Belgian rent indexation calculator (2026)

The legal formula (art. 1728bis of the old Civil Code) is: base rent × new index / initial index. The base rent is the rent stated in the lease, excluding costs and charges. The new index is the health index of the month before the anniversary of the lease taking effect. The initial index is the health index of the month before the lease was signed — except in Flanders: for residential leases concluded from 1 January 2019 it is the month before the lease took effect.

That is why the calculator asks for both dates. Signing and taking effect often fall in the same month, but if you sign in March a lease starting 1 May, the initial month (outside the Flemish decree) is February and the anniversary month is April.

Since 1994 the health index has been published in several base years (1988, 1996, 2004, 2013 and, since 2026, 2025 = 100). Both index figures must come from the same base series; the calculator automatically picks the right base from the signing date.

During the energy crisis all three regions restricted indexation for one year for homes with a poor energy rating. In Flanders (anniversaries from October 2022 through September 2023) a home with EPC label D could pass on only half of the indexation, and a home with label E, F or without a valid EPC nothing at all. Brussels capped label E at half and F and G at zero; Wallonia used 75% for D, 50% for E and zero for F, G or without a PEB certificate.

That restriction is never “caught up”: what you could not index then stays out of the rent for good. Flanders and Brussels formalise this as a correction factor you must apply to the standard formula at every subsequent indexation. Wallonia rebases the calculation instead: the restricted 2022–2023 rent and the index of that freeze year become the new basis for every later indexation. The calculator applies this automatically once you select the EPC label the home had during the restriction.

In Brussels there is an additional absolute condition since 14 October 2022: without a valid EPB certificate (and a registered lease) the rent may not be indexed at all.

You may index once a year, at the earliest on the anniversary of the lease taking effect. It does not happen automatically: you must request the new rent from your tenant in writing. If you wait too long, the indexation works back no more than three months from your request — money you forfeit for good after that.

For residential and commercial leases you may index as soon as the lease is in writing (unless the lease explicitly excludes indexation). For rentals outside those regimes — a garage, an office, a second home — indexation is only allowed if the lease contains an indexation clause. For Flemish student housing, indexation during the contract is prohibited.

The calculator computes the legal maximum rent from the health index and the regional correction rules. It does not handle rent revision (renegotiating the base rent itself), costs and charges, or social housing. Anniversaries are assumed to fall on the first of the month; only for Brussels leases with an anniversary between 14 and 31 October can the freeze year differ as a result. For conclusive legal advice, consult a lawyer or your landlords’ association.