Find the scale for the map you’re going to use – it might be a ruler bar scale or a written scale, in words or numbers.
If you’re using a “normal” road map, you’ll usually find the scale near or in the map legend.
Use a ruler or the Smartruler I featured in my last review (shown below) to measure the distance between the two places. If the line is quite curved, use a string to determine the distance and then measure the string.
If the scale is a representative fraction (and looks like 1/100,000 or 1:100,000), multiply the distance of the ruler by the denominator, giving distance in the ruler units.
If the scale is a word statement (i.e. “One centimeter equals one kilometer”) then determine the distance: 1cm:1KM or 1/100,000
For a graphic scale, you’ll need to measure the graphic and divide the scale into the measured units on the ruler.
Convert your units of measurement into the most convenient units for you (i.e. convert 63,360 inches to one mile).
If you have the data connectivity, you can use the Map Scale Calculator found here.