Inputs
Example data table
| Diameter | Depth (y/D) | Slope (%) | n | Estimated Q (L/s) | Estimated V (m/s) |
|---|---|---|---|---|---|
| 200 mm | 0.60 | 0.80 | 0.011 | ~21 | ~0.83 |
| 300 mm | 0.75 | 0.50 | 0.013 | ~56 | ~1.06 |
| 450 mm | 0.70 | 0.40 | 0.013 | ~140 | ~1.10 |
| 600 mm | 0.80 | 0.30 | 0.013 | ~270 | ~1.20 |
Formula used
This calculator uses the Manning equation for gravity flow: Q = (1/n) · A · R^(2/3) · S^(1/2)
- Q = discharge (m³/s)
- n = Manning roughness (dimensionless)
- A = flow area (m²)
- R = hydraulic radius = A/P (m)
- P = wetted perimeter (m)
- S = slope (dimensionless)
For partially full circular pipes, A and P come from circular-segment geometry using a central angle θ (radians).
How to use this calculator
- Select your pipe diameter and unit.
- Choose a flow condition: partially full (enter y/D) or full pipe.
- Enter slope in %, ‰, or as a direct ratio (m/m).
- Select a material preset, or choose custom and edit Manning n.
- Pick output units for flow and velocity.
- Press Calculate. Results appear above the form and below the header.
- Use Download CSV/PDF to save outputs for documentation.
Sewer flow design notes
1) Why gravity flow estimates matter
Gravity sewers are typically sized to convey peak wet-weather flow without surcharge, while still maintaining adequate velocity during dry-weather periods. This tool helps you see how diameter, slope, roughness, and depth ratio combine to influence capacity and velocity.
2) Typical pipe sizes and service ranges
Common municipal laterals start near 150–200 mm, while collectors and trunks often range from 300–1200 mm. Larger diameters increase area rapidly, but they can reduce velocity at low flow, which may affect settling risk and maintenance frequency.
3) Practical slope guidance
Slopes often fall between 0.2% and 2.0%. Steeper grades generally raise velocity and help transport solids, but they can increase excavation depth and energy at drop structures. Flatter grades may require smaller diameters or closer grade control to maintain performance.
4) Roughness values and real-world aging
Smooth plastic pipes may use n ≈ 0.011, while many concrete and clay installations are near n ≈ 0.013. Deposits, root intrusion, joint offsets, and corrosion can increase effective roughness. For conservative checks, engineers often test a slightly higher n to represent aging.
5) Partially full operation is normal
Unlike pressurized lines, gravity sewers commonly flow partially full, leaving headspace for ventilation and transient wet-weather surges. The depth ratio y/D changes the flow area and wetted perimeter, which changes the hydraulic radius and therefore changes the Manning discharge.
6) Velocity targets and self-cleansing
Many practices aim for about 0.6 m/s (roughly 2 ft/s) or higher at key flow conditions to reduce deposition. This calculator highlights a simple self-cleansing check. If velocity is low, consider slope adjustments, diameter selection, or operational strategies.
7) Allowances for infiltration and peak factors
Capacity checks typically include allowances for infiltration/inflow and a peak factor that converts average sanitary flow to a design peak. Peak factors often range from 2 to 4 for small basins, while inflow during storms can dominate short durations. Record assumptions in design notes so future operators understand the basis for capacity and surcharge checks. If you have a target flow, you can iterate diameter or slope until the estimated capacity exceeds the required peak with a reasonable margin.
8) Interpreting results for constructability
Use the discharge and velocity together. A pipe that meets peak capacity but runs too slow at normal flow may experience sediment buildup. Also consider cover requirements, minimum slopes, manhole spacing, and local standards before finalizing dimensions and grades.
FAQs
1) What does the depth ratio y/D represent?
It is the flow depth divided by pipe diameter. A value of 0.75 means the water depth is 75% of the diameter, which changes area, wetted perimeter, and hydraulic radius.
2) Should gravity sewers be designed as full pipes?
Most gravity sewers are designed to run partially full under normal conditions. Full-flow checks are still useful for extreme events, but long-term operation typically remains below full depth.
3) Which Manning n should I use?
Start with a material-based n, then consider aging and deposits. Smooth plastic may use about 0.011, while many concrete or clay lines use about 0.013. Increase n for conservative checks.
4) Why does velocity matter in sewer design?
Velocity helps move solids and reduces settling. Low velocities can increase maintenance and odor issues. Many designs aim near 0.6 m/s or higher at key operating conditions, depending on standards.
5) What slope unit should I select?
Use percent for common grade notation, permil for finer resolution, or ratio if you already have a dimensionless slope. The calculator converts these consistently into S for the flow equation.
6) Can I use this tool for pressurized force mains?
This tool is for gravity flow using Manning. Force mains are pressurized and are usually analyzed with different methods such as Hazen–Williams or Darcy–Weisbach, including pump curves and losses.
7) How can I size a pipe to meet a required flow?
Enter your slope and roughness, then adjust diameter and depth ratio until the predicted discharge exceeds your required peak. Check velocity at typical flow depths to avoid overly slow operation.
Accurate sewer sizing starts with clear flow assumptions always.