Slope

How to calculate terrain slope: percent, degrees and permille

10 min read June 18, 2026

Slope looks like a simple ratio, but the elevation source, horizontal distance and field line matter.

Field takeaways

Percent slope is vertical difference divided by horizontal distance, multiplied by 100.
Permille is useful for drainage and pipe grades.
Degrees are angular slope and should not be confused with percent.
GNSS height and DEM elevation need control for precise work.
MapLab Survey helps read two-point and along-line slope in the field.

Basic formula

Percent slope is vertical difference divided by horizontal distance, multiplied by 100. A 2 m rise over 50 m gives 4 percent slope. Permille multiplies the same ratio by 1000. Degrees are the angular expression of that ratio.

Elevation source matters

Phone altitude, RTK ellipsoidal height, orthometric elevation and DEM elevation are not the same. For drainage, road grades and foundation levels, the elevation reference must be clear.

MapLab Survey workflow

Draw or select a line, read slope and profile, and mark suspicious breaks with photos and notes. Low-grade drainage work needs tighter vertical control.

In the field · MapLab Survey

Measure, check and deliver in the right format.

MapLab Survey is a mobile field app for RTK/GNSS points, drawings, area/volume calculations, coordinate conversion and export to DXF, KML/KMZ, GeoJSON and CSV. Capture, calculation and export can work offline; live NTRIP corrections and online basemaps need connectivity.

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Frequently asked questions

How is percent slope calculated?

Vertical difference divided by horizontal distance, multiplied by 100.

Is percent the same as degrees?

No. Degrees are angular slope.

What is permille slope?

The same ratio multiplied by 1000.

Is phone elevation enough?

For reconnaissance only. Precise work needs control.

Technical references

The field guidance in this article is aligned with the technical and official references below.