How to Measure Distances on a Map

Woman's hands measuring on navigational chart.
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Maps are useful for more than just directions. They can also help you determine the distance between two (or more) places. The scales on a map can be different types, from words and ratios to pictorial. Decoding the scale is the key to determining your distance.

Here's a quick guide on how to measure distances on a map. All you'll need is a ruler, some scratch paper, and a pencil. 

Here's How

  1. Use a ruler 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.
  2. 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.
  3. If the scale is a word statement (i.e. "1 centimeter equals 1 kilometer") then determine the distance by simply measuring with a ruler. For example, if the scale says 1 inch = 1 mile, then for every inch between the two points, the real distance is that number in miles. If your measurement is 3 5/8 inches, that would be 3.63 miles.
  4. If the scale is a representative fraction (and looks like 1/100,000), multiply the distance of the ruler by the denominator, which denotes distance in the ruler units. The units will be listed on the map, such as 1 inch or 1 centimeter. For example, if the map fraction is 1/100,000, the scale says centimeters, and your points are 6 centimeters apart, in real life they'll be 600,000 centimeters apart or 6 kilometers. 
  5. If the scale is a ratio (and looks like this 1:100,000), you'll multiply the map units by the number following the colon. For example, if you see 1:63,360, that is 1 inch = 1 mile on the ground.
  6. For a graphic scale, you'll need to measure the graphic, for example, white and black bars, to determine how much ruler distance equates to distance in reality. You can either take your ruler measurement of the distance between your two points and place that on the scale to determine real distance, or you can use scratch paper and go from the scale to the map.
    To use paper, you'll place the edge of the sheet next to the scale and make marks where it shows distances, thus transferring the scale to the paper. Then label the marks as to what they mean, in real distance. Finally, you'll lay the paper on the map between your two points to determine the real-life distance between them.
  7. After you've found out your measurement and compared with the scale, you'll convert your units of measurement into the most convenient units for you (i.e., convert 63,360 inches to 1 mile or 600,000 cm to 6 km, as above).

Look Out

Watch out for maps that have been reproduced and have had their scale changed. A graphic scale will change with the reduction or enlargement, but other scales become wrong. For example, if a map was shrunk down to 75 percent on a copier to make a handout and the scale says that 1 inch on the map is 1 mile, it's no longer true; only the original map printed at 100 percent is accurate for that scale.