Contour lines are lines drawn on a map that indicate the elevation level of a geographical area. Contour intervals are the spaces between these contour lines. To prevent the map from becoming too cluttered, contour intervals are not always explicitly labeled with the interval that they represent. Instead, only the occasional contour line is labeled and it is assumed that the elevation level that any intermediate contour lines represent can be calculated from these. Being able to calculate contour intervals allows you to determine the elevation level of any intermediate point on a map from the contour lines that surround it.

Locate two contour lines on the map that are labeled with a specific elevation.

Calculate the difference between the two elevation levels that they represent. To calculate the difference, subtract the number representing the line with the lesser elevation from the number representing the line with the higher elevation.

Count the number of intermediate contour lines between the two lines chosen in Step 1.

Add one to the number of intermediate lines calculated in Step 3. For example, if you counted five intermediate lines between your chosen contour lines you would add one to get six.

Divide the result from Step 2 by the result from Step 4. The quotient of these two numbers is the contour interval.
References
 Photo Credit http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contou7.gif