| Scenario | Area (ha) | Rate (L/s/ha) | Allowance (L/s) | ADWF (L/s) | Total (L/s) |
|---|---|---|---|---|---|
| Residential catchment | 35.0 | 0.25 | 8.75 | 12.50 | 21.25 |
| Mixed-use corridor | 22.0 | 0.35 | 7.70 | 10.00 | 17.70 |
| Rehab priority zone | 15.0 | 0.45 | 6.75 | 6.80 | 13.55 |
These numbers are illustrative. Always use your authority’s required inflow criteria.
- Direct: ADWF is entered as a flow rate.
- Population-based: ADWF = (Population × Per‑capita) ÷ 86,400.
- Per‑capita accepts L/person/day or gpcd (US gallons).
- Area: Allowance = Area × Rate.
- Length: Allowance = Length × Rate.
- Percent: Allowance = ADWF × (Percent/100).
- Adjusted: Allowanceadj = Allowance × Adjustment Factor.
- Total Design Flow = ADWF + Allowanceadj.
- PWWF = Total × Peak Factor.
- Harmon: PF = 1 + 14 / (4 + √Pk) with Pk in thousands.
It lets you reflect local risk drivers—groundwater, aging joints, surcharge history, and rainfall intensity—without changing the base allowance standard.
- Choose how you will define ADWF: direct flow or population-based.
- Select the allowance method that matches your design guideline.
- Enter the required values and confirm units for each field.
- Set an adjustment factor to reflect condition and uncertainty.
- Select peaking mode, then click Calculate Allowance.
- Review results shown above the form, then download CSV or PDF.
For compliance work, verify rates and peaking factors against local standards and project specifications.
Design purpose and allowance context
Inflow and infiltration allowances protect collection systems during wet weather. This calculator adds an explicit allowance to average dry-weather flow (ADWF) so pipes, pumps, and downstream units are sized with a transparent safety margin. Use it for preliminary layouts, rehabilitation comparisons, and design checks where standards specify an allowance separate from peak factors.
Dry-weather flow inputs and unit control
ADWF can be entered directly in L/s, m³/day, gpm, MGD, or cfs, or derived from population and per‑capita demand. The population method converts per‑capita flow to liters per day, then divides by 86,400 seconds. Unit conversions are built in, so mixed sources can be normalized before applying allowance rules.
Allowance methods aligned to field data
Select an allowance based on the same basis used by your authority. Area-based inflow multiplies service area by an allowance rate (for example, 0.25 L/s/ha). Length-based inflow multiplies pipe length by a rate (for example, 0.15 L/s/km). Percent-based inflow applies a percentage of ADWF, useful when only flow records exist. Direct entry supports fixed allowances from reports.
Adjustment factor and peaking options
An adjustment factor scales the allowance for asset condition, groundwater exposure, or rainfall risk without rewriting the base standard. The calculator then combines ADWF and adjusted allowance to form total design flow. For peak wet-weather flow, choose a constant peak factor or the Harmon approach, which relates peaking to population size and helps prevent overdesign for larger service areas.
Results, reporting, and quality checks
Outputs include ADWF, allowance, total design flow, and peak wet-weather flow in both L/s and m³/day. Export buttons create CSV and PDF summaries for reviews, submittals, and calculation packages. Validate inputs by checking that units match the governing criteria, that population is provided when Harmon peaking is selected, and that the adjustment factor reflects documented site assumptions. Record the selected method, rates, and conversion units so reviewers can reproduce your design basis.
Frequently Asked Questions
What does the calculator treat as “allowance”?
Allowance is the additional inflow/infiltration flow added to ADWF before peaking. It represents wet-weather intrusion expected from area, pipe length, or policy percentages, then can be scaled by an adjustment factor.
Which allowance method should I select?
Use the method required by your local standard. Area and length methods match asset-based criteria. Percent of ADWF works when you have reliable flow records. Direct input fits fixed allowances from studies or permit conditions.
How are flow units converted?
All inputs are normalized internally to liters per second. The tool converts common engineering units such as m³/day, gpm, MGD, and cfs, then reports results back in L/s and m³/day for quick checking.
What is the adjustment factor used for?
It scales the allowance to reflect risk, condition, and uncertainty—such as high groundwater, poor joints, or intense rainfall. Apply it only when you can justify the uplift with inspection findings, rainfall history, or policy guidance.
How does Harmon peaking work here?
Harmon peaking estimates a peak factor from population size and applies it to the total flow (ADWF plus adjusted allowance). If population is missing, the calculator falls back to the constant peak factor you entered.
What do the CSV and PDF exports include?
Exports summarize the selected method, ADWF, adjusted allowance, total design flow, peak factor, and peak wet-weather flow. They are designed for review packages and can be attached to calculation sheets or submittal records.