How to calculate a rent indexation: step-by-step guide

Answer in brief

The calculation of a rent indexation in Belgium follows a unique federal formula, defined in article 1728^bis of the Civil Code. It applies in four steps: identify the two Statbel health indexes, apply the formula, possibly correct with the regional EPC coefficient, verify the validity conditions (registered lease, written request, anniversary date). Brussels and Flanders keep an EPC correction; in Wallonia, the capping window ended on 31 October 2023, but a permanent adapted method remains for EPC D-G leases capped in 2022-2023.

Step 1: identify the right health indexes

The classic pitfall of the indexation calculation is in the index selection. Two dates count, and none is the one most landlords spontaneously choose.

• Base index: health index of the month preceding the lease signature. Not the signature month itself. A lease signed on 14 June 2021 takes the index of May 2021.
• New index: health index of the month preceding the anniversary date of the effective start. For an indexation on 1 July 2026, it is the index of June 2026.

Statbel publishes the raw health index every month, generally around the 26th. As long as the required month’s index is not published, the indexation cannot be notified — the calculation would be premature and legally disputable.

Signature date	Base index month	Anniversary date	New index month
14 June 2021	May 2021	1 July 2026	June 2026
1 September 2018	August 2018	1 September 2026	August 2026
15 March 2024	February 2024	15 March 2026	February 2026
28 February 2020	January 2020	1 March 2026	February 2026

When the effective start date differs from the signature date (frequent case: lease signed in March for entry in June), it is the effective start date that drives the anniversary date — not the signature one. This nuance, codified by the case law of the justices of the peace, is the second major pitfall of the calculation.

Step 2: apply the federal formula

The calculation formula is in article 1728^bis of the Belgian Civil Code, introduced by the Law of 6 August 1993 and specified by the Royal Decree of 24 December 1993. It is identical in all regions:

Indexed rent = Base rent × (New index ÷ Base index)

The base rent is the initial rent inscribed in the lease, never the last indexed rent. One always indexes relative to the contract rent — not relative to the previous indexation. This rule avoids cumulated discrepancies in case of small successive errors.

Complete worked example. Lease signed on 14 June 2021 with initial rent of €800. Indexation on 1 July 2026.

1. Base index: May 2021 → 110.99
2. New index: June 2026 → 132.47
3. Calculation: 800 × (132.47 ÷ 110.99) = €954.85

This cumulated increase over 5 years reflects Belgian inflation over the period. It is fully due, but only from the receipt of the written request by the tenant: indexation never has retroactive effect before notification, except catch-up limited to 3 months (see our after 5 years guide).

Step 3: apply the regional EPC coefficient

Since the inflation surge in 2022, the three regions have introduced — then adjusted — an EPC coefficient intended to penalise energy-wasting properties. The situation as of 4 May 2026 is as follows:

Wallonia

Capping window ended late 2023, but a permanent adapted method remains for capped EPC D-G leases. The window mechanism of the Walloon decree of 19 October 2022 ended on 31 October 2023. Since then, the federal standard formula applies — except for EPC D, E, F or G leases whose anniversary fell within the window of 1 November 2022 to 31 October 2023: their indexation restarts from the adapted (capped) rent of the window, with the index of the month preceding that anniversary as base index (special method of the decree of 19 October 2022, still in force). Other Walloon leases are calculated directly according to step 2.

Brussels

The EPC coefficient applies only to leases signed before 14 October 2022, the effective date of the Brussels ordinance of 11 October 2022. For later leases, the federal formula applies without correction.

Flanders

The EPC coefficient is permanent since the decree of 4 July 2023. It applies to all main residence leases, without condition of signature date.

Corrected formula and frequent error

When a coefficient applies, it bears on the increase of indexation, never on the total rent. The corrected formula is:

Indexed rent = Base rent + (Theoretical increase × EPC coefficient)
where Theoretical increase = Base rent × ((New index ÷ Base index) − 1)

Example. Brussels lease signed on 1 June 2020 (so before 14 October 2022, coefficient applicable), initial rent €800, EPC E. On 1 July 2026:

1. Federal theoretical increase: 800 × (132.47 ÷ 110.99) − 800 = €154.85
2. EPC coefficient E in Brussels: 75 %
3. Corrected increase: 154.85 × 0.75 = €116.14
4. Indexed rent: 800 + 116.14 = €916.13

Applying the coefficient on the total rent (not on the increase) would give 800 × 0.75 × (132.47 ÷ 110.99) ≈ €716.13, that is a rent lower than the initial rent. It is absurd, and legally wrong: the EPC coefficient never aims to decrease an existing rent, but to moderate the indexation increase.

Step 4: verify the validity conditions

The right calculation is not enough. Four cumulative conditions must be met for the indexation to be legally due:

1. Registered lease with the competent registration office within two months of the signature. An unregistered lease purely and simply removes the right to indexation, regardless of the calculation.
2. Written request to the tenant, ideally by registered mail for proof of receipt. Free form is allowed but must mention the calculation detail, the indexes used and the effective date (see our letter template).
3. Anniversary date strictly respected: indexation takes effect at the earliest on the anniversary date of the effective start. An early notification does not advance the effective date.
4. Once a year only, and retroactivity limited to the 3 months preceding the request (article 1728bis, §1 of the Civil Code and equivalent regional provisions).

Our when to index guide details the optimal notification windows and calendar pitfalls to avoid.

Special case: change of statistical base

Statbel has successively published the health index on five bases: 1988, 1996, 2004, 2013 and 2025. When changing base, the indexes are no longer directly comparable — an official connection coefficient is needed to move from one series to another.

Base	Application period	Concerned leases
1988	1989–1998	Leases signed before 1998
1996	1998–2006	Leases 1998-2005
2004	2006–2014	Leases 2006-2013
2013	2014–2025	Leases 2014-2025
2025	2026–…	Leases 2026+

Legal rule: your lease keeps the base corresponding to its signature date. A 2018 lease remains indexed on base 2013; a March 2026 lease starts from base 2025. When the indexation period crosses a base change, the new index is converted into the lease’s original base using the connection coefficient published by Statbel.

Our calculator automatically selects the right base and applies the connection coefficients, which removes this recurring source of error — including for old leases still in force.

Frequent errors to avoid

• Confuse signature date and effective start date when choosing the anniversary date (always the effective start date).
• Use the smoothed health index instead of the raw one. The smoothed is for wages and pensions; only the raw applies to rents.
• Forget the EPC coefficient for Brussels (pre-14/10/2022 leases) and Flanders (all leases).
• Apply the EPC coefficient on the total rent instead of the increase — this can produce an indexed rent lower than the initial rent.
• Index more than once on the same anniversary year.
• Notify an indexation on an unregistered lease — the operation is void even if the calculation is correct.
• Claim a catch-up beyond 3 months: the retroactivity limit is strict (art. 1728bis, §1 CC).

Calculate in 30 seconds with our tool

To avoid all these pitfalls and get the exact indexed rent for your lease, our calculator automatically applies the federal formula, the regional EPC coefficient and the right Statbel index based on the signature date.

Going further

• Indexation after 5 years: catch-up rules
• When can a rent be indexed?
• Complete methodology and sources
• Index table: monthly health indexes
• Tenant notification letter template