Check a measured drainage profile

Understand the four equivalent ways to state fall

Total fall is start level minus end level. A positive result means the entered end is lower in the selected start-to-end direction. Millimetres per metre equals total fall in millimetres divided by horizontal run. Percentage grade is the same decimal slope multiplied by 100.

A 1:N ratio means one unit of vertical fall across N units of horizontal run. The calculator reports this only for a positive fall. Level or reverse profiles need correction or a deliberately different direction; showing a negative 1:N value would be easy to misread.

Use horizontal run, not sloping pipe length, for ordinary grade calculations. Measure levels to the same reference point, such as pipe invert or finished surface. Mixing an invert at one end with top-of-pipe or surface level at the other produces a meaningless result.

Enter project limits from the correct design source

The minimum and maximum fields are user inputs because drainage requirements vary by system. Sanitary pipe diameter, fixture load, system type and adopted plumbing rules matter. Stormwater design also depends on catchment, rainfall, pipe or channel capacity, overflow and lawful discharge. Paving, ramps and wet areas have different performance and access constraints.

The result is “within entered range” only when fall is positive, grade is at least the entered minimum and, when a non-zero maximum is entered, no greater than that maximum. This is a comparison to your numbers, not a code lookup.

NCC Volume Three contains plumbing and drainage requirements and references AS/NZS 3500 pathways. The ABCB’s verification material demonstrates that gradient interacts with diameter, flow, filling and venting. A slope alone cannot size or approve a drain.

The target total-fall range multiplies run by the entered grades. Target end levels subtract that fall from the start level. When no maximum is entered, the page reports only the minimum target rather than inventing an upper bound.

Survey tolerance can be large relative to a shallow fall

If each endpoint can vary by the entered tolerance, the difference between them can vary by twice that amount in the worst opposing direction. The tolerance grade band subtracts and adds twice the endpoint tolerance from total fall, then converts those limits to grade. Negative limits are retained because they reveal when measurement uncertainty spans, or remains entirely within, reverse fall.

This sensitivity is not statistical confidence and does not replace calibrated survey equipment. It shows why short runs and shallow grades require careful control. A few millimetres of error may consume much of the intended fall.

Use stable benchmarks, record instrument setup, and distinguish design level, set-out level and as-built level. Check intermediate points because correct endpoints can still hide a sag or high spot. Pipe sockets, bedding and deflection can change the effective invert between surveyed points.

Use intervals to set out a straight profile, then verify every segment

The interval outputs divide total horizontal run and measured fall evenly. They are useful for a straight-line check at stakes, stringline points or laser targets. Real drainage may contain bends, junctions, pits, vertical drops or deliberate grade changes, so a single interval schedule cannot describe every segment.

Create a chainage table for construction. At each chainage, record design invert, tolerance, pipe or surface reference and as-built measurement. Keep arithmetic signs consistent: levels decrease in the positive fall direction. A midpoint is shown as a quick linear cross-check.

Do not pull a flexible pipe down at pits while leaving an unsupported sag between them. Bedding, side support, compaction and cover protect the profile and pipe. Excavation near existing footings requires appropriate assessment and control.

Surface drainage must manage water beyond the arithmetic line

NCC Housing Provisions address surface water around Class 1 buildings and the need to divert it away from the building, subject to the applicable pathway and limitations. Finished levels must also preserve slab, cladding, damp and termite clearances. One calculated fall between two points does not confirm all of these interfaces.

Crossfall, local depressions, thresholds, joints and construction tolerances affect actual flow. A path that falls longitudinally may still pond if its cross-section traps water. Grates and pits need capacity, safe placement and accessible maintenance.

Stormwater systems need an overflow route for events beyond pipe capacity. Do not direct water onto neighbours or rely on a doorway threshold as the only flood protection. Check local authority rules, easements, detention and connection requirements.

Sanitary drainage grade works with flow, pipe size and venting

Too little grade can reduce transport and increase blockage risk. A steep pipe is not automatically acceptable either; system performance, permitted drops, connections and venting require coordinated design. NCC verification examples show different gradient conditions for different system arrangements and fixture connections.

Only licensed or authorised practitioners should perform regulated plumbing work where required. They must select materials, diameter, junctions, inspection openings, bedding, cover, testing and connection. The calculator does not generate a compliance certificate or hydraulic calculation.

Keep inspection and test records. Verify invert before backfilling, after initial bedding and after compaction where appropriate. Photograph junctions and measure as-built chainage and levels for future maintenance.

A practical profile-check workflow

Keep units explicit. Survey levels are often in metres while construction falls are discussed in millimetres. A misplaced decimal can change the intended fall by ten or one hundred times. Have another person independently check critical set-out.

Check cover, crossings and outlet level before accepting the grade

A mathematically suitable fall may still be impossible to build. The upstream connection, downstream outlet, required pipe cover, structural beams, footings, other services and property boundary can constrain the vertical corridor. Plot these items on the long section with their clearance requirements rather than discovering the conflict during excavation.

Where gravity drainage cannot reach a lawful outlet with the required profile, do not simply reduce cover or flatten the grade. A designer may need to change route, level, pipe system, pit arrangement or pumping strategy. Pumped systems introduce storage, power, alarms, maintenance and overflow risks that this calculator does not assess.

Crossings beneath driveways and areas subject to loads need suitable pipe strength, bedding, cover and protection. Changes in material or diameter can change invert continuity at sockets and structures. State whether levels refer to upstream invert, downstream invert, pit floor or surface grate.

At handover, keep the chainage table, benchmark, photographs, test results and as-built plan. Drains need accessible inspection and maintenance points. Sediment, roots, construction debris and damaged outlets can defeat a correctly graded line, so operation and maintenance remain part of drainage performance.