What the 135 LPCD standard actually covers
LPCD stands for litres per capita per day — the water one person is expected to use in a day. IS 1172:1993, the Indian standard for water supply in buildings, fixes the residential figure at 135 litres per person per day where the house is on a municipal (piped town) supply, and a higher 150 LPCD where the house draws on its own borewell, because an owned source tends to be used more freely.
That 135 litres is not just drinking water. The standard breaks the daily household demand down roughly like this:
- Drinking & cooking — about 5 litres. The only water that must be potable-clean, but the smallest slice.
- Bathing — about 55 litres. The single largest use in most Indian homes — a bucket bath is ~20 L, a shower far more.
- Flushing — about 30 litres. Every WC flush is 6–10 L; this adds up fast across a family.
- Washing & cleaning — about 45 litres. Clothes, utensils, floors, and the general housekeeping load.
Add those up and you land near 135 litres for a person on municipal supply. The figure is a design minimum, not a luxury allowance — it is what a plumbing system must be able to deliver and store so the house never runs dry under normal use.
Why storage is two-thirds of a day, not a full day
A common instinct is to size the tank to a full day of demand. IS 1172:1993 is more economical than that: it sets the total stored capacity at two-thirds of the daily demand — the 0.67 factor this calculator uses. The reasoning is that a house on a working supply is refilled during the day, so it does not need to hold a whole day’s water at once; it only needs enough buffer to ride through the hours between fills and the peak-use mornings and evenings.
storage (litres) = occupants × LPCD × 0.67 × storage days
The 0.67 factor and the per-capita rates are the exact locked IS 1172:1993 values used inside NirmanShastra’s paid PlumbingPro tool — this free calculator shares the same engine for the single-day case, so the two never disagree.
Why intermittent-supply areas need a bigger buffer
The two-thirds rule assumes water comes every day. Large parts of India do not have that. In many towns and peri-urban areas the municipal line runs only on alternate days, or for a couple of hours every second or third day, or a tanker arrives on a schedule. In those places a tank sized for a single day empties before the next supply arrives, and the household is dry — exactly the failure the standard warns against.
The fix is the storage-days multiplier in this tool. This is a genuinely new addition, not an IS clause: it simply scales the standard two-thirds-of-a-day storage by the number of days you must survive between supplies. Set it to 2 for an alternate-day supply, 3 for a twice-a-week line, and the required storage grows in proportion. A continuous, reliable supply stays at 1 day and collapses back to the plain IS 1172:1993 figure.
Because the difference between “enough” and “dry taps” is one-sided — running out is far worse than a slightly larger tank — the tool always rounds the requirement up to the next size actually sold off the shelf (500, 1000, 1500, 2000, 3000, 5000 litres). It never rounds down.
Worked example — a family of eight on municipal supply
Take the calculator’s default inputs: 8 occupants, a municipal supply, and 1 day of storage. The daily demand is 8 × 135 = 1080 litres. Applying the two-thirds storage rule gives 1080 × 0.67 = 723.6, which rounds to 724 litres of required storage.
8 × 135 = 1080 L/day → 1080 × 0.67 × 1 = 724 L → buy 1000 L
724 litres falls between the 500 L and 1000 L standard sizes, so the tool rounds up to a 1000 litre tank — the smallest off-the-shelf tank that will not leave this family short. If the same house sat on an alternate-day supply, setting storage days to 2 would raise the requirement to 1448 litres and push the recommendation to a 1500 litre tank.
Schematic for estimation reference only. Sizing uses the IS 1172:1993 per-capita demand and two-thirds storage rule with a storage-days buffer; actual requirements vary with occupancy patterns, fittings, gardens, and local supply reliability. All storage tanks must be food-grade HDPE, covered, and insect-proof per IS 12701. Not a substitute for a licensed plumber’s design.