Domestic Water Demand
Daily potable and non-potable demand per IS 1172 and NBC 2016 Part 9.
Free · no sign-in requiredPurpose
Establish the daily water requirement of a building so that storage tanks, the incoming connection, treatment plant and the pumping system can be sized, and so the drainage and sewage load can be derived.
Inputs required
Occupancy or population by category, the per-capita allowance applicable to each use, and additional demands such as landscape irrigation, HVAC make-up water, swimming pool make-up and process water.
Standards followed
IS 1172 for basic requirements of water supply, drainage and sanitation, and National Building Code of India 2016 Part 9 Section 1 for plumbing services. Cooling-tower make-up is derived from evaporation and drift with the design cycles of concentration.
Inputs
Results
- Potable (domestic) demand
- 46,155 L/d
- Non-potable (flushing / irrigation)
- 39,395 L/d
- Raw total
- 85,550 L/d
- With allowance
- 94,105 L/d
- Design volume
- 94.11 m³/d
- Non-potable fraction
- 46 %
- Residents
- 54,000 L/d (P 36,180 / NP 17,820)
- Staff / office
- 11,250 L/d (P 5,625 / NP 5,625)
- Visitors
- 4,500 L/d (P 1,350 / NP 3,150)
- Canteen / kitchen
- 3,000 L/d (P 3,000 / NP 0)
- Landscape irrigation
- 4,800 L/d (P 0 / NP 4,800)
- HVAC make-up
- 8,000 L/d (P 0 / NP 8,000)
- Group demand = occupants × lpcd
- Flushing demand = group demand × flushing share
- Extra demand = area × rate | meals × L/meal
- Design demand = Σ demands × (1 + allowance %)
- Effective lpcd = total demand / population
- Per-capita rates follow IS 1172:1993 Table 1; confirm against the local water authority's byelaw, which may mandate higher rates.
- Flushing, irrigation and HVAC make-up are treated as non-potable and may be served from a treated/recycled water system where permitted.
- Cooling-tower make-up should be taken from the actual tower evaporation + blowdown calculation, not a rule of thumb.
- Fire-fighting water is a static reserve and is excluded from daily consumption; size it separately per NBC Part 4.
Engineering reference & methodology
Splitting potable from non-potable demand
Total demand must be separated by water quality, not treated as one figure. Potable demand covers drinking, cooking and washing. Flushing, landscape irrigation and cooling-tower make-up can be met with treated recycled water or harvested rainwater. Separating them at the demand stage determines the size of the flushing tank, the treated-water tank and the recycling plant, and it is the single decision that most affects a building's net water draw.
Per-capita allowances
Allowances differ by occupancy: residential per resident, offices per occupant on a working day, hospitals per bed, hotels per bed with additional laundry and kitchen demand, schools per student, and factories per worker per shift. Where a building has several occupancies, calculate each separately with its own population and allowance rather than applying one blended figure, and confirm which allowances the local authority enforces — many municipalities mandate values above the code minimum.
Population is the dominant uncertainty
The allowance is published, but the population is assumed. A residential population derived from apartment type, an office population derived from net area per person, or a shift pattern that changes after handover will move the answer far more than any refinement of the allowance. State the population basis explicitly and test the result against a credible upper case before committing tank volumes.
Cooling-tower and process make-up
In an air-conditioned building the cooling-tower make-up can rival the domestic demand. It is governed by the evaporation rate, which follows the rejected heat, plus drift and the blowdown needed to hold the design cycles of concentration. Operating at low cycles wastes water; operating at high cycles risks scaling and demands better water treatment. Irrigation demand should be based on the planted area, the plant type and the local evapotranspiration rate rather than a flat allowance.
From daily demand to storage and drainage
Storage is typically sized for one day of demand, split between an underground raw or treated tank and overhead tanks, with the split set by the pumping strategy and the authority's requirement. Sewage generation is usually taken as a high proportion of the water supplied, so the demand figure also drives the sewage treatment plant capacity and the drainage design. Fire-water storage is a separate dedicated reserve and must never be counted as available domestic storage.
Failure consequences and verification
Underestimating demand causes storage to run dry during the daily peak, pumps to cycle against an empty tank, and a treatment plant that cannot keep up — faults that appear only after occupancy and are expensive to correct in a completed building. Overestimating it produces oversized tanks that occupy revenue space and hold water long enough to stagnate and lose residual disinfectant. Before use, verify the population basis with the architect's schedule, confirm the allowances against the local authority's current requirement, and reconcile the resulting daily figure with the supply the water utility will actually commit to.
Results are preliminary design guidance. Verify every value against the edition of each standard adopted by your project or authority having jurisdiction, and against manufacturer performance data, before using it in design or procurement.
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