A percentage slope becomes a real elevation difference over distance. Establish the endpoints before excavation.
1. Establish a datum and both endpoints
Choose a stable elevation datum and measure the upstream outlet, downstream inlet and ground profile. Work with pipe invert levels—the inside bottom of the pipe—rather than vague trench or pipe depths.
Record horizontal route length separately from elevation difference. A laser or level and a written profile reduce cumulative errors that are difficult to correct after bedding is placed.
2. Convert the specified slope into fall
A 2% slope means 2 units of vertical fall per 100 equal units of horizontal run. Multiply horizontal length by slope divided by 100. The calculator also adds that fall to an upstream invert depth.
Do not assume one universal slope. Pipe diameter, fixture type, wastewater characteristics and local plumbing rules affect what is permitted. Excessive or irregular grade can also create installation and performance problems.
3. Check cover, crossings and the receiving system
Plot the calculated invert against ground level, required cover, frost exposure, foundations and other utilities. Include bedding thickness when converting pipe elevation to trench depth.
EPA explains that septic-system layout depends on factors including lot slope, soil, groundwater and local rules. The tank inlet elevation and access for future service can constrain the entire route.
EPA · Frequently asked questions about septic systems ↗4. Transfer and verify the grade on site
Set repeatable level marks, prepare uniform bedding and check each section before backfill. Support fittings and protect penetrations so local settlement does not create a low spot.
Record as-built invert levels, cleanouts and changes in direction. Stop if excavation conditions, groundwater or unknown utilities differ from the plan.
From profile to trench
| Quantity | Reference | Do not confuse with |
|---|---|---|
| Ground elevation | Site datum | Surface appearance |
| Pipe invert | Inside bottom of pipe | Top or centreline |
| Required cover | Top of pipe to surface | Invert depth |
| Bedding | Below pipe | Pipe cover |
| Fall | Difference between inverts | Sloped pipe length |
Worked example
A 15-metre drain at 2%
The upstream invert is 0.60 m below the chosen datum.
Fall = 15 × 0.02 = 0.30 m = 300 mm Downstream invert depth = 0.60 + 0.30 = 0.90 m
Cover and trench depth still depend on pipe diameter, bedding, ground profile and local requirements.
What to have ready
- Stable site datum and level survey
- Required pipe size and slope
- Endpoint invert elevations
- Cover, bedding, crossings and cleanouts
Common mistakes
- Measuring slope along the pipe instead of horizontally.
- Using top-of-pipe depth as the invert.
- Excavating before checking the downstream elevation.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.