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Drain Slope Calculator

Audit source-labelled drain-segment fall, slope and percent grade against an already documented comparison slope—without selecting a minimum, pipe, drainage method or compliance.

Use this result well

Inputs that matter
One de-identified reviewed drain segment; named geometry, common-datum/direction and comparison-slope records; one paired feet/inches/in-per-foot or metres/millimetres/mm-per-metre unit system; positive horizontal run; and finite start/end elevations plus documented comparison slope
Output to expect
Source-preserving signed fall, signed slope, signed percent grade, documented comparison slope and signed reproduced-minus-comparison difference
How it works
Subtract endpoint elevation from start elevation on the named datum, divide by horizontal run and compare only with the entered source-selected slope; do not insert a quarter-inch minimum, infer direction, select pipe/drainage method or declare compliance
  • Confirm start/end direction, common datum, horizontal projection versus developed route and field measurement tolerance; verify uniform alignment/grade across the complete installed segment.
  • Verify pipe diameter/material, DFU load, applicable adopted table and exceptions, fittings/cleanouts, grease-interceptor or combination-waste-and-vent conditions, pump/ejector/backwater needs, installation and authority approval separately.

Choose your path

Built around the job you need to finish

Reproduce signed drain-segment fall, slope and grade from a source-labelled horizontal run and common-datum elevations, then compare an already documented slope without selecting a minimum, pipe or compliance.

Plumbing designer auditing one drain segment

Check geometry without a fixed quarter-inch threshold replacing pipe/load/system classification.

Name the geometry, datum/direction and comparison-slope records; choose one paired unit system; inspect signed fall/slope and difference.

Can trace the segment while diameter, DFU loading, special conditions and approval remain in the complete design.

Field reviewer reconciling an as-built survey

Preserve datum and direction so endpoint elevations are not silently reversed.

Select field/as-built basis, copy horizontal projection and two elevations, then compare the reproduced sign with the submitted slope record.

A negative or positive difference is arithmetic, not an automatic pass/fail or pump instruction.

Apprentice comparing drain geometry

Learn in/ft, mm/m and percent-grade relationships without mistaking one threshold for every drain.

Run separate US and metric education records, including a signed rise, and review explicit exclusions.

Can reproduce geometry and list pipe/load, uniformity, fittings, special systems and authority checks the tool omits.

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Reference & details

How it works

Preserve Geometry, Datum and Comparison Sources

Horizontal run and both endpoint elevations must share an explicit measurement record and datum. The comparison slope remains owned by a separate responsible design/review; the tool never inserts a code-table value.

Record = geometry source + datum/direction source + comparison-slope source

Reproduce Signed Geometry

Positive, zero and negative signs are preserved. US records pair feet with inch elevations and in/ft; metric records pair metres with millimetre elevations and mm/m. No flow direction or drainage suitability is inferred from the sign.

Signed fall = start − end; signed slope = fall ÷ horizontal run

Compare Without a Verdict

The signed difference is record-reconciliation arithmetic only. Diameter, DFU loading, uniform alignment, special systems, fittings, installation tolerance and adopted requirements determine whether the complete system is acceptable.

Difference = reproduced signed slope − documented comparison slope

Updated: August 2026

Example Scenarios

A designer copies horizontal projection, endpoint elevations and the separately selected design slope. The tool exposes the arithmetic while pipe size, loading and applicable provisions remain in the design record.

A reviewer records a common datum and intended start-to-end direction before comparing surveyed elevations with the submitted slope. Uniformity between endpoints still requires field verification.

An apprentice enters metres, millimetres and mm/m for a segment whose endpoint is higher. The negative sign becomes a review question, not an automatic pump/ejector recommendation.

FAQ

No. Applicable slope depends on pipe/system classification, diameter, load, special conditions, adopted requirements and responsible review. Copy the selected comparison slope from its named source.

Swapping endpoints reverses the sign, while elevations from different datums are not comparable. The record must state which endpoint is start and which direction is being reviewed.

No. Equal endpoint difference can hide bellies, high spots or alignment changes. Inspect and measure the complete installed route using the responsible field method.

Only that the entered endpoint elevation exceeds the entered start elevation in the stated direction and datum. It does not by itself select a drainage method or equipment response.

No. Diameter/material, DFU load, applicable table/exceptions, fittings/cleanouts, special drainage conditions, installation tolerances and authority approval are outside the geometry comparison.

About Drain Slope Calculator

Reproduce one reviewed drain-segment geometry record without turning three measurements into a code verdict. Name the geometry, common datum/direction and comparison-slope sources; use one complete US or metric unit system; and inspect signed fall, signed slope, percent grade and the signed comparison difference. Pipe/load classification and approval remain in the complete design.