Plan sewer sizes with flow and slope inputs. Check velocity limits and depth targets fast. Export results for reports and team reviews easily today.
Choose a flow method, then size a gravity sewer using slope, roughness, and a target depth ratio.
Sample inputs and typical output for a quick check.
| Design Q (L/s) | Slope | Material n | y/D | Result |
|---|---|---|---|---|
| 60 | 0.005 | 0.011 | 0.80 | Often ~300 mm (site dependent) |
| 120 | 0.004 | 0.013 | 0.80 | Often ~450 mm (site dependent) |
| 250 | 0.003 | 0.013 | 0.85 | Often ~750 mm (site dependent) |
Use your local standards for minimum diameter, cover, and corrosion allowances.
This calculator uses Manning’s equation for gravity flow:
Q = (1/n) · A · R2/3 · S1/2
For partially full circular pipes, the calculator computes A and P from the segment angle based on y/D. It then checks capacity and velocity against your limits.
Always verify final sizing with local codes, minimum diameter rules, cover requirements, and corrosion/abrasion considerations.
Use a verified peak design flow whenever available. If you estimate from population, combine average wastewater generation with a peaking factor, then add infiltration and any project-specific allowances. Consider industrial discharges, groundwater seasonality, and inflow from cross-connections. Apply a safety factor to cover growth, uncertainty, and operational variability without oversizing excessively.
Gravity sewers depend strongly on slope and pipe roughness. A smoother lining lowers the roughness coefficient and increases capacity at the same diameter. Many systems are checked at partial depth (often around 0.7–0.85 of diameter) to preserve ventilation space and accommodate future increases. Where surcharging is expected, confirm the hydraulic grade line separately.
Adequate velocity helps prevent sediment deposition and odor issues, while excessive velocity can increase abrasion at bends and manholes. Use your local self-cleansing criterion for the minimum velocity and a practical upper limit for material durability. If velocity is low, consider a steeper slope, smaller diameter, or staged construction. If velocity is high, consider energy dissipation measures.
The candidate table compares each diameter’s capacity and velocity at the selected depth ratio. A “PASS” indicates both requirements are satisfied for the same diameter, which is typically preferred for constructability and compliance. If no option meets velocity limits, review constraints, depth target, and slope before accepting a capacity-only selection. Also check minimum code diameters and maintenance requirements.
Example inputs: Q = 85 L/s, slope S = 0.0045, n = 0.011, y/D = 0.80, safety factor = 10%, velocity limits = 0.60–3.00 m/s. The calculator will scan standard sizes and may select a mid-range diameter (often 375–450 mm, site dependent). Confirm manhole spacing, cover, bedding class, and alignment constraints. Export the CSV or PDF to capture assumptions and the selected size.
1) What does the depth ratio y/D control?
y/D sets how full the pipe is during the design check. It changes flow area and hydraulic radius, which affects both capacity and velocity. Many sanitary sewers are checked around 0.7–0.85 to keep air space.
2) Should I size a sewer for full flow?
Not usually for sanitary systems. Full-flow checks can be used for surcharge scenarios, but routine sizing often targets partial flow to allow ventilation and growth. Use local criteria and confirm surcharging with a separate hydraulic grade analysis.
3) How do I choose Manning’s n?
Start with the pipe material and lining used on the project, then adjust for age, joint condition, deposits, and expected roughness. When uncertain, use a conservative n and document the assumption in your design notes.
4) What safety factor is appropriate?
Common practice ranges from 5% to 20% depending on data quality, growth expectations, and infiltration risk. Use the smallest factor that still covers uncertainty, and verify that the selected diameter also meets minimum code and maintenance rules.
5) Why is my velocity below the minimum?
Low velocity usually comes from a large diameter, shallow slope, high roughness, or a low design flow. Try reducing diameter, increasing slope where feasible, or rechecking allowances. Field constraints and minimum sizes may limit adjustments.
6) Can this be used for storm drainage pipes?
The math applies to gravity flow, but storm design often uses different return-period flows, inlet controls, and tailwater conditions. Use this tool for preliminary checks, then confirm with your stormwater method and local drainage standards.
7) What should I verify before finalizing the size?
Confirm minimum diameter, cover and clearance, bedding class, corrosion/abrasion risk, manhole spacing, and constructability. Recheck with surveyed slopes, future tie-ins, and any surcharge requirements. Always align outputs with your jurisdiction’s standards.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.