Water Demand Calculator
Size potable water systems by estimating occupant usage, peak hour demand, storage requirements, and landscape irrigation needs based on standard demand factors.
Typical residential demand is 60–80 gallons per person per day.
Use 2.5–4 for small systems; lower for large campuses.
Lawns often need 0.5–1.5 inches per week depending on climate.
Backup storage is often sized for 4–12 hours of peak demand.
Daily demand
7,800 gal/day
Peak hour demand
13.54 gpm
Storage requirement
4,875 gallons
Daily irrigation demand
1,001.25 gallons
Annual water demand
3,212,456.25 gallons
How to Use This Calculator
Define occupant usage
Enter the building population and daily water use per person or per unit based on design guidelines.
Adjust peak factors and irrigation
Use plumbing or municipal standards to set peak hour multipliers and landscape irrigation requirements.
Size storage and distribution
Review storage and annual demand outputs to size tanks, pumps, and water supply connections.
Formula
Daily Demand = Occupants × Gallons per Person
Peak Hour (gpm) = Daily Demand ÷ 24 ÷ 60 × Peak Factor
Storage = Peak Hour × 60 × Storage Hours
Irrigation = Area × Inches/week × 0.623 ÷ 7
Example: 120 occupants at 65 gal/day require 7,800 gal/day, 13 gpm peak flow (factor 2.5), and ~4,800 gallons of storage for 6 hours of supply. A 15,000 sq ft landscape needing 0.75 inches/week adds 1,000 gallons per day.
Modify peak factors for schools, hospitals, or industrial processes per design manuals.
About the Water Demand Calculator
Engineers and facility managers rely on accurate demand estimates to design resilient water systems. This calculator offers quick sizing for projects ranging from multifamily buildings to campuses, incorporating indoor and outdoor consumption.
When to Use This Calculator
- Preliminary design: Develop quick estimates before detailed hydraulic modeling.
- Municipal planning: Forecast demand for new developments or zoning changes.
- Water conservation: Evaluate savings from fixture upgrades or xeriscaping.
- Emergency planning: Size storage for backup supply and resilience scenarios.
Why Use Our Calculator?
- ✅ Integrated factors: Combines indoor demand, peak flow, storage, and irrigation.
- ✅ Flexible inputs: Adapt to residential, commercial, or institutional contexts.
- ✅ Engineer-informed: Uses standard coefficients from plumbing and landscape design guides.
- ✅ Actionable outputs: Provides the key numbers needed for design decisions.
Common Applications
Architects: Provide utilities with demand estimates during permitting.
Facility operators: Benchmark existing consumption against design assumptions.
Consultants: Evaluate the impact of water-saving retrofits on peak flows and storage needs.
Tips for Best Results
- Segment occupants (residents, staff, visitors) to apply tailored demand factors.
- Incorporate seasonal irrigation adjustments or smart controller savings.
- Validate assumptions with meter data when retrofitting existing buildings.
- Account for fire flow separately; this tool focuses on potable and irrigation demand.
Frequently Asked Questions
How do I choose the peak factor?
Use tables from the Uniform Plumbing Code or local regulations. Small systems have higher diversity factors, while large systems peak less.
Can I include process water?
Yes. Add expected process demand to daily usage or treat as a separate line item added to the final result.
Does the irrigation formula assume irrigation every day?
It spreads weekly irrigation evenly across the week. Adjust for actual watering schedules if different.
How do I size pumps from this data?
Use the peak hour demand (gpm) plus safety factors to size pumps and distribution piping.