A map is a hardcopy representation of part of the earth's surface and it is essential that a scale bar and scale ratio be present on the map to convey the reduction factor. This enables the map user to measure a distance on the map and determine the distance on the ground. These days maps are usually produced under the metric system which makes it easy to perform this task. Maps are usually produced at standard scale factors of (say) 1:10,000 or 1:50,000 or 1:100,000 and all you need to do is take a measurement on the map (in mm) and multiply that figure by the scale factor to determine the distance on the ground.

   Calculation:  Measure  466 mm on a map which is scale 1 : 50,000  ...
       466 x 50,000 = 23,300,000 mm
       ie  23,300 metres
       ie  23.3 kilometres

You may, however, come across an older map from before the mid 1960's when imperial measurements were the norm. These maps displayed scale ratios but often they were of the form 60 chains to 1 inch or 2 miles to 1 inch etc

In this case measure mm on the map and multiply by the scale factor below ...

   40 Chains = 1 Inch  >>  40 x 22 x 36  >>  1 : 31,680  (scale factor)
   60 Chains = 1 Inch  >>  60 x 22 x 36  >>  1 : 47,520   (  "  )
   1 Mile = 1 inch     >>  63,360 x 1    >>  1 : 63,360   (  "  )
   2 Miles = 1 Inch    >>  63,360 x 2    >>  1 : 126,720  (  "  )
   4 Miles = 1 Inch    >>  63,360 x 4    >>  1 : 253,440  (  "  )

   Calculation:  Measure 132mm on a map which is scale 60 Chains to 1 Inch ...
       132 x 47,520 = 6272,640 mm
       ie 6,272.6 metres
       ie 6.27 kilometres