Inputs
Example Data Table
| Case | Design Flow | Slope | n | y/D | Expected Output |
|---|---|---|---|---|---|
| A | 12.0 L/s | 1.0% | 0.013 | 0.80 | Diameter ~ 200–250 mm, verify velocity |
| B | 25.0 L/s | 0.8% | 0.011 | 0.85 | Diameter ~ 250–315 mm, adequate capacity margin |
| C | 80 gpm | 1.5% | 0.009 | 0.75 | Diameter ~ 4–6 in, strong self-cleansing |
These examples are illustrative. Confirm final sizing using your applicable code and project criteria.
Formula Used
This calculator uses the Manning equation for open-channel flow in a circular conduit at a selected depth ratio.
- Q = (1/n) · A · R^(2/3) · S^(1/2)
- V = Q / A
- S is slope (m/m). n is Manning roughness.
- A and wetted perimeter are computed from circular segment geometry for the chosen y/D.
How to Use This Calculator
- Select SI or US units to match your drawings.
- Choose Design Flow or DFU input, then enter values.
- Enter slope and select a suitable roughness coefficient.
- Set depth ratio and velocity limits for performance checks.
- Use standard diameters for quick sizing, or check a custom size.
- Press Calculate to see the recommended diameter and metrics.
- Download CSV or PDF to attach results to submittals.
Design Flow and Fixture Loading
Building sewer sizing starts with a peak flow. If you have fixture units, convert DFU to flow with an adjustable factor and review the implied peak. Example: 120 DFU × 0.10 L/s per DFU = 12.0 L/s. Keeping the conversion visible avoids hidden assumptions when tenant counts or fixture schedules change.
Slope Control and Capacity
Slope is entered as percent and used as grade in the hydraulic check. In Manning flow, capacity varies with the square root of slope, so small grade changes can reduce required diameter or restore velocity. Where cover limits grade, test a few realistic slopes and compare the capacity margin for design resilience.
Partial-Flow Hydraulics in Circular Pipes
Sanitary sewers often run partially full. At a chosen depth ratio y/D, the calculator computes wetted area and hydraulic radius using circular-segment geometry, then applies Manning’s equation. Many designs check y/D from 0.70 to 0.90 to preserve air space and reserve capacity. The reported area and radius support clear peer review.
Velocity Targets and Solids Transport
Minimum velocity helps limit deposition, while a maximum velocity check limits abrasion on steep reaches. Compare computed velocity at the chosen y/D to your targets. If velocity is low, consider a slightly steeper slope, a lower depth ratio, or a smaller standard size that still meets capacity. These checks reinforce a self-cleansing intent.
Diameter Selection and Constructability
The standard size list matches common supply chains and simplifies submittals. For unusual materials or inner diameters, a custom check evaluates one diameter directly. Example check: 12.0 L/s, 1.0% slope, n=0.013, y/D=0.80 typically falls near 200–250 mm. Confirm cleanouts, bends, and connection elevations against local code requirements.
- A: 12.0 L/s, 1.0% slope, n=0.013, y/D=0.80.
- B: 25.0 L/s, 0.8% slope, n=0.011, y/D=0.85.
- C: 80 gpm, 1.5% slope, n=0.009, y/D=0.75.
FAQs
What does the calculator size for?
It checks a circular sewer at a selected depth ratio and finds the smallest candidate diameter that meets design flow and your velocity limits, using Manning-based partial-flow geometry.
Can I size from fixture units?
Yes. Choose the DFU option and set a DFU-to-flow factor that reflects your design method. The tool shows the converted peak flow so you can validate the assumption.
Why is y/D important?
Depth ratio controls wetted area and hydraulic radius. A higher y/D can increase capacity but may reduce air space. Typical checks use 0.70–0.90 depending on criteria.
What roughness value should I pick?
Select a value that matches pipe material and condition. Smoother materials use lower n, while older or rougher conduits use higher n. Confirm with your specifications.
What if velocity is below the minimum?
Try a steeper slope, a smaller diameter, or a lower depth ratio. If constraints prevent changes, document the risk and consider maintenance access, flushing, or other mitigation.
Does it replace code-based tables?
No. It is a transparent engineering check. Always confirm fixture-unit conversions, minimum diameters, slopes, and venting requirements against the governing plumbing and sewer standards.
How accurate are the PDF and CSV exports?
They export the computed result fields shown on screen, including flow, slope, diameter, capacity, and velocity. Use them as calculation records and attach them to reviews or submittals.