Sewer Invert Level Calculator

• Invert level: IL = CL − D (cover level minus depth).
• Fall over length: F = L × S, where S is slope (m/m).
• Percent slope: S(%) = (F ÷ L) × 100.
• 1 in N: N = L ÷ F (use absolute values).
• Downstream invert: IL₂ = IL₁ − F for a falling grade.
• Obvert level: OL = IL + Ø where Ø is pipe diameter in meters.

1. Select the calculation method that matches your field notes.
2. Enter levels in meters, using a consistent datum reference.
3. For slopes, use either percent or 1 in N format.
4. Click Calculate to view results above the form.
5. Download CSV or PDF to attach to schedules and checks.
6. Verify results against design minimum slopes and clearances.

1) Why invert levels matter on site

Sewer inverts control gravity flow, cover depth, and clash risk with other services. A small level error can change fall, reduce capacity, and create ponding. Many projects target grades between 0.5% and 2.0% for common DN150–DN450 lines, then adjust for self-cleansing and constraints.

2) Inputs that drive the calculation

The core field set is cover level (or finished ground), depth to invert, and a consistent datum note. This tool also accepts pipe diameter in millimeters to estimate obvert level, which helps confirm clearance inside manholes and avoid conflicts with incoming branches.

3) Working with slope formats

Crews often receive grades as percent or “1 in N.” A 1.25% grade equals 0.0125 m/m and is the same as 1 in 80. The calculator converts both ways so design, survey, and construction teams can compare values without recalculating.

4) Practical checks and tolerances

Always confirm that cover and invert values reference the same benchmark. Check computed depth is reasonable: many urban installs sit roughly 1.0–4.0 m deep, while deeper runs may require shoring and additional safety controls. Use the “check” outputs to spot inconsistent entries quickly.

5) Example workflow for manhole scheduling

Start with the upstream invert, enter the centerline distance, and apply the nominated grade to compute the next invert. Repeat for each reach to build a manhole schedule. Export CSV for quantity checks and coordination reviews, and keep the PDF output with your inspection records for traceability.

1) What is an invert level?

It is the internal bottom level of a pipe where flow begins. In manholes, it is measured at the pipe’s lowest inside point and used to set grades and connect incoming lines.

2) What units should I use?

Enter cover and invert levels in meters. Enter pipe diameter in millimeters. Keep all levels on the same datum to avoid offsets that can hide grade problems.

3) How do I choose percent or 1 in N?

Use whichever format you were issued. Percent is convenient for calculations; 1 in N is common on drawings. The calculator converts both, so you can cross-check quickly.

4) Why is my depth negative?

Depth is cover minus invert. A negative depth usually means values are reversed, or the datum changed between measurements. Re-check which level is higher and confirm the benchmark.

5) What is obvert level and when is it useful?

Obvert is the top inside of the pipe. It helps verify headroom and incoming pipe clashes within manholes, especially when multiple pipes converge at different elevations.

6) Can I use this for rising mains?

Yes. In the distance-and-slope mode, select “Rise” to add the vertical change instead of subtracting. Still confirm the system design intent and hydraulic requirements.

7) How should I verify results before construction?

Compare outputs to design grades, minimum cover requirements, and clearance rules. Check fall over each reach, then review the chain of inverts to confirm no unintended backfalls exist.