Indexation calculation error: how to correct it?

Brief answer

An indexation calculation error generally results from one of these 7 causes: wrong statistical index base, wrong reference month, forgotten EPC coefficient, direct calculation instead of cascade, health index / CPI confusion, wrong rounding, wrong lease origin date. Quick verification is done via our calculator. Correction depends on the direction of the error (overcalculation = refund, undercalculation = renotification with retroactive effect limited to 3 months).

The 7 most frequent calculation errors

Error 1: Wrong statistical index base

The Statbel health index has known several successive bases: 1996, 2004, 2013, 2025. Each lease uses the base in force at the signature date. Using the wrong base gives an aberrant result.

How to avoid: our calculator automatically selects the right base according to the signature date. For manual calculation, see our methodology.

Lease signed between	Base to use
Before 1997	Base 1988
1997-2005	Base 1996
2006-2013	Base 2004
2014-2025	Base 2013
Since 2026	Base 2025

Error 2: Forgetting « the preceding month »

The formula uses the indexes of the month preceding the signature and the month preceding the anniversary. Many landlords take the indexes of the exact month of signature and anniversary — which shifts by one month and slightly distorts the result.

Concrete example: lease signed on 15 June 2018.

• ❌ Wrong: use the June 2018 index as base index
• ✅ Correct: use the May 2018 index (month preceding June)

The gap is generally on the order of 0.2 to 1% of the indexed rent — a few euros per month, but cumulated over several years, the effect becomes significant.

Error 3: Regional EPC coefficient forgotten or wrong

This is the most costly error financially. In Brussels (leases prior to 14/10/2022) and Flanders (all leases), a reducing coefficient applies according to the EPC class (75%, 50% or 0%).

Consequence if forgotten: indexation overestimated by 25 to 100%.

Region	Lease	Coefficient to apply
Wallonia (since 31/10/2023)	All	100% (no coefficient)
Brussels	Lease < 14/10/2022	EPC coefficient (75% D-E, 0% F-G)
Brussels	Lease ≥ 14/10/2022	100%
Flanders	All leases	EPC coefficient (75% C, 50% D-F, 0% G)

See our complete EPC-classes tableau for detail.

Error 4: Direct calculation instead of cascade (recovery)

For a retroactive recovery, many calculate flat: Initial rent × (Current index ÷ Base index). This is mathematically wrong for a lease that has crossed several anniversaries.

The correct method is the cascade: each year, the indexation is calculated on the previous indexed rent, not on the initial rent.

Worked example (2018 lease, initial rent 800 €):

Method	1st indexation 2019	Year 2026 (8 indexations)
❌ Direct calculation	800 × (109.21 ÷ 106.87) = 817.53 €	800 × (137.30 ÷ 106.87) = 1,027.80 €
✅ Correct cascade	800 × (109.21 ÷ 106.87) = 817.53 €	Identical on the final year (snowball effect neutralised by the ratio)

For standard annual indexations, the cascade gives the same result as direct calculation only on the final year considered. The chain detail changes, and that is what counts to reconstruct a history.

Our calculator automatically applies the cascade.

Error 5: Health index / general CPI confusion

Rent indexation uses the health index, not the general CPI (Consumer Price Index). Both are published by Statbel and differ slightly (the health index excludes fuels, tobacco, alcohol).

Consequence: using the general CPI gives a result shifted by 0.1 to 0.5% compared to the health index. Punctual, but cumulated over 5-9 years, the gap can reach 1-3%.

How to avoid: on Statbel, verify that the series used is indeed entitled « health index » (in NL: « gezondheidsindex »).

Error 6: Wrong rounding

The legal practice in Belgium is to round the indexed rent to the cent (2 decimals). Some landlords round to the upper euro, others to the half-euro. These approximations are technically contestable.

Example: 800 × (132.47 ÷ 128.12) = 827.1518… → 827.15 € (correct), not 828 € nor 827.50 €.

The gap from the correct rounding is marginal (a few cents), but in case of formal contestation, the tenant is entitled to demand exact rounding.

Error 7: Wrong lease origin date

For a lease that has been renewed, assigned or transferred (sale, inheritance), the origin date for the indexation calculation remains that of the initial signature between the original landlord and the original tenant. Not the renewal date, not the property purchase date.

Consequence if error: using a more recent date as « base » severely underestimates the chain of possible indexations.

See our owner change guide for the precise continuity rules.

How to correct an error already made

Case A: Overcalculation (tenant paid too much)

The tenant has paid a rent higher than what was legally due. Procedure:

1. Recalculate the right amount via the calculator
2. Notify the tenant of the error (simple letter or explicit email suffices)
3. Refund the overpayment:
   • Either by direct transfer (preferred for amount > 100 €)
   • Or by deduction on the next month’s rent (clear mention in the bank communication)
4. Formalise in writing the agreement on the new correct amount

Tenant prescription period: 5 years (article 2277 CC for repetition of the undue). This is much longer than the landlord’s retroactivity (3 months of arrears maximum, article 1728^bis, §1 CC), which clearly favours the tenant.

Case B: Undercalculation (landlord collected too little)

The landlord has claimed a rent lower than what was legally due. Procedure:

1. Recalculate the right amount
2. Renotify by registered mail mentioning the error and the right amount
3. The new amount takes full effect for the future upon receipt
4. Recovery limited to the 3 months preceding the month of the request (article 1728^bis, §1 CC) — beyond that, past months are lost, see our retroactive indexation guide

Asymmetry unfavourable to the landlord: the retroactive effect is limited to 3 months, contrary to the tenant who can recover 5 years of overpayment.

Case C: Old error propagated to the entire chain

If the error dates back several years and has served as a basis for the following indexations (cascade effect), the entire chain must be recalculated from the error:

1. Identify the year of the original error
2. Recalculate each subsequent year on the correct basis
3. Accumulate the annual gaps
4. According to the direction (overcalculation or undercalculation), apply procedure A or B

This is generally the most complex case — see our regularisation guide for the complete procedure.

Verification tool: the official calculator

Our calculator indexation-loyer.be automatically applies:

• The right index base according to the signature date
• The correct preceding months for the indexes
• The appropriate regional EPC coefficient according to region and lease date
• The legal roundings to the cent
• The complete cascade for multi-year recoveries

To verify a received calculation, simply enter the parameters and compare. It is the fastest and most reliable method to detect an error.

Practical tips to avoid errors

Tip	Benefit
Always use the calculator rather than a manual calculation	Eliminates 95% of errors
Keep a written copy of each calculation (screenshot)	Proof in case of subsequent dispute
Verify the property’s EPC class before any indexation	Avoids the most costly error
Note the exact signature date on a reference document	Avoids confusion upon owner change
Recalculate all indexations from the start at each verification	Detects errors propagated in cascade
For a portfolio > 5 properties, delegate to an agency	Externalises the risk (with recourse in case of fault)

Special case: conscious error vs involuntary error

If the error results from characterised bad faith (for example, the landlord intentionally omitted the EPC coefficient to inflate the rent), the tenant can invoke in addition to repetition of the undue:

• Damages for prejudice
• The annulment of the indexations tainted with fraud (rare but existing case law)
• If applicable, a criminal complaint for fraud (very rare in rental matters)

This is why, in case of doubt, it is always better to rectify quickly an error as soon as detected — the landlord’s good faith is then presumed and amicable regularisation suffices.

To go further

• How to calculate an indexation — step-by-step method
• Calculate my indexation — official verification tool
• Regularisation: procedure — correct after error
• EPC classes and indexation — coefficient to apply
• Indexation by agency — recourse if agency error
• Retroactive indexation — recovery rules
• Methodology — legal sources and official formula
• Notification template — correct form letter

A calculation error is never serious if detected early and corrected fast. The official calculator is the most profitable prevention tool of rental management.