# Water Demand Calculator: Daily Demand and Tank Storage for a Building

> Domestic water demand is the water a building's occupants use in a day for drinking, washing, cleaning and flushing, estimated as people × litres per person per day. NBC 2016 Part 9 tabulates per-person figures by building type, 45 litres a day for offices. Storage is the daily demand times the days of supply held, split between an overhead tank and an underground sump.

## What it calculates

The calculator estimates a building's **domestic water demand** and the **storage** needed
to hold it:

- the **daily demand** in litres and cubic metres, from occupants and visitors;
- the **total domestic storage** for the number of days of supply you want to hold;
- the **split between the overhead tank and the underground sump**;
- optionally, the **split between flushing and other domestic use**.

It covers domestic water only. **Fire-fighting water is not included**: it is sized
separately under NBC 2016 Part 4 by the fire consultant, and nothing on this page should be
used to estimate it.

The calculator is for early planning: checking whether a building's tanks are in the right
range for a new headcount, setting aside roof and basement space for tanks, or briefing a
plumbing consultant.

## Inputs

**Building type** fills in the litres per person per day with the figure NBC 2016 Part 9
gives for that type. The field stays editable; choose "Other" to enter your own.

**Water per person per day** is the per-capita demand. The default of 45 litres is the NBC
2016 Part 9 figure for offices. Confirm the value for your building type with your plumbing
consultant.

**Occupants** is the number of people in the building on a normal day: staff in an office or
factory, residents in a hostel.

**Visitors a day** is the number of people who are in the building for part of the day.
Leave it at zero where visitors are few.

The **assumptions** panel holds the water allowed for each visitor, the days of storage,
the share of storage held in the overhead tank and the optional share of demand used for
flushing.

## Method

- **Daily demand** = occupants × litres per person + visitors × litres per visitor.
- **Storage** = daily demand × days of storage.
- **Overhead tank** = storage × overhead share; **sump** = storage − overhead tank.
- If a flushing share is entered: **flushing** = daily demand × flushing share, and **other
  domestic use** = daily demand − flushing.

All figures are in litres, with cubic metres alongside (1 m³ = 1,000 litres).

## Assumptions

| Assumption | Default | Basis |
| --- | --- | --- |
| Water per person per day | 45 litres | NBC 2016 Part 9 figure for offices |
| Water per visitor | 15 litres | An allowance for washroom use by part-day visitors |
| Days of storage | 1 | Set from how reliable the municipal or tanker supply is |
| Overhead tank share | 33% | A common practice split, with the rest in the sump; not a code figure |
| Flushing share | 0% | Not split unless you enter a share |

The per-person figure is the one that matters most. NBC 2016 Part 9 tabulates different
figures for different building types, and some are counted per bed or per seat rather than
per person, so hospitals, hotels, restaurants and cinemas need the right basis as well as
the right number. Canteens, showers, landscape irrigation, cooling towers and process water
are not in the per-person figure and need their own allowance.

Days of storage is a judgement about supply. A building on a reliable municipal connection
may hold one day; one that depends on tankers or a borewell may hold more. Local bye-laws
and the water authority's conditions can also set storage requirements.

## Limitations

- **It gives water volume, not tank size.** A tank is bought larger than the water it holds,
  to allow for the space above the overflow and the water below the outlet.
- **It does not size pipes or pumps.** Pipe sizes come from the flow at peak use, not the
  daily total, and pump duty depends on the height and the refill time.
- **It excludes fire-fighting water.** That storage is set under NBC 2016 Part 4 by the fire
  consultant.
- **It excludes non-domestic uses.** Canteen kitchens, cooling towers, irrigation and process
  water each need their own figure.
- **It does not judge water quality or treatment.** Filtration, softening and any recycling
  of grey water are separate design decisions.

## Worked example

An office with 200 staff, no allowance for visitors and one day of storage, using the
default assumptions:

| Step | Calculation | Result |
| --- | --- | --- |
| Occupants | 200 × 45 | 9,000 litres a day |
| Visitors | 0 × 15 | 0 litres a day |
| Daily demand | 9,000 + 0 | 9,000 litres (9 m³) |
| Storage | 9,000 × 1 day | 9,000 litres |
| Overhead tank | 9,000 × 33% | 2,970 litres (2.97 m³) |
| Underground sump | 9,000 − 2,970 | 6,030 litres (6.03 m³) |

The building needs **9,000 litres a day**, held as about **2,970 litres overhead** and
**6,030 litres in the sump**, before any allowance for tank freeboard and before any fire
storage.

Add 100 visitors a day and raise the storage to two days, and the daily demand becomes
10,500 litres, the storage 21,000 litres, the overhead tank 6,930 litres and the sump
14,070 litres. If you enter a flushing share, purely as an example 40%, the original 9,000
litres a day splits into 3,600 litres for flushing and 5,400 litres for other domestic use.

## How engineers use it

A plumbing consultant starts from the same per-capita figures, but builds the demand from
the actual uses in the building: occupants and visitors by area, the canteen by meals, any
showers, landscaping and HVAC make-up water. The storage is set from the supply conditions
and the authority's requirements, divided between sump and overhead tank to suit the
structure and the pumping arrangement, and compartmented so one half can be cleaned while
the other stays in use. Fire-fighting storage is designed separately by the fire
consultant.

From the demand, the consultant sizes the pumps, the risers and the distribution pipework
for the peak flow, and the drainage that takes the water away again.

## When a professional design must replace it

Use the calculator to check whether existing tanks suit a new headcount, to reserve space
for tanks early in a project, and to brief a consultant. Replace it with a plumbing design
by a qualified consultant before tanks are built or bought, before pumps and pipes are
sized, whenever the building has canteens, process water or cooling towers, and wherever
the authority sets its own storage conditions. Fire-fighting storage always comes from the
fire consultant's design under NBC 2016 Part 4.

## Frequently asked questions

### How much water does an office use per person per day?

NBC 2016 Part 9 gives 45 litres per person per day for offices, which covers drinking, hand washing, cleaning and flushing. A building with a staff canteen, showers, gardens or cooling towers uses more, and those uses need their own allowance. Confirm the figure for your building with the plumbing consultant.

### How do I work out the size of an overhead tank?

Work out the daily demand, multiply by the number of days of storage you want, and decide what share sits on the roof. With 200 office staff at 45 litres each and one day of storage, the building holds 9,000 litres; a 33% share overhead gives 2,970 litres of water in the overhead tank, and the tank itself is bought slightly larger to allow for the space above the water and below the outlet.

### Why split storage between an overhead tank and a sump?

The underground sump receives the municipal or tanker supply and holds the bulk of the water, where its weight sits on the ground. The overhead tank holds a smaller share and feeds the building by gravity, and pumps refill it from the sump. The split is a design choice, not a code figure, and depends on the structure, the pumps and how the building is supplied.

### Does this include water for fire fighting?

No. Fire-fighting water is sized separately under NBC 2016 Part 4 by the fire consultant, depending on the building's height, occupancy and fire protection systems. It must not be estimated from this calculator, and the domestic figures here do not include it.

### Should flushing water be calculated separately?

Where flushing uses treated or recycled water, or a separate tank, yes. Enter the share of demand used for flushing and the calculator splits the daily figure into flushing and other domestic use. The right share depends on the building type and fittings, so take it from your plumbing consultant.

## Sources

- [NBC 2016, Part 9](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Plumbing services: water supply
- [IS 1172](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of basic requirements for water supply, drainage and sanitation
- [NBC 2016, Part 4](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Fire and life safety (fire-fighting water storage, sized separately)

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Source: https://hagerstone.com/calculators/water-demand-calculator