Methodology: how we calculate rent indexation
Full transparency on the formula applied, the data sources, the regional coefficients and the limits of the tool. Every calculation can be audited and reproduced manually from the indices published by Statbel.
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. The base formula is unique and identical throughout the country:
Indexed rent = Base rent × (Health index of the month preceding the anniversary date ÷ Health index of the month preceding signature)
Regional variants
Since 2022, two regions apply a corrective coefficient based on the energy performance of the property:
- Brussels (art. 224/2 § 1er/2 of the Brussels Housing Code, ordinance of 11 October 2022): for leases signed before 14 October 2022, an EPC coefficient applies based on the energy class. Monthly correction factor according to the anniversary month (official tables).
- Flanders (decree of 4 July 2023): permanent EPC coefficient, applicable to all leases. More restrictive than Brussels on intermediate classes.
- Wallonia (decree of 19 October 2022, still in force): the capping window ended on 31 October 2023, but a permanent adapted method remains for EPC D-G leases capped during the window. Other leases follow the standard federal calculation.
The complete formula with EPC correction is:
Indexed rent = Base rent + (Theoretical increase × EPC coefficient)
with Theoretical increase = Base rent × (New index / Base index − 1)
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: for leases signed between 1988 and 1995
- Base 1996 = 100: 1996 — 2003
- Base 2004 = 100: 2004 — 2013
- Base 2013 = 100: 2014 — 2025 (currently the base in force for the majority of leases)
- Base 2025 = 100: starting 1 January 2026 for new leases
The transition between bases is automatic according to the date the lease was signed — the user never has to choose themselves.
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
Leases signed from 14/10/2022: 100% all classes. A valid EPC certificate provided to the tenant is required to index. Different percentages applied during the 2022-23 window.
Wallonia (historical 2022-2023)
D-G leases with an anniversary in the 11/2022-10/2023 window: indexation computed from the window's adapted rent (decree of 19/10/2022). Other leases: 100%.
In Wallonia, these percentages correspond to the capping window (1/11/2022 → 31/10/2023), which has now ended. The decree of 19 October 2022 remains in force, however: EPC D-G leases capped during the window have since followed a permanent adapted method (indexation rebased on the adapted rent of the window).
Flanders (permanent)
Label G does not exist in the Flemish EPC. Contracts from 1/10/2022: 100% all classes.
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 does one thing: apply the legal indexation formula with Statbel indices and regional EPC coefficients. It does not:
- 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 (free and quick procedure for most rental disputes).
Frequently asked questions
Why not use the smoothed health index? + −
The smoothed health index (arithmetic average of the last 4 months) is used for salaries, pensions and social benefits. The Law of 6 August 1993 specifies that rent indexation uses the raw health index, month by month. Confusing the two can distort the calculation by around ten euros per month.
What sets your calculation apart from other calculators? + −
Three things: (1) Statbel data updated daily, (2) handling of multiple base years (1988, 1996, 2004, 2013, 2025) with automatic selection based on the lease date, (3) accurate regional EPC coefficients for Brussels and Flanders, month by month. Most online calculators ignore at least one of these aspects.
Is the source code auditable? + −
Yes. The calculation logic is open — on request via the contact form, we provide the Python backend code (FastAPI + PostgreSQL) and the tabulated EPC coefficients. The repository will eventually be published as open source on GitHub.
How is the monthly index update handled? + −
A cron job on our VPS calls the Statbel API every day. When a new monthly index is published (typically around the 26th), it is ingested into our PostgreSQL database and immediately available via /v1/indices. No manual intervention required.
What happens if Statbel issues a correction to a published index? + −
Statbel occasionally publishes revisions (rare). Our cron systematically re-imports the last 12 months to capture any revisions. Values stored in our database are therefore always aligned with Statbel within 24 hours.
Can the calculator make mistakes? + −
The applied formula is strictly legal and the indices are copied from Statbel. The risk of error lies in user input: if the user confuses the signature date and the entry-into-force date, or selects the wrong EPC, the result will be mathematically correct but legally incorrect. That is why the calculator has 3 distinct date fields and an explicit selector for the energy class.
Page updated on 04/05/2026. Sources: Statbel, Law of 6 August 1993, Royal Decree of 24 December 1993, Brussels ordinance of 11 October 2022, Walloon decree of 15 March 2018, Flemish decree of 4 July 2023.