Quick answer

The formula is: Base rent x (New health index / Starting health index). The base rent is from the original lease (not the last rent paid). The starting index is from the month before signing. The new index is from the month before the anniversary of entry into force.

Each element has a precise legal definition:

  • Base rent: the amount agreed in the original lease (not a previously indexed amount)
  • Starting health index: the health index published by Statbel for the month preceding the signing
  • New health index: the health index for the month preceding the anniversary of the lease’s entry into force
  • Result: rounded to the nearest cent

Step-by-step calculation

Example: lease signed 15 March 2022, entry into force 1 April 2022, base rent 850 EUR.

  1. Find the starting index: February 2022 (month before signing) = 128.57
  2. Find the new index: March 2023 (month before first anniversary) = 134.12
  3. Apply the formula: 850 x (134.12 / 128.57) = 886.69 EUR
  4. New monthly rent: 886.69 EUR (increase of 36.69 EUR)

Where to find the indices: the official health index is published monthly by Statbel.

Practical tip

Use our indexation calculator to get the result automatically with the latest official indices. This eliminates calculation errors and provides a formatted notification letter for your tenant.

Common calculation mistakes

Using the previously indexed rent as the base: this is the most frequent error. The law requires using the original base rent, not last year’s indexed amount. Using the indexed amount creates a compounding effect that is not authorised.

Confusing signing date and entry into force: the starting index relates to the signing date, while the new index relates to the entry into force anniversary. These dates may differ.

Using the wrong index: always use the health index (indice sante), not the consumer price index (CPI). The health index excludes tobacco, alcohol and fuels.

Regional specifics

Brussels-Capital Region

The formula is identical but EPC restrictions may limit or block the result under the Ordinance of 27 July 2017.

Wallonia

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

Flanders

The Flemish Housing Rental Decree of 9 November 2018 applies less restrictive EPC limitations (only E and F labels affected).

Legal basis

Article 1728bis of the Belgian Civil Code — Coordinated text on Justel.

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