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

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

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:

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.