What is the rent indexation formula?

The legal rent indexation formula

Each element has a precise legal definition:

• Base rent: the amount agreed in the original lease (not the last rent paid or a previously indexed amount). If the lease was revised between triennial periods, the revised rent becomes the new base.
• Starting health index: the health index published by Statbel for the month preceding the month the lease was signed.
• New health index: the health index for the month preceding the anniversary date of the lease’s entry into force.

Indexation can only be requested once per year, on the anniversary date of entry into force, and only upon written request from the landlord.

Concrete calculation example

Take a lease signed on 15 March 2022 for a rent of 850 EUR, entering into force on 1 April 2022. On 1 April 2023, the landlord wishes to index:

The rent increases from 850 EUR to 886.69 EUR, an increase of 36.69 EUR per month.

Steps to calculate yourself:

1. Find the base rent in your original lease
2. Identify the starting health index on Statbel (month preceding signing)
3. Find the new health index (month preceding the anniversary date)
4. Apply the formula: divide the new index by the starting index, then multiply by the base rent
5. Round to the nearest cent

Common calculation mistakes

Using the previously indexed rent as the base: this is the most common error. The law requires using the original base rent, not last year’s indexed amount. Using the indexed amount would create an unauthorised compounding effect.

Confusing signing date and entry into force: the starting index is linked to the signing month, while the new index is linked to the entry into force month. These two dates may differ, particularly for leases signed mid-month.

Using the wrong index: the health index (indice sante) must be used, not the consumer price index (CPI). The health index excludes certain products such as tobacco, alcohol and fuels.

Regional specifics

The indexation formula is identical across all three regions, but EPC restrictions may modify the final result:

Brussels-Capital Region

The Ordinance of 27 July 2017 provides that properties with an EPC E, F or G face indexation restrictions or a freeze.

Wallonia

The Walloon Decree of 15 March 2018 applies similar restrictions for EPC labels E, F and G.

Flanders

The Flemish Housing Rental Decree of 9 November 2018 provides less restrictive EPC rules — only EPC E and F labels see their indexation reduced.