Several indexation tools have recently removed base 88 from their tables, causing concern among some landlords who were still using it in their calculations. There is no cause for alarm: you are absolutely not required to stick to it. Here is the simple rule that avoids any indexation error.
The principle to remember
In Belgium, indices are regularly "rebased" (base 88, base 2004, base 2013, base 2025, etc.), but this does not change the result of the indexation.
Only the relationship between the start index and the current index matters
It does not matter which base is used. If both indices (start and current) are expressed in one and the same base, the indexation result will be strictly identical, regardless of which base is chosen.
In other words, you can perfectly well use a more recent base for your calculations, even if you previously relied on base 88. This often simplifies the reading and use of the indices, with no impact whatsoever on the indexed rent amount.
The golden rule: the same base for both indices
Critical point: all indices used in the calculation must without exception come from the same base.
Correct examples
- Start index base 2013 + current index base 2013 = OK
- Start index base 2025 + current index base 2025 = OK
- Start index base 88 + current index base 88 = OK (if your lease predates 1996)
The incorrect example to avoid at all costs
- Start index base 88 + current index base 2013 = distorted calculation
This type of mixing systematically distorts the calculation and can lead to indexation that is:
- Over-stated (risk of challenge by the tenant + repayment + interest)
- Under-stated (a net loss for the landlord over the full duration of the lease)
In practice: which base should you use?
For leases signed from 1996 onwards
It is perfectly acceptable to use more recent bases (2013 or 2025), which also corresponds to current practice. This is the most common situation today.
For leases predating 1996
These leases remain tied to base 88. In this case, you have two options:
- Continue using base 88 if you still have access to the corresponding index tables
- Convert the indices to a more recent base (2013, for example), taking care to rebase the start index in the same base as the current index
Several indexation services remain available for older leases — notably to assist landlords with the calculation when base 88 is no longer directly accessible in their usual tools.
The basic indexation formula (reminder)
For reference, the legal formula for indexing a residential rent in Belgium is:
New rent = Base rent × (New health index / Start health index)
Where:
- Base rent: the rent agreed in the contract (excluding charges)
- New health index: the index for the month preceding the lease anniversary
- Start health index: the index for the month preceding the signing of the lease
Both indices must be expressed in the same base.
Practical example: a 2022 lease
Take a lease signed on 1 February 2022 with a base rent of EUR 850.
- Signing: February 2022 → health index January 2022: 118.21 (base 2013)
- Anniversary: February 2026 → health index January 2026: 137.37 (base 2013)
Calculation: 850 × (137.37 / 118.21) = 850 × 1.1621 = EUR 987.79
The indexed rent for 2026–2027 is EUR 987.79/month.
If the bases had been mixed by mistake (for example, start index in base 2004 and current index in base 2013), the calculation would be completely distorted — this is precisely the trap to avoid.
Tools to help you
Several online tools allow you to verify your calculations:
- Statbel (Belgian official statistics): index tables in all bases
- SPF Economie / FPS Economy: official indexation calculator
- Professional federation services: verify the legal calculation and prepare the adaptation letters
For small portfolios (a few properties), Excel or a spreadsheet is sufficient using the formula above. For larger portfolios, a centralised management tool avoids human error.
In practice: what you need to do
- Identify the base of your start index (the one indicated in the lease or in the indexation annex)
- Retrieve the current index in the same base
- Apply the formula
- Send the adaptation letter by registered post to the tenant, at least 3 months before the date of application
If you have several older leases (pre-1996) and no longer have access to base 88, do not panic: convert all your indices to a more recent base (2013 recommended). The mathematical result will be identical.
In conclusion
The disappearance of base 88 from certain indexation tables is not a problem:
- It only affects very old leases (pre-1996)
- More recent bases (2013, 2025) give exactly the same mathematical result
- The only absolute rule: the same base for both the start index and the current index
Keep the golden rule close to hand when running your indexation calculations, and you will avoid 99% of the errors that end up in front of the juge de paix.
Further reading
For a broader overview of the Belgian rental market in 2025, see our article on the Federia rental market report 2025 — it details rent trends by region and property type.
If you manage a mixed portfolio (residential and co-tenancy), indexation in co-tenancy deserves particular attention: see our article on the legal pitfalls of Walloon co-tenancy.
Automate your rent indexations with Seido — automatic calculation at the anniversary date, dispatch of adaptation letters, indexation history per lease, and alerts if the start index and current index are not in the same base.