Methodology: how we calculate rent indexation

Our calculation formula

Rent indexation in Belgium is governed by the Law of 6 August 1993 and the Royal Decree of 24 December 1993 which sets out its application, and today by the regional texts (art. 1728bis of the former Civil Code and art. 224/2 of the Brussels Housing Code; art. 26 of the Walloon decree of 15 March 2018; art. 34 of the Flemish Housing Rental Decree). The ordinary formula is the same in all three regions:

Indexed rent = Base rent × (New index ÷ Starting index)
New index = health index of the month preceding the anniversary of the lease's entry into force.

Starting index: signature or entry into force?

Only the starting index differs from one region to another:

• Brussels and Wallonia: health index of the month preceding the signature (conclusion) of the lease.
• Flanders: index of the month preceding the entry into force for leases concluded since 1 January 2019; the month preceding conclusion for older written leases.

Anniversaries, the Walloon 2022-2023 period and the regional cut-off dates (14 October 2022 in Brussels, 1 October 2022 in Flanders), on the other hand, are always determined by the entry into force. That is why the calculator asks for both dates. Example: for a lease signed on 10 March 2021 that entered into force on 1 April 2021, the starting index is that of February 2021 (110.39) in Brussels and Wallonia, but that of March 2021 (110.56) in Flanders.

Regional variants

Since 2022, all three regions take the energy performance of the property into account:

• Brussels (art. 224/2 § 1/2 of the Brussels Housing Code, ordinances of 13 October 2022 and 13 October 2023): for leases that entered into force before 14 October 2022 with EPC E, F or G, a correction factor set by anniversary month (official tables) — roughly 0.945 to 0.990 for E, 0.891 to 0.980 for F-G. Applied since 14 October 2023, with no end date. A registered lease and an EPC certificate provided to the tenant are required to index.
• Flanders (decree of 3 October 2022, amended on 7 October 2022 and 21 April 2023): for contracts that entered into force before 1 October 2022, a correction factor of roughly 0.945 to 0.980 for EPC D and 0.891 to 0.960 for EPC E, F or no EPC. Applied since 1 October 2023, with no end date. No EPC is required to index; label G does not exist.
• Wallonia (decree of 19 October 2022 amending art. 26 of the decree of 15 March 2018, still in force): no factor, but a special method for leases with EPC D to G or no certificate that had an anniversary between 1 November 2022 and 31 October 2023. Other leases follow the ordinary formula at 100%.

In Brussels and Flanders, the factor multiplies the full result of the ordinary formula — it does not apply to the increase alone:

Indexed rent = (Base rent × New index ÷ Starting index) × Correction factor
Example (Brussels, EPC E, rent of €850, lease signed on 10 March 2021, entered into force on 1 April 2021, indexation on 1 April 2026): 850 × 137.78 ÷ 110.39 = €1,060.90; × 0.965766823 = €1,024.58.

Calculating on a share of the increase (half of it, for example) only existed during the temporary periods of 2022-2023; it is no longer used today.

Where the indices come from

All indices come exclusively from Statbel — the Directorate-General Statistics of the FPS Economy. We perform no calculation or interpolation: we retrieve and store the official values, month by month.

Multiple base years

Statbel publishes the health index on several successive base years according to the start date of each series. Our system handles selection automatically:

• Base 1988 = 100: leases signed before 1 January 1991
• Base 1996 = 100: leases signed from 1 January 1991 to 31 December 1998
• Base 2004 = 100: leases signed from 1 January 1999 to 31 December 2008
• Base 2013 = 100: leases signed from 1 January 2009 to 31 December 2025 (the majority of current leases)
• Base 2025 = 100: leases signed from 1 January 2026

This breakdown is the convention applied by our calculator, based on the date the lease was signed — the user never has to choose themselves. What matters legally is using two indexes expressed in the same base: it is their ratio that determines the indexation.

Raw index, not smoothed

Statbel publishes two variants of the health index:

• The raw health index: value for the current month. This is the one used for rent.
• The smoothed health index: arithmetic average of the last 4 raw indices. Used for salaries, pensions and social benefits. Not for rent.

Confusing the two is a frequent error. The difference can amount to several euros per month, especially during periods of rapid variation.

The regional EPC factor

Brussels

The criterion is the lease's entry-into-force date (before 14 October 2022), not its signature date. The factor depends on the anniversary month of the entry into force; October is split (1-13 / 14-31). During the temporary period from 14 October 2022 to 13 October 2023 the rule was different: EPC E limited to 50% of the indexation, EPC F, G and no certificate without indexation.

Wallonia (special method)

In Wallonia there is no factor. During the period from 1 November 2022 to 31 October 2023, indexation was limited to 75% of the increase for EPC D, to 50% for EPC E, and excluded for EPC F, G and no certificate. The decree of 19 October 2022 has not been repealed: since 1 November 2023 and until the end of the tenancy, the leases concerned are calculated from the rent as adapted during that period, with as starting index that of the month preceding the anniversary that fell within that period. Official example from the Walloon administration (SPW): €600, lease signed in September 2019, entered into force on 1 November 2019, EPC E → adapted rent of €665.24 on 1 November 2022, then €667.22 on 1 November 2023.

Flanders (permanent)

The criterion is the contract's entry into force before 1 October 2022. An expired EPC is treated as no EPC (E/F factor). The factor ceases if the landlord obtains a new EPC A+, A, B or C, or in the event of a new contract. During the temporary period from 1 October 2022 to 30 September 2023, an EPC D allowed only half of the increase to be passed on, and an EPC E, F or no EPC none of it.

Data updates

Our VPS runs a daily cron job that queries the Statbel API and synchronises our PostgreSQL database. The process:

• Daily cron at 6 a.m. UTC
• Retrieval of the last 12 months (to capture any Statbel revisions)
• Insert / update in the database with consistency checks
• Email notification to the maintainer in case of failure

Statbel typically publishes the new monthly index around the 26th of each month. The calculator integrates the new value within the following hours.

The source code is open

The entire backend (Python / FastAPI / PostgreSQL) and frontend (Astro) is open. We will publish the GitHub repository publicly after stabilisation. In the meantime, the source code is auditable on simple request via the contact form.

Tech stack:

• Backend: Python 3.11, FastAPI, Pydantic v2, PostgreSQL 16, weasyprint for PDFs
• Frontend: Astro 6, Tailwind 4, strict TypeScript, vanilla JS for interactive components
• Hosting: Hostinger VPS (Docker + Traefik) for the API, Cloudflare Pages for the front-end
• Data: daily ingestion via Statbel API, PostgreSQL storage, CSV/JSON distribution

Limits of the tool

The calculator covers ordinary residential leases in the three Regions: it applies the legal indexation formula with Statbel indices and the regional EPC rules (correction factors, Walloon method). It does not:

• Cover the German-speaking Community, which has had its own housing competence since 1 January 2020 and a distinct EPC regime
• Handle, in Brussels, dwellings let by a social housing company (SISP) or by the Housing Fund, which are outside the correction factor — the form does not ask for the type of landlord
• Handle, in Flanders, social housing (the factor targets the private rental market), nor a new EPC obtained during the year, which ends the factor on the day of the new certificate with a pro rata calculation — the form only knows the current class
• Verify that the base rent entered is the revised rent where the rent was revised (triennial revision, court decision, addendum): that revised amount is the one to enter
• Perform contract analysis (specific clauses of your lease)
• Provide legal advice (in case of dispute, contact a lawyer or the justice of the peace)
• Verify lease registration (to be done at the registration office)
• Assess the market (an indexed rent may remain very far from market prices)
• Provide complete property management (issuing receipts, deposit handling, etc.)

For these aspects, consult a professional: lawyer specialising in real estate law, syndic, estate agent, or the justice of the peace of the property's commune (low-cost procedure, no lawyer required, for most rental disputes).

Frequently asked questions

Page updated on 21/09/2026 — rules last verified: 21 September 2026. Sources: Statbel, Law of 6 August 1993, Royal Decree of 24 December 1993, Brussels ordinances of 13 October 2022 and 13 October 2023 (correction factors, be.brussels), Walloon decree of 15 March 2018 as amended by the decree of 19 October 2022 (wallonie.be), Flemish decree of 3 October 2022, amended on 7 October 2022 and 21 April 2023 (correction factors, vlaanderen.be). This content is informative and does not constitute legal advice.