Calculator Inputs
Enter fixture counts and expected use assumptions. The page estimates peak total, hot, and cold water demand.
Formula Used
This educational calculator uses a WDC-style probabilistic model. It is not a substitute for the official adopted calculator or local approval.
- Expected demand: μ = Σ(n × q × p)
- Demand variance: σ² = Σ(n × q² × p × (1 - p))
- Peak demand: Q = min(connected flow, μ + zσ)
- Hot demand uses q × hot share. Cold demand uses q × cold share.
- Hazen-Williams pressure check: head loss per 100 ft = 4.52 × Q^1.85 ÷ (C^1.85 × d^4.87)
Here n is fixture count, q is flow rate, p is adjusted probability, and z is the selected confidence score.
How to Use This Calculator
- Enter the number of dwelling units.
- Select a confidence level for the peak estimate.
- Enter fixture counts, flow rates, probabilities, and hot shares.
- Enter pipe length, available pressure drop, C factor, and velocity limit.
- Press Calculate Demand.
- Review total, cold, and hot demand above the form.
- Download the CSV or PDF summary for project records.
Example Data Table
| Fixture | Example count | Flow rate | Use probability | Hot share |
|---|---|---|---|---|
| Lavatory faucet | 4 | 1.2 gpm | 1.5% | 60% |
| Kitchen sink | 1 | 1.8 gpm | 2.0% | 55% |
| Shower head | 2 | 2.0 gpm | 3.0% | 70% |
| Tank toilet | 3 | 2.5 gpm | 1.2% | 0% |
| Clothes washer | 1 | 2.2 gpm | 1.5% | 50% |
About This IAPMO Water Demand Calculator
Water pipe sizing has changed. Older sizing methods often use conservative fixture unit curves. Those curves can oversize residential systems. The IAPMO Water Demand Calculator approach looks at modern fixtures and real use patterns. This page provides a practical estimator for designers, students, and planners. It does not replace local code review. It helps you compare peak flow, connected flow, hot demand, and cold demand before final pipe sizing.
Why Peak Demand Matters
Peak demand is the short period when several fixtures may run together. It controls service pipes, risers, mains, and branches. If demand is too low, users may see weak pressure. If demand is too high, pipe sizes may become larger than needed. Larger pipes cost more. They also hold more water. Extra water volume may increase wait time for hot water. It may also reduce turnover inside the system.
How This Tool Helps
This calculator uses fixture counts, flow rates, use probabilities, and hot water shares. It estimates average flow first. It then adds a percentile allowance for simultaneous use. You can choose a confidence level. You can also enter equivalent pipe length, available pressure drop, friction coefficient, and velocity limit. The tool recommends a nominal pipe size using a simple Hazen-Williams pressure loss check.
Interpreting Results
The total demand result is useful for the building supply. Cold demand helps size cold branches. Hot demand helps size heater outlets, hot mains, and recirculation planning. The Hunter number shows the average number of busy fixtures during the peak period. Probability of no flow shows how often a segment may sit idle under the entered assumptions. Review each fixture row. A high count, high flow, or high probability can strongly affect the final result.
Design Notes
Use realistic fixture data. Avoid mixing code tables without checking their scope. Confirm whether the project is single family or multifamily. Check local adoption before submitting drawings. Include meter loss, valve loss, elevation, heater limits, and special equipment in a full design. Save the CSV or PDF report for review. Keep assumptions with each project file.
The best output comes from careful inputs. Recheck unusual fixtures, fixture groups, and branches before ordering pipe or preparing permit notes.
FAQs
Is this the official IAPMO calculator?
No. This is an educational web estimator with transparent assumptions. Use the official adopted calculator, current code text, and local authority review for permit work.
What projects is it best for?
It is most useful for early residential demand checks, branch comparisons, classroom examples, and quick design reviews before formal sizing.
What does use probability mean?
Use probability is the chance that one fixture runs during the peak moment. Higher values increase the mean demand and peak allowance.
Why are hot and cold demand different?
Each fixture has a hot water share. Toilets use cold water only. Showers and tubs usually place more load on hot piping.
Which confidence level should I use?
A higher confidence level gives a larger peak estimate. Use the level required by your design policy, reviewer, or local standard.
Can it size the final pipe alone?
No. Final sizing should include meter loss, elevation, valves, fittings, equipment limits, material rules, pressure requirements, and local code conditions.
Why is connected flow higher than peak demand?
Connected flow assumes every fixture runs at once. Peak demand uses probability and diversity because simultaneous full operation is usually unlikely.
What should I export?
Export the final result, fixture assumptions, pipe settings, and recommended sizes. Keep the file with project notes and drawings.