Daily Per Capita Water Demand

It is a unit reconciliation. Supply is normally measured in cubic metres per day (m³/d), but per capita demand is expressed in litres per person per day. One cubic metre contains 1,000 litres, so multiplying the supply by 1,000 converts it into litres per day. Dividing by the population then yields litres per person per day. Without the factor of 1,000 the result would be a thousand times too small — a city using 150 L/c/d would appear to use 0.15.

Use the population actually served by the network — the connected population, plus any standpipe users the network supplies. If you divide a metered supply figure by a census population that includes large unconnected areas, you will overstate the per capita demand and under-size the network for the people who will eventually connect. Conversely, if you use the connected population now but design for the full population later, you must apply a growth factor. Most design manuals therefore specify the "mid-design-year served population" — usually 30 years out — as the denominator for sizing calculations.

Non-revenue water (NRW) is the difference between the water put into the network and the water that is billed to customers. It includes physical leakage from pipes and tanks, illegal connections, unbilled authorised use, and meter under-registration. NRW is typically 10–25 % in well-managed systems and 40–60 % in poorly managed ones. Because the PCD formula uses the production figure, it captures the sum of real consumption and NRW. A city with 50 % NRW that reports 300 L/c/d is actually consuming about 150 L/c/d and losing the rest. This is why a water-balance audit must accompany any demand calculation.

Multiply litres per capita per day by 0.264172 to get US gallons per capita per day. So 150 L/c/d = 39.6 gpcd, and 400 L/c/d = 105.7 gpcd. The UK imperial gallon is larger (4.546 L rather than 3.785 L), so if you need imperial gallons per capita per day, multiply by 0.219969 instead. In practice, gpcd is almost always US gallons in the water industry, and 100 gpcd ≈ 379 L/c/d.

Average day demand is the annual total divided by 365 — the figure this calculator computes. Maximum day demand is the highest single day of the year, typically 1.5–2.0 times the average, and it sets the size of the treatment works and the source works. Peak hour demand is the highest single hour, typically 1.5–2.5 times the maximum day, and it sets the size of the distribution mains and the booster pumps. Service reservoirs are sized to balance the difference between the near-constant supply rate and the fluctuating demand rate. All three come from the same per capita figure, multiplied by different factors.

Increasingly, yes. Rising temperatures increase outdoor water use in temperate cities and increase evaporation from open reservoirs. Longer dry spells reduce the security of source yield just when demand is highest. Many utilities now prepare demand projections under multiple climate scenarios and build in additional headroom for the upper percentile. Others add a "climate factor" of 5–15 % to the design demand in regions expected to see significant warming or drying. The calculation itself is unchanged — only the projected input value moves.

It is only as accurate as its weakest input. Production is usually well metered at a modern treatment works, so the supply figure is typically reliable to within a few percent. Population, on the other hand, is often a planning estimate that can be out by 10–30 % in fast-growing areas. A single year of data is also misleading because of weather, so utilities usually work with a rolling three-year average. The practical rule is to treat any PCD figure as a planning estimate with a band of ±15 %, and to revisit it annually as new meter readings and population estimates become available.