To calculate rent indexation in Belgium, you apply one stable formula: new rent = base rent × (new health index ÷ starting health index). The "base rent" is the rent written in your lease, the "starting health index" is the health index tied to the month your lease began, and the "new health index" is the value for the month before your lease anniversary. Belgium's official health index is published every month by Statbel, the national statistics office — you look up the two index figures there, divide, and multiply. That is the whole calculation. Below is a worked example with clearly-labelled illustrative numbers, a step-by-step checklist, and the regional cap you need to watch for.

What exactly is the health index in Belgium?

The health index (indice santé / gezondheidsindex) is a version of the consumer price index that excludes a few products such as tobacco, alcohol and motor fuels. It is the figure Belgian residential leases use for indexation — not the ordinary CPI. Statbel publishes a new value near the end of every month, so you never guess: for a real calculation you look up two specific monthly figures on Statbel and use them exactly as published.

What is the formula for rent indexation in Belgium?

The legal formula for most leases signed since 1991 is:

Indexed rent = base rent × (new health index ÷ health index at the start of the lease)

• Base rent — the monthly rent agreed in the lease.
• Health index at the start of the lease (the "base index") — the health index of the month before the month the lease took effect.
• New health index — the health index of the month before the lease's yearly anniversary.

Notice that the base rent and the base index never change for the life of the lease. Every year you only swap in a fresh "new health index" and recalculate from the original starting point. That is why indexation compounds gently over the years rather than resetting.

Can you show a worked example?

Here are illustrative numbers only — not the current official index. Suppose:

• Base rent = €1,000 per month
• Health index at the start of the lease = 128.00 (an example figure)
• New health index (month before the anniversary) = 131.20 (an example figure)

Apply the formula:

€1,000 × (131.20 ÷ 128.00) = €1,000 × 1.025 = €1,025.00

So in this example the rent rises from €1,000 to €1,025.00 — an increase of €25, or 2.5%. Swap in the real Statbel figures for your own months and the method is identical.

How do you calculate it step by step?

1. Find your lease start date and base rent in the contract.
2. Identify your base index: the health index for the month before the lease took effect. Look it up on Statbel.
3. Work out the month before your lease anniversary this year, and look up that month's health index — this is your new index.
4. Divide the new index by the base index to get the indexation coefficient (for example, 131.20 ÷ 128.00 = 1.025).
5. Multiply your base rent by that coefficient to get the indexed rent.
6. Check the regional rules (below), then notify your tenant in writing. Indexation is generally applied once a year, from the anniversary, and is not automatically backdated for long-forgotten years.

Do the three Belgian regions calculate it differently?

The core formula is the same nationwide, but Flanders, Brussels and Wallonia each set their own conditions — and in recent years all three have limited indexation for homes with a poor energy performance certificate (EPC / PEB / EPB) label. In practice this means a property with a weak energy rating may only be indexed partially, or not at all, while a well-rated home can be indexed in full. Because the exact treatment depends on the region, the EPC label and the year, always confirm the current regional rule for the property's location before you finalise the figure. For the bigger picture on what increases are actually allowed, see our pillar guide below.

Is there an easier way than doing this by hand?

Yes. ImmoDesk stores each lease's start date and base rent, pulls the correct health index for the right months, and applies the formula automatically — so you get the indexed rent, the coefficient and a tenant-ready notice without any manual lookups. You can try the maths yourself with our rent indexation calculator, read how the feature works on the rent indexation feature page, or, for the rules on how much a landlord may raise the rent overall, read the pillar: how much can a landlord increase the rent in Belgium.

Try ImmoDesk free — no credit card and let your leases index themselves each year.

This is general information, not legal advice. Belgian rent indexation rules differ by region and change over time; always confirm the current health index on Statbel and the regional rules for your property before acting.