized correctly for the number of users.

How often does a septic tank need to be pumped out?

How often depends on the tank size, the number of users, and the reserve volume designed for sludge. In general, check the tank’s condition periodically and pump it out when you notice wastewater draining more slowly than usual.

If a house connects to a village’s central treatment system, does it still need its own septic tank?

It depends on that central system’s requirements. Some projects still require each house to have a preliminary septic section before discharging into the shared pipe, to reduce the load on the central treatment system, so check with the project’s central system operator before deciding to skip one.