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 correction factor, calculation from the last indexed rent instead of the base rent, health index / CPI confusion, wrong rounding, wrong lease origin date (including confusing signature and entry into force). 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: 1988, 1996, 2004, 2013, 2025. Both indexes in the formula must be expressed in the same base: mixing two bases gives an aberrant result.

How to avoid: our calculator automatically selects the base from the signature date (convention below) and takes both indexes in it. For manual calculation, see our methodology.

Lease signed	Base used by our calculator
Before 1 January 1991	Base 1988
From 1 January 1991 to 31 December 1998	Base 1996
From 1 January 1999 to 31 December 2008	Base 2004
From 1 January 2009 to 31 December 2025	Base 2013
From 1 January 2026	Base 2025

Error 2: Forgetting « the preceding month »

The formula uses the index of the month preceding the signature (in Flanders, for leases concluded since 2019: the month preceding the entry into force) and that of the month preceding the anniversary of the entry into force. Many landlords take the indexes of the exact month — which shifts by one month and slightly distorts the result.

Concrete example: Brussels or Walloon 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 correction forgotten or wrong

This is the most costly error financially. In Brussels and Flanders, for poorly rated leases that entered into force before October 2022, the full result of the formula is multiplied by a correction factor; in Wallonia, a special method restarts from the adapted rent of 2022-2023.

Consequence if forgotten: claimed rent too high, by up to about 11% of the formula’s result with the lowest factors (≈ 0.891).

Region	Leases concerned	Correction to apply
Brussels	Entered into force before 14/10/2022, EPC E, F or G	Factor: E ≈ 0.945-0.990; F-G ≈ 0.891-0.980
Brussels	Entered into force from 14/10/2022, or EPC A to D	100%
Flanders	Entered into force before 1/10/2022, EPC D, E, F or no EPC	Factor: D ≈ 0.945-0.980; E, F, no EPC ≈ 0.891-0.960
Flanders	Entered into force from 1/10/2022, or EPC A+ to C	100% (label G does not exist)
Wallonia	EPC D to G or no certificate, with an anniversary between 1/11/2022 and 31/10/2023	Adapted-rent method (no factor)
Wallonia	Other leases	100%

Example: Brussels lease concluded on 20 May 2021 and entered into force on 1 June 2021, base rent 800 €, EPC E, indexed on 1 June 2026. Ordinary formula: 800 × (139.22 ÷ 110.93) = 1,004.02 €; × factor 0.972124068 = 976.03 €. Forgetting the factor means claiming 27.99 € too much every month.

The reverse error also exists: still applying the rules of the temporary periods, when only part of the increase could be claimed (for example half of it, for an EPC E in Brussels from 14 October 2022 to 13 October 2023, for an EPC D in Flanders from 1 October 2022 to 30 September 2023, or for an EPC E in Wallonia from 1 November 2022 to 31 October 2023). Those rules have ended: the calculation no longer bears on a share of the increase.

See our complete EPC-classes table for detail.

Error 4: Starting from the last indexed rent instead of the base rent

The legal formula restarts every year from the base rent and the lease’s starting index: Base rent × (New index ÷ Starting index). The classic error is to apply that same index ratio to the last, already indexed rent: the inflation of previous years is then counted twice, and the claimed rent exceeds the legal rent.

Worked example (Walloon lease, EPC B, concluded on 15 June 2018 and entered into force on 1 July 2018, base rent 800 €, starting index May 2018: 106.99):

Indexation	Correct calculation	Result
1 July 2019	800 × (109.02 ÷ 106.99)	815.18 €
1 July 2026	800 × (139.08 ÷ 106.99)	1,039.95 €

❌ Starting from 815.18 € (or from any other already indexed rent) and multiplying it by 139.08 ÷ 106.99 gives an amount above the legal rent. ✅ In 2026 as in 2019, one restarts from the contract’s 800 €. Practical consequence: after several years without indexation, a single calculation is enough to get today’s rent, with no « cascade » to reconstruct. Only exception: the Walloon special method, which restarts from the adapted rent of 2022-2023.

Our calculator automatically restarts from the base rent.

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 × (139.08 ÷ 110.99) = 1,002.4686… → 1,002.47 € (correct), not 1,003 € nor 1,002.50 €.

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.

Frequent variant: confusing signature and entry into force. The two dates do not play the same role. The new index is always that of the month preceding the anniversary of the entry into force. The starting index is that of the month preceding the signature in Brussels and Wallonia, but that of the month preceding the entry into force in Flanders for leases concluded since 1 January 2019. Example: 800 € lease (EPC B) signed on 10 March 2021 and entered into force on 1 June 2021, indexed on 1 June 2026 (new index: May 2026, 139.22). In Brussels, the starting index is February 2021 (110.39): 1,008.93 €. In Flanders, it is May 2021 (110.99): 1,003.48 €. Picking the wrong date therefore distorts the rent by 5.45 € per month.

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. In Wallonia (article 25 of the decree of 15 March 2018), restitution covers the amounts paid in the 5 years preceding the request, and the action is time-barred after 1 year.

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 applies for the future from the anniversary date, once requested in writing
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 (base rent × that year’s index ÷ starting index)
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: our calculator

Our calculator indexation-loyer.be automatically applies:

• The right index base according to the signature date
• The correct preceding months for the indexes, with each region’s own starting index (signature or entry into force)
• The EPC correction factor (Brussels, Flanders) or the Walloon adapted-rent method, according to the region, the class and the lease’s entry-into-force date
• The legal roundings to the cent
• The calculation from the base rent, even after several years without indexation

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 and entry-into-force dates on a reference document	Avoids a wrong starting index and confusion upon owner change
Recalculate each indexation from the base rent at each verification	Detects errors propagated from year to year
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 correction factor 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 — verification from your data
• Regularisation: procedure — correct after error
• EPC classes and indexation — correction 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. Our calculator is the most profitable prevention tool of rental management. This information is provided for guidance only and does not constitute legal advice.