Each year on the anniversary of the rental agreement, the global rent of the house is indexed using the Index figures published by the Belgian Government Institution Statbel.

What is rent indexation? Every year, on the date on which the contract entered into force, the rent for your accommodation may be adjusted to the cost of living.

As stated by law in article 1728bis, §1, 1e lid BW and article 7 in your rental agreement, the global rent and hence the room rents are indexed by using the following formula:
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​Basic rental price x new (standard of living) index / Basic (standard of living) index

The basic index is that of the month preceding the conclusion of the rental agreement. The new index is that of the month preceding the start date of the rental agreement.

The indexation is performed on the net rent and will take effect the month after the announcement of the indexation by email.
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Below is an example:
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The date on which the house has been launched: 1st of December 2020
Current date: December 2021
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Person A: Moved in on the 1st of December 2020. Total Rent: 600 (net rent) + 150 (charges) = €750

Base index: 109.91
New index: 115.20

Calculations indexation net rent

Room A: €628.88 = €600.00 * 115.20/ 109.91

New rent

Person A: €628.88 + €150 = €778,88 => From the 1st of January 2021

The new rent will always be applied one month after announcement.

To calculate the indexation you can use Rent Calculator Tool of the Belgian Government.