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

Answer in brief

The calculation of a rent indexation in Belgium follows the same formula in the three regions (article 1728^bis of the former Civil Code, taken over by article 26 of the Walloon decree of 15 March 2018 and article 34 of the Flemish Residential Tenancy Decree). It runs in four steps: identify the two Statbel health indexes, apply the formula, possibly correct according to the EPC and the region, verify the validity conditions (written lease — also registered in Brussels and Wallonia —, written request, anniversary date). Brussels and Flanders multiply the result by a correction factor for certain leases predating October 2022; Wallonia applies a permanent special method to 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 — the conclusion of the lease and its entry into force — and they do not play the same role in every region.

• Starting index: in Brussels and Wallonia, the health index of the month preceding the conclusion of the lease. Not the month of conclusion itself: a lease concluded on 14 June 2021 takes the index of May 2021. In Flanders, for leases concluded since 1 January 2019, it is the index of the month preceding the entry into force (older Flemish leases stay on the month preceding the conclusion).
• New index: in the three regions, the health index of the month preceding the anniversary of the entry into force. 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.

Region	Conclusion	Entry into force	Starting index	Anniversary	New index
Wallonia	14 June 2021	1 July 2021	May 2021	1 July 2026	June 2026
Flanders	14 June 2021	1 July 2021	June 2021	1 July 2026	June 2026
Brussels	10 March 2021	1 June 2021	February 2021	1 June 2026	May 2026
Flanders	10 March 2021	1 June 2021	May 2021	1 June 2026	May 2026

When the entry-into-force date differs from the date of conclusion (frequent case: lease concluded in March for entry in June), the anniversary date is always that of the entry into force, in the three regions; only the starting index depends on the region. The gap is not trivial: for the €800 lease concluded on 10 March 2021 above, the formula gives €1,008.93 in Brussels (starting index 110.39) against €1,003.48 in Flanders (110.99) on 1 June 2026. This is the second major pitfall of the calculation.

Step 2: apply the federal formula

The calculation formula comes from article 1728^bis of the former Civil Code, which still applies in Brussels (together with article 224/2 of the Brussels Housing Code) and was taken over by article 26 of the Walloon decree of 15 March 2018 and by article 34 of the Flemish Residential Tenancy Decree. It is identical in all regions:

Indexed rent = Base rent × (New index ÷ Starting 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. Only exception: the Walloon special method described in step 3, which restarts from the adapted rent of 2022-2023.

Complete worked example. Walloon lease (EPC B) concluded on 14 June 2021 and entered into force on 1 July 2021, base rent of €800. Indexation on 1 July 2026.

1. Starting index: May 2021 (month preceding the conclusion) → 110.99
2. New index: June 2026 → 139.08
3. Calculation: 800 × (139.08 ÷ 110.99) = €1,002.47

The same lease located in Flanders would start from the index of June 2021 (month preceding the entry into force, 111.31): 800 × (139.08 ÷ 111.31) = €999.59.

This cumulated increase over 5 years reflects Belgian inflation over the period. It is only due upon the landlord’s written request: indexation is never automatic, and a late request only reaches back to the 3 months preceding the month of the request (see our after 5 years guide).

Step 3: apply the regional EPC correction

Since the inflation surge in 2022, the three regions have limited the indexation of energy-wasting properties — first through temporary measures (2022-2023), then through a permanent regime. The situation as of 17 September 2026 is as follows:

Wallonia

Capping period ended late 2023, but a permanent special method remains for capped EPC D-G leases. The temporary limitation of the Walloon decree of 19 October 2022 ended on 31 October 2023. Since then, the ordinary formula applies — except for leases rated EPC D, E, F, G or without a certificate, which entered into force before 1 November 2022 and had an anniversary between 1 November 2022 and 31 October 2023: their indexation restarts from the adapted rent of that period, with the index of the month preceding that anniversary as starting index (decree of 19 October 2022, still in force, amending article 26 of the decree of 15 March 2018). Other Walloon leases are calculated directly according to step 2.

Official example of the Walloon administration (SPW). Rent of €600, lease concluded in September 2019 and entered into force on 1 November 2019, EPC E. On 1 November 2022, the ordinary formula gave €703.69; as the previous rent was €626.79, only half of the increase could be applied: adapted rent of €665.24. On 1 November 2023, the calculation restarts from that amount, with the index of October 2022 (127.92) as starting index: 665.24 × (128.30 ÷ 127.92) = €667.22, against €705.79 with the ordinary formula.

Brussels

A correction factor applies to leases that entered into force before 14 October 2022 with an EPC E, F or G (ordinance of 13 October 2022, then ordinance of 13 October 2023, in force on 14 October 2023 — article 224/2, § 1/2 of the Brussels Housing Code). Since 14 October 2023, the full result of the formula is multiplied by this factor, which depends on the anniversary month: about 0.945 to 0.990 for an EPC E, about 0.891 to 0.980 for an EPC F or G. Classes A to D, and every lease that entered into force from 14 October 2022, are indexed at 100%.

The criterion is indeed the lease’s date of entry into force, not the date on which it was concluded. And in Brussels, no indexation is due if the lease is not registered or if the EPC certificate has not been provided to the tenant.

Flanders

Same logic: since 1 October 2023, a correction factor applies to contracts that entered into force before 1 October 2022 with an EPC D (about 0.945 to 0.980) or E, F or no EPC (about 0.891 to 0.960) — decree of 3 October 2022, article 4. Labels A+, A, B and C, and every contract that entered into force from 1 October 2022, are indexed at 100%. Label G does not exist in Flanders. No EPC is required to index, but a missing (or expired) EPC triggers the lowest factor; the factor stops applying once the landlord obtains a new EPC A+, A, B or C.

Corrected formula and frequent error

When a correction factor applies (Brussels, Flanders), it multiplies the full result of the ordinary formula — not just the increase. The corrected formula is:

Indexed rent = (Base rent × New index ÷ Starting index) × Correction factor

Example. Brussels lease concluded on 20 May 2021 and entered into force on 1 June 2021 (so before 14 October 2022, factor applicable), base rent €800, EPC E. On 1 June 2026:

1. Starting index: April 2021 (month preceding the conclusion) → 110.93
2. New index: May 2026 → 139.22
3. Ordinary formula: 800 × (139.22 ÷ 110.93) = €1,004.02
4. Correction factor EPC E (anniversary in June): 1,004.02 × 0.972124068 = €976.03

The same lease in Flanders, with an EPC D, starts from the index of May 2021 (110.99): 800 × (139.22 ÷ 110.99) = €1,003.48; × 0.972124068 = €975.51.

The frequent error is to keep applying the percentages of the temporary 2022-2023 periods, when only part of the increase could be claimed (in Brussels, from 14 October 2022 to 13 October 2023, at most half of the indexation for an EPC E and no indexation for an EPC F, G or no EPC; in Flanders, from 1 October 2022 to 30 September 2023, half of the increase for an EPC D and no indexation for an EPC E, F or no EPC). Those rules have ended: since then, the factor bears on the full indexed rent. Conversely, forgetting the factor here means claiming €1,004.02 instead of €976.03, that is €27.99 too much every month.

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. Written lease in the three regions and, in Brussels and Wallonia, a registered lease: without registration, no indexation is due, regardless of the calculation. In Brussels, the EPC certificate must also have been provided to the tenant; in Wallonia the EPC is not a general condition, and in Flanders no EPC is required to index.
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 entry into force. 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	Leases concerned (signature date)
1988	Signed before 1 January 1991
1996	Signed from 1 January 1991 to 31 December 1998
2004	Signed from 1 January 1999 to 31 December 2008
2013	Signed from 1 January 2009 to 31 December 2025
2025	Signed from 1 January 2026

Convention of our calculator: the base is picked from the lease’s signature date. A 2018 lease is calculated in base 2013; a March 2026 lease starts from base 2025. What matters legally is using two indexes expressed in the same base: it is their ratio that determines the indexation. Never mix a starting index from one base with a new index from another base.

Our calculator automatically selects the base and takes both indexes in it, which removes this recurring source of error — including for old leases still in force.

Frequent errors to avoid

• Confusing the date of conclusion and the entry-into-force date: the anniversary date (and thus the new index) always follows the entry into force; the starting index follows the conclusion in Brussels and Wallonia, the entry into force in Flanders (leases concluded since 2019).
• Using the smoothed health index instead of the raw one. The smoothed is for wages and pensions; only the raw applies to rents.
• Forgetting the correction factor in Brussels (leases that entered into force before 14/10/2022, EPC E, F or G) and in Flanders (contracts that entered into force before 1/10/2022, EPC D, E, F or no EPC) — or the adapted-rent method in Wallonia.
• Still applying the 2022-2023 percentages to the increase: since October 2023, the factor multiplies the full indexed rent.
• Indexing more than once on the same anniversary year.
• Notifying an indexation on an unregistered lease in Brussels or Wallonia — no indexation is due, even if the calculation is correct.
• Claiming 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 formula, each region’s own starting index, the EPC correction factor (or the Walloon method) and the right Statbel index. These explanations are informative and do not constitute legal advice.

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