The calculator estimates design peak flow by combining average dry-weather flow, a peaking factor, and any infiltration/inflow (I/I) allowance:
Qavg= population × per-capita × return factor ÷ 86400, or entered directlyQpeak_sanitary=Qavg × PFQtotal=Qpeak_sanitary + QI/IQdesign=Qtotal × (1 + safety%)
When Harmon is selected, the peaking factor is:
PF = 1 + 14 / (4 + √(P/1000)), with P as population.
Bounds are applied to avoid unrealistic values.
- Choose an average flow method (population-based or direct).
- Select a peaking factor method and set reasonable bounds.
- Include I/I if wet-weather allowances are required.
- Add a safety or growth factor for planning horizons.
- Pick the output unit you need for hydraulic checks.
- Click Calculate, review notes, then export CSV or PDF.
| Scenario | Population | Qavg (L/s) | PF | I/I (L/s) | Safety | Qdesign (L/s) |
|---|---|---|---|---|---|---|
| Residential district | 25,000 | 31.2500 | 3.292 | 4.6875 | 10% | 118.4 |
| Mixed-use corridor | 60,000 | 75.0000 | 2.777 | 6.0000 | 15% | 256.1 |
| Small subdivision | 4,000 | 4.1667 | 4.000 | 1.0000 | 10% | 19.5 |
Design basis for peakflow
Sanitary sewer peakflow is the design driver for gravity mains and pump stations. This calculator starts with average dry‑weather flow (Qavg) and applies a peaking factor (PF) to represent diurnal variation. For preliminary work, typical residential per‑capita rates range from 80 to 200 L/person/day, while return‑to‑sewer factors commonly fall between 75% and 100% depending on irrigation and reuse.
Population, Qavg, and unit consistency
When you use the population method, Qavg is computed as population × per‑capita × return factor ÷ 86,400 to convert daily volume to L/s. If you already have measured or modeled average flow, enter it directly and the tool converts it internally to L/s. Output can then be reported in L/s, m³/s, m³/day, gpm, MGD, or cfs without changing the underlying math.
Peaking factor selection
Harmon’s approach relates PF to population size, giving higher peaks for smaller systems. As population increases, PF reduces, reflecting diversified usage. The calculator also offers a rule‑of‑thumb banded PF and a fixed PF option. Min/max bounds provide QA/QC control; for example, many designers cap PF between 1.8 and 6.0 to avoid extreme peaks that can oversize pipes.
Infiltration and inflow allowances
Wet‑weather allowances can be represented as a percentage of Qavg, as a length‑based rate (L/s·km), or as a direct flow. Length‑based inputs are useful for master planning where network length is known; for instance, 0.10 to 0.50 L/s·km is often used for comparative studies. Always align I/I assumptions with local standards and observed groundwater conditions.
Design flow, margins, and reporting
The final design peak flow equals (Qavg × PF + QI/I) × (1 + safety%). A 5% to 20% safety margin is common for near‑term sizing, while larger margins may be justified for phased growth. Use the optional minimum peak floor for very small contributing areas. Export CSV for calculations and PDF for submittals, ensuring transparent documentation of chosen assumptions. For sensitivity, rerun with alternative PF and I/I values, then compare velocity and depth checks against criteria.
What does this calculator output for design?
It reports average flow, selected peaking factor, I/I allowance, and the final design peak flow in your chosen unit, plus a breakdown table suitable for checking and exporting.
When should I use Harmon versus a fixed peaking factor?
Use Harmon when population is known and you want a population‑sensitive peak. Use a fixed factor when your client standard specifies one, or when you are calibrating to monitored diurnal data.
How is I/I handled during wet weather sizing?
I/I can be added as a percent of average flow, as a length‑based allowance, or as a direct flow. The value is added to peak sanitary flow before applying the safety margin.
Why are min and max bounds applied to the peaking factor?
Bounds prevent unrealistic peaks from dominating the design. They support QA/QC by keeping PF within a plausible range for your system while still allowing sensitivity studies.
Can I size small laterals with a minimum peak floor?
Yes. Enable the minimum peak floor to force a baseline design flow for small contributing areas. This helps maintain practical pipe sizes when computed demand is extremely low.
What should I include in submittals or design notes?
State the average flow basis, peaking method, I/I method and values, safety margin, and output units. Export the PDF for documentation and attach the CSV if reviewers want calculations.