The concentration index for a Dutch municipality is the ratio of the percentage of Belgian inhabitants in the municipality to the percentage of Belgian inhabitants in the total population of the Netherlands. The index for Dutch people in Belgian municipalities is calculated in the same way.
For example: about 1 percent of the total population in Belgium is Dutch. In Turnhout municipality more than 3 percent of the population is Dutch. The concentration index for Turnhout is therefore: 3/1 *100 = 300.