The principle of the indexation formula
The rent indexation formula in Belgium is enshrined in the Belgian Civil Code (Article 1728bis). It is designed to adjust the rent in line with the cost of living, without allowing an increase exceeding actual inflation.
Indexed rent = Base rent x (New health index / Starting health index) x EPC factor
This formula is a matter of public policy for primary residence leases: the parties cannot deviate from it. The result constitutes the maximum amount the landlord may claim. No additional surcharge is permitted under the heading of indexation.
For the complete guide: rent indexation guide. For automated calculation: indexation calculator.
Each variable of the formula explained
Base rent
The base rent is the first amount agreed between the landlord and the tenant. It appears in the original lease contract, under the “monthly rent” heading. This amount never changes in the formula, even if the rent has been indexed several times in the past.
One exception: if a rent revision has been formalised (amicable agreement or decision by the justice of the peace), the new amount replaces the base rent in the formula.
Starting health index
The starting health index corresponds to the health index published for the month preceding the signing of the lease. This value is fixed and invariable for the entire duration of the contract.
| Signing date | Starting index |
|---|---|
| 15 January 2020 | Health index for December 2019 |
| 1 March 2022 | Health index for February 2022 |
| 28 June 2023 | Health index for May 2023 |
New health index
The new index is the health index for the month preceding the anniversary of the lease’s entry into force. This value changes every year.
The distinction between the signing date and the date of entry into force is essential. A lease signed on 15 March but coming into force on 1 April uses the March index for the new index (month preceding April).
Find all values in the health index table.
The EPC correction factor in the formula
Since 2022-2023, the formula incorporates a third element: the correction factor linked to the energy performance of the building (EPC). This factor is a multiplier between 0 and 1.
How the EPC factor modifies the result
Without the EPC factor (leases predating 2022 or EPC A-D), the calculation remains identical to the classic formula because the factor equals 1.00. For poorly rated properties, the factor reduces or cancels the increase.
| Scenario | Classic formula | With EPC F factor (Wallonia) |
|---|---|---|
| Base rent | 800 euros | 800 euros |
| Index ratio | 1.16 | 1.16 |
| EPC factor | 1.00 | 0.00 |
| Indexed rent | 928 euros | 800 euros (no indexation) |
The EPC factor varies by region. Consult EPC and indexation for the exact values.
Step-by-step application of the formula
Step 1: identify the dates
Find in the lease the signing date and the date of entry into force. These two dates may be identical or different.
Step 2: find the starting index
Look up the health index for the month preceding the signing. This value sometimes appears in the contract itself. Otherwise, consult the index table or the Statbel website.
Step 3: find the new index
Look up the health index for the month preceding the anniversary of entry into force. This index is published monthly by Statbel, generally within the first 10 working days of the following month.
Step 4: identify the EPC factor
Check the energy class of the property on the EPC certificate and apply the corresponding factor for your region.
Step 5: apply the formula
Multiply the base rent by the index ratio, then by the EPC factor. Round to the nearest cent. The result is the new monthly rent.
Step 6: notify the tenant
Send a letter to the tenant with the calculation details. An indexation letter template is available.
Special cases in applying the formula
Renewed lease
In the case of renewal with a new rent, the formula starts from zero with the new base rent and new indices. In the case of tacit renewal, the original indices remain valid.
Rent revision during the lease
If the rent has been revised (by agreement or by the justice of the peace), the revised amount becomes the new base rent in the formula. The reference indices also change.
Commercial lease
For commercial leases, the formula is the same but an explicit indexation clause is mandatory. EPC restrictions generally do not apply to commercial leases. See commercial lease indexation.
Subletting
The subtenant is subject to the conditions of the main lease. Indexation of the sublease follows the same formula, with the indices specific to the sublease.
Complete calculation guide — Common indexation errors — Indexation calculator.
Frequently asked questions
Is the formula the same in all 3 regions?
Yes. The base formula is identical. Only the regional EPC factor differs. See 3-region comparison.
Lease without an indexation clause?
For a primary residence lease, indexation applies by default. For other leases, a written clause is required.
Can the formula be modified in the contract?
No. The official formula is a matter of public policy. Any derogatory clause (for example indexation based on the CPI instead of the health index) would be void.
Does the starting index change upon renewal?
Yes in the case of formal renewal with a new rent. No in the case of tacit renewal.