Invert elevation is the elevation of the inside bottom of a pipe at a specific location. For a gravity sewer, storm drain, or culvert, the invert is the lowest interior point of the pipe — where water flows. Engineers specify invert elevations on construction plans so field crews can verify that pipes are installed at the correct depth and slope to maintain gravity flow.
Gravity sewer and storm drain systems work because water flows downhill. Every pipe in the system must be installed at a precise elevation and slope to keep water moving. If a pipe is installed even slightly above design invert, water ponds in the pipe, solids accumulate, and the system eventually fails — backups, overflows, and costly emergency repairs.
Inspectors verify invert elevation at every structure (manhole, catch basin, cleanout, junction box) and often at pipe joints along each run. A failing invert check stops work until the pipe is adjusted or replaced — which on an installed pipe means cutting, re-excavating, and relaying. Verifying invert elevation during installation, before backfill, is far cheaper than discovering a problem after the trench is closed.
At any structure — manhole, catch basin, junction box — pipes enter from the upstream side and leave from the downstream side. The invert of the pipe entering the structure is called invert in. The invert of the pipe leaving is invert out. For gravity flow, invert out is always lower than invert in.
The difference between invert in and invert out at a structure is the manhole drop. A minimum drop of 0.1 ft (1.2 inches) is typically required at manholes where flow changes direction, to account for headloss through the structure. On a straight-run manhole, the drop is typically the pipe slope times the manhole diameter — continuous grade maintained through the structure.
When multiple pipes enter a structure at different inverts, the designer specifies each separately: "IE IN (12" from N) = 98.42, IE IN (8" from E) = 98.55, IE OUT (15" to S) = 97.89."
Compare to the design grade on the plans. If the design calls for 1.0% and you measure 1.0%, the pipe is correctly installed. If you measure 0.3%, the pipe is too flat — gravity flow may be inadequate and the inspector will require adjustment.
Invert elevations appear as callouts at each structure on the utility plan. A typical manhole callout reads:
Breaking this down: RIM is the top of the manhole frame and cover — the elevation a pedestrian would stand on. IE IN is the invert of the incoming pipe, with pipe size and direction noted in parentheses. IE OUT is the invert of the outgoing pipe. The 0.05 ft drop from 98.42 to 98.37 maintains grade through the manhole on a straight run.
On profiles (the side-view drawing), invert elevations are labeled at the centerline of each structure along the pipe run. The profile shows the slope of the pipe and the depth of cover at each manhole.
Most common for active pipe installation. Set in the upstream manhole on a bracket, dial in the design grade, and project a beam down the pipe run. The crew lays pipe to the beam. At each joint, verify the target reading matches design.
Shoot the invert elevation directly from outside the trench. Requires line of sight to the inside bottom of the pipe at the structure. More common for final as-built verification after backfill is complete.
Measure the vertical distance from the manhole rim (known elevation from survey) to the inside bottom of the pipe using a tape measure. Subtract from rim elevation. Quick field check — less accurate than laser or total station.
In construction, "invert" refers to the inside bottom of a pipe, channel, or conduit — the lowest interior point where fluid flows. Invert elevation is the surveyed elevation of that inside bottom at a specific location. Engineers specify invert elevations on utility plans to ensure pipes are installed at the correct depth and slope for gravity flow. The term originates from arch construction where the "inverted arch" forms the base of a conduit.
Three methods: (1) Pipe laser — set in the upstream manhole, project a grade beam, read the invert elevation from the display. (2) Total station or GPS rover — shoot the invert directly with line of sight to the inside bottom. (3) Tape from rim — measure from the manhole rim (known elevation) down to the pipe invert, subtract from rim elevation. The pipe laser is most common during active installation; total station is most common for final as-built survey.
Invert in is the elevation of the pipe entering a structure from the upstream (higher) direction. Invert out is the elevation of the pipe leaving in the downstream (lower) direction. For gravity flow, invert out must always be lower than invert in. The difference is the "manhole drop" — typically 0.1 ft minimum at direction-change manholes, or continuous grade at straight-run manholes.
Grade % = (Invert In − Invert Out) ÷ Pipe Length × 100. Example: upstream invert = 98.42, downstream invert = 97.89, length = 53 ft. Grade = (98.42 − 97.89) ÷ 53 × 100 = 1.0%. Compare this measured grade to the design grade on the plans. Most projects specify a tolerance of ±0.05% — if the measured grade is outside that range, the pipe needs adjustment before the inspection will pass.
Minimum slopes depend on pipe size and the governing plumbing/design code. Under the International Plumbing Code (IPC): 4-inch pipe = 1/4 inch per foot (2% slope minimum); 6-inch pipe = 1/8 inch per foot (1% minimum). These minimums ensure a minimum flow velocity of 2 fps (feet per second) at full flow — the self-cleaning velocity that prevents solids from accumulating. Larger pipes can flow at lower slopes while still meeting the 2 fps minimum due to larger hydraulic radius.
Auto-calculated grade %, pass/fail verification, and inspector-ready as-built reports.