# Percentages » Index numbers

## What are index numbers?

A percentage that is relative to a base value is called an index number. Behind the index number you do not have to write a %-symbol.

Often historical date is compared over a couple of years. The base value corresponds to a certain year, this year is called the **base year**.

*Example*

In the table, the year 2015 has been chosen as the base year.

The value of 2015 is therefore the base value and is 100%.

The index numbers can be found in the last column.

Year | Value | In- or decrease compared to 2015 in % | Index number |

2012 | 53 412 | –9.1 | 1 90.9 |

2013 | 53 726 | –8.5 | 1 91.5 |

2014 | 56 243 | –4.3 | 1 95.7 |

2015 | 58 743 | –0 | 100 |

2016 | 61 467 | –4.6 | 104.6 |

2017 | 59 551 | –1.4 | 101.4 |

### Calculating with index numbers

Because an index number is a percentage, you can calculate these the same way.

*Example 1*

See the table above.

The value that corresponds to 2018 is 60 342.

Calculate the index number for the year 2018.

Answer:

60 34258 743 × 100 ≈ 102.7

*Example 2*

See the table above.

It is known that the index number of the year 2011 is 89.7.

Calculate the value for the year 2011.

Answer:

89.7 : 100 × 58 743 ≈ 52 692