The slope of a straight line is the rate of change of the dependent variable (the variable plotted on the Y-axis) relative to the change of the independent variable (the variable plotted on the X-axis). A straight line has one slope (constant slope), however the slope of a curve is changing at different points on the curve.

To determine the slope of a straight line, take two points on the line and determine the X and Y coordinates of each of these points. The slope can be determined from the following relationship:

The semilog graph paper allows plotting the data at logarithmic intervals without the need for logarithmic transformation.

In order to determine the slope of a line drawn on a semilog graph paper it is necessary to do logarithmic transformation for the y values. The slope of the line in this case will be determined from the following relationship.