Sizing your water tank from the people in the house
135 litres per person per day is the standard. That number sizes the sump, the tank and the septic tank, all from one answer.
How to size your water tanks without guessing
Most people guess the size of their water tanks. A builder suggests a generic volume because that is what they cast on the last job. A neighbour tells you what they installed on their plot.
When you guess water storage, you pay for it twice. A tank that is too small runs dry between supply cycles and leaves you waiting for water tankers. A tank that is too large costs more to excavate, takes up ground area, and holds stagnant water.
The ground sump and the overhead tank perform different jobs. Sizing both from a single design basis keeps your water fresh and keeps your construction budget under control.
135 litres per person: the code standard
Building standards do not leave water storage to guesswork. Indian Standard IS 1172 sets domestic water consumption at 135 litres per person per day.
This 135-litre figure covers daily basic human needs inside the house. It accounts for drinking, cooking, bathing, washing clothes, washing utensils, and flushing toilets.
Your storage capacity must derive directly from your total occupant count. You calculate your baseline daily requirement by multiplying your occupant count by this 135-litre standard.
The same occupant count also controls the size of your waste system. Under IS 2470 Table 1, septic tank capacity is sized directly from the number of occupants rather than picked from a catalogue size. One accurate head count establishes the baseline for incoming water and outgoing waste.
How water storage and drainage rules apply to your build
Your daily demand does not sit in one single container. A residential water system divides volume across ground storage and overhead distribution.
Your underground sump acts as the primary buffer. It receives water from municipal mains or a borewell and holds the total daily reserve. Your overhead tank sits at the high point of the building to create gravity pressure for your plumbing fixtures.
Drinking water must remain uncontaminated by general domestic reserves. Our practice requires that potable water is stored separately from general water. You build two distinct tanks rather than one tank divided by an internal partition wall. A single partition wall can develop hairline cracks, allowing non-potable water to seep into your drinking supply.
The table below outlines how these water and sanitation rules align across Indian Standards and building practice:
| System element | Design standard | Source |
|---|---|---|
| Daily domestic water demand | 135 litres per person per day | IS 1172 |
| Potable water storage | Two separate tanks, not one divided tank | PRACTICE |
| Septic tank capacity | Sized from occupant count | IS 2470 Table 1 |
| Septic tank to borewell separation | Minimum 18 m clearance | TNCDBR 2019; IS 2470 |
| Septic tank to building clearance | Minimum 15 m clearance, or vent rises 2 m above the roof | IS 2470 |
When you arrange these storage and waste units on your plot, site geometry dictates their position:
- The underground sump sits where municipal connections or supply lines enter the property.
- Potable water remains in its own dedicated tank structure to prevent cross-contamination.
- The overhead tank sits directly above structural supports, distributing water downward to wet areas.
- The septic tank and its soak pit sit far from any ground water source to safeguard your supply.
The placement mistakes that ruin your water
The most expensive water mistake is placing waste collection too close to clean water sources. Under both IS 2470 and the Tamil Nadu Combined Development and Building Rules 2019, the minimum distance between a septic tank and a borewell is 18 m. The same 18 m separation applies between a dispersion trench or soak pit and your borewell.
If blackwater leaches through soil into your subsoil water, your borewell becomes unusable. Fixing a contaminated borewell requires drilling a new well or installing filtration systems.
On small urban sites, maintaining an 18 m distance requires strict planning. For example, a 40 × 60 ft plot has a diagonal measurement of about 22 m. On that plot, an 18 m clearance is possible only if you place the borewell in one corner and the septic tank in the opposite corner. On a 30 × 40 ft plot, an 18 m separation usually cannot fit at all.
Building proximity brings another strict clearance. Under IS 2470, a septic tank must sit at least 15 m away from the building. If your site does not allow a 15 m clearance, IS 2470 permits a closer placement only if the foul vent pipe extends at least 2 m above the roof level.
In our own drawing practice, automated checks caught a septic tank placed 2.6 m from a house wall due to an unadjusted plot fraction rule. Automated checking moved that foul drainage to the plot corner furthest from the borewell.
What this means for your plan
Check your drawing to confirm that your water capacity matches 135 litres per person per day and that your septic tank matches the occupant count in IS 2470 Table 1. Ask your designer to verify that potable water uses an isolated tank structure rather than a shared divider wall. Confirm that your borewell and your septic tank maintain the full 18 m statutory clearance on your site layout. The tool applies these capacity baselines and boundary separation distances automatically when it draws your plan.
- IS 1172: 135 litres per person per day
- PRACTICE: potable water stored separately from general water - two tanks, not one divided
- IS 2470 Table 1: septic tank capacity sized from occupant count
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