Rise and horizontal run are perpendicular legs of a right triangle. The measured surface or path along the slope is the hypotenuse and cannot replace run without a geometric conversion.
Name all three sides
Rise is vertical change, run is horizontal projection, and slope length joins their endpoints. Percent slope divides the first by the second.
Convert length only with enough information
If you know inclined length and angle, run = length × cos(angle). If you know inclined length and rise, use the Pythagorean theorem. SlopeCalc intentionally asks for run directly.
Normalize units before division
One metre over 20 metres and 100 centimetres over 20 metres describe the same 5% slope. SlopeCalc applies exact rational unit factors first.
Record how each distance was measured
Label the vertical change, horizontal projection, and inclined length before entering numbers. A short note about the measurement direction and units makes the calculation reproducible and exposes an accidental hypotenuse value before it changes the result.