Water Supply Expected Demand Formula

Overview

Engineering reference data for Water Supply Expected Demand Formula in water systems.

Key Formulas

Hydrostatic Pressure

$P = \rho g h$

Pressure due to water column.

Flow Rate

$Q = A v$

Area × velocity.

Variables

Expected Demand Formula

The expected total water flow ($q_{et}$) for a water supply system accounts for the intermittent nature of fixture use and provides a more realistic estimate than the simple sum of all fixtures. The formula is:

$q_{et} = q_{nl} + 0.015 (\Sigma q_n - q_{nl}) + 0.17 (\Sigma q_n - q_{nl})^{1/2}$

Where:

• $q_{et}$ = Expected total water flow (l/s)
• $q_{nl}$ = Demand of the largest consumer/fixture (l/s)
• $\Sigma q_n$ = Total theoretical water flow, the sum of all fixture demands (l/s)

Important Property: The expected total water flow ($q_{et}$) can never be less than the demand of the largest single fixture ($q_{nl}$).

Total vs. Expected Demand Reference

The following table shows expected demand values for various total theoretical demands, based on the formula above with a maximum fixture load of 0.2 l/s.

Application Example

Scenario: Main water supply to a nursing home.

• Total theoretical demand ($\Sigma q_n$) = 50 l/s
• Largest fixture demand ($q_{nl}$) = 0.4 l/s

Calculation:

q_{et} &= 0.4 + 0.015 (50 - 0.4) + 0.17 (50 - 0.4)^{1/2} \\ &= 0.4 + 0.744 + 1.156 \\ &= 2.3 \text{ l/s} \end{aligned}$$ ## Important Limitations This formula is **valid for ordinary systems** with consumption patterns found in homes, offices, and nursing homes. **Caution:** The formula should **not** be used for systems serving large groups where use is intermittent and synchronized. This includes applications such as: - Hotels - Hospitals - Schools - Theaters - Wardrobes/locker rooms in factories For these applications, it is likely that many fixtures (e.g., all showers) are used simultaneously. Using the formula blindly will result in undersized supply lines. A separate, more detailed analysis is required for such peak-demand scenarios. ## Interactive Charts <InteractiveChart columns={[ { key: "col0", label: "Total Theoretical Demand (Σqₙ)", type: "number", unit: "l/s" }, { key: "col1", label: "Expected Demand (qₑₜ)", type: "number", unit: "l/s" } ]} rows={[ { col0: 0.2, col1: 0.2 }, { col0: 0.8, col1: 0.4 }, { col0: 1.6, col1: 0.5 }, { col0: 4, col1: 0.6 }, { col0: 8, col1: 0.85 }, { col0: 15, col1: 1.1 }, { col0: 20, col1: 1.5 }, { col0: 30, col1: 1.8 }, { col0: 40, col1: 2.1 }, { col0: 65, col1: 2.8 }, { col0: 70, col1: 2.9 }, { col0: 100, col1: 3.7 } ]} xKey="col0" title="Water supply - expected demand" source="engineeringtoolbox.com" /> ## References - [Original Source](https://www.engineeringtoolbox.com/water-supply-expected-demand-formula-d_1076.html)