Water Demand Calculator
Estimate daily demand, peak demand, diversity effect, and storage requirement.
Open ToolDomestic Hot Water Demand Calculator
Free domestic hot water demand calculator for fixture demand, storage volume, recovery load, heater capacity, diversity, and preliminary plumbing design.
Domestic hot water demand calculator
This domestic hot water demand calculator estimates hot water storage and recovery load from occupancy, fixtures, demand factors, storage duration, and temperature rise. It is useful for apartments, hotels, offices, gyms, staff facilities, kitchens, and mixed-use buildings during early plumbing design. The calculator helps engineers compare whether storage volume or heater recovery capacity is likely to control the design. Final domestic hot water sizing must still consider fixture schedules, peak draw patterns, local plumbing code, ASHRAE or CIBSE guidance, legionella control, recirculation losses, plant redundancy, heat pump or calorifier data, and project specifications. Load the sample first, then adjust fixture demand, diversity, storage time, and temperature rise to understand the controlling assumption.
Inputs
Forms are empty on load. Use sample data to preview the workflow or enter your own project values.
Learn more
Formula logic, use cases, assumptions, and limits for this tool.
How this tool helps
Use this to estimate water heater storage and recovery capacity. Final design must check fixture profile, usage diversity, Legionella controls, and local code.
Assumptions and limits
This tool provides a planning-level result. It does not replace detailed engineering design, approved software, manufacturer data, authority requirements, or project specifications.
Typical use cases
- Early estimation and tender support.
- BOQ sanity checking and technical review.
- Site discussion before final engineering verification.
- Comparison of alternatives using consistent assumptions.
Result interpretation
Read the recommendation, warning, and confidence sections together. A result without warnings is still not an approval; it only means the entered values did not trigger built-in planning alerts.
Quick answer for common searches
Domestic hot water demand depends on users, usage pattern, storage time, recovery rate, and temperature rise. A good planning check separates daily volume from peak recovery capacity.
Useful search phrases covered by this page
- domestic hot water demand
- hot water demand calculator
- domestic hot water sizing
Next checks
Input definitions and typical values
These notes explain what each input means and which values usually produce a more reliable planning result.
Code / standard profile
Selects the planning profile used for conservative defaults. Use the project authority or client standard where known; otherwise use the closest regional profile. Options: GCC project planning, IPC planning.
Occupants / users people
Expected number of people using the space. Use realistic peak occupancy, not only average daily occupancy.
Hot water per user L/user
Project input used directly in the calculation. Use measured, scheduled, manufacturer, or authority-approved values where available.
Fixture/process allowance L
Project input used directly in the calculation. Use measured, scheduled, manufacturer, or authority-approved values where available.
Temperature rise deg C
Temperature input used by the calculation. Use local design or average operating values in Celsius where possible.
Storage factor %
Percentage adjustment used for diversity, derating, margin, reserve, or contingency. Use project-specific assumptions where available.
Recovery time hours
Project input used directly in the calculation. Use measured, scheduled, manufacturer, or authority-approved values where available.
Practical verification checklist
- Confirm the selected standard profile is suitable for the project location and authority.
- Replace sample values with project drawings, schedules, equipment data, or measured site values.
- Review warnings, methodology, and PDF report before using the result outside early planning.
- Verify final decisions against official standards, manufacturer data, consultant specifications, and local approvals.
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Open ToolCalculation methodology
Peak hot water demand is adjusted by storage factor; heating energy and recovery kW use water temperature rise.
Inputs are treated as planning assumptions. The calculator shows warnings when entered values fall outside practical thresholds or selected standard-profile limits.
Version history
- v1.3.6-prelaunch: Added methodology, standard profile support, version notes, and direct PDF report download.
- v1.3.5: Calculator page added with sample data, warnings, and print-ready report layout.
Stage 2 roadmap: optional saved projects, dashboards, and shared calculation history without forcing login for free use.
Trust and verification
Every result includes assumptions, warning logic, and a downloadable report. Final engineering decisions still need qualified review, local authority approval, manufacturer data, and project specifications.
Worked example
Use the sample data button to load a realistic scenario, then download the PDF report to review inputs, results, methodology, warnings, and disclaimers in one place.