Water Demand Calculator

What Is a Water Demand Calculator?

Water utilities and planners need reliable estimates of how much water a population uses each day. This calculator computes average daily demand (ADD), maximum daily demand (MDD), and peak hour flow from population size and per-capita consumption. It supports both liters and US gallons.

Water Demand Formulas

The average daily water demand is calculated as:

$$\text{ADD} = \text{LPCD} \times P \times 1.1$$

Where LPCD is liters (or gallons) per capita per day and P is population. The 10% factor accounts for water waste in the distribution system. Maximum daily demand and peak hour flow use standard multipliers:

$$\text{MDD} = 1.4 \times \text{ADD}$$

$$\text{Peak (L/s)} = \frac{2.3 \times \text{ADD}}{86{,}400}$$

The default LPCD is 225 L/(capita·day), equivalent to 60 gal/(capita·day). You can adjust the MDD and peak ratios in the additional parameters to match local design standards.

Example

For a city of 100,000 residents at 225 L/(capita·day): ADD = 225 × 100,000 × 1.1 = 24.75 million L/day. MDD = 1.4 × 24.75 = 34.65 million L/day. Peak flow = 2.3 × 24,750,000 / 86,400 ≈ 659 L/s. For related tools, see the Tap Water Calculator, the Rainwater Collection Calculator, or the Hydroelectric Power Calculator.