Rent calculators
Revise a rent under the rules of the place the building stands in.
France indexes on the IRL, the three Belgian regions on the health index under three different codes, and no US state has an index at all: a few cap the rise by statute and the rest leave it to the lease. Pick where the building is and the calculator asks the questions that jurisdiction actually makes worth asking.
Letting law is written below the country here, so the rules differ inside it.
Belgium indexes the rent the lease states, excluding charges, never last year's indexed figure. Leave it empty to use the rent above.
It fixes the anniversary, and where an index applies, the starting index.
The anniversary you are claiming.
A poor rating blocks or dampens the rise in some places
The index for Aug 2023, the month before the lease start you gave. The lease should state it, with its base year.
The index for Aug 2026, the month before the anniversary, on the same base year.
Type the two index figures and the answer appears here. Dardaris holds no copy of the series, so it works from the figures you enter and shows them in the working.
What governs in Belgium, Flanders
- Index
- Gezondheidsindex (health index), base 2013 = 100
- Formula
- new rent = original base rent × health index of the month before the anniversary ÷ health index of the month before the lease was signed
Nothing is stored and nothing is sent. This runs the same calculation engine the product itself runs, in this browser.
The same calculator, under other law.
A rent rises differently in each of these, and the page for each one says how. Anywhere else, the calculator still runs on that jurisdiction's own pack.
France, worked through in full
A revision is one ratio, between two quarters a year apart.
The IRL is the rent reference index published by INSEE every quarter. A revision compares the quarter your lease names with the same quarter a year earlier, and applies that ratio to the rent. Everything difficult about it is in which two numbers you pick.
The two figures, and where they sit
One is the latest figure for the quarter your lease names. The other is the figure for that same quarter a year earlier. The rent is multiplied by the first and divided by the second.
Twelve months apart
The same quarter, a year earlier
The reference quarter, for this revision
Rent × the later figure ÷ the earlier figure
Neither figure is printed on this page, because Dardaris holds no copy of the series. You enter both off the publication, and the calculator above shows its working with the figures you typed.
The quarter is the lease's, not the latest one
The reference quarter is written into the lease. Reaching for the newest published figure instead is the commonest error, and it produces an increase you are not entitled to.
You revise the rent being paid, not the original
This is the opposite of the Belgian rule, where the original base rent is indexed every time. In France each year builds on the last, so applying the ratio to the original rent loses you the compounding permanently.
The lease has to allow it at all
Without a revision clause there is no right to revise, and adding one later needs the tenant to agree. The clause is also where the reference quarter is named.
The three that cost money
Three rules turn a routine increase into nothing at all.
None of these announces itself. Nobody writes to tell you the window has closed or that the rating blocks you, and the calculator above enforces all three rather than handing you a figure you are not allowed to charge.
Twelve months, then the year is gone
A revision must be claimed within twelve months of falling due. Miss it and that year is forfeited outright: you cannot claim it later and you cannot roll it into the next one.
Forfeited, not deferred
A rating of F or G blocks it entirely
Since August 2022 a dwelling rated F or G cannot have its rent revised in metropolitan France. Class G has been unlettable since January 2025, and F follows in January 2028.
Refused, whatever the lease says
The wrong base rent, compounding
Indexing the original rent each year instead of the rent in payment looks conservative and is simply wrong in France. The shortfall is permanent and grows with every year you repeat it.
Quiet, and it never corrects itself
12 months, and there is no back-claiming
The window opens on the anniversary the lease sets and closes 12 months later. There is no partial claim and no catching up: a revision claimed late runs forward from the day you claim it, never from the anniversary you missed. The months before that are not owed to you.
Which is why the expensive version of this mistake is not miscalculating. It is remembering in month thirteen.
What a forgotten year costs
The loss is not this year's increase. It is every year after it.
Because France revises the rent in payment, a year you never claimed lowers the base that every future revision is worked from. The gap does not close on its own and nothing in the lease brings it back.
A year claimed late is a year lost
There is no back-claiming in France. The revision runs from the day you claim it, so the months between the anniversary and your letter are simply not owed.
And the base stays lower for good
Next year's revision is worked from this year's rent. A year skipped is not one year of increase missed, it is a permanently lower starting point for every revision after it.
Or you charge what you may not
Reaching for the newest quarter instead of the one the lease names, or revising a dwelling the DPE blocks, produces an increase the tenant can challenge and recover.
When, and in what order
Four things, once a year, in this order.
A revision is not difficult. It is a date you have to remember eleven months after the last time you thought about it, which is a different problem and a harder one.
- Step 1
Find the anniversary and the quarter
Both are in the lease. The anniversary sets when the window opens; the reference quarter sets which two index figures the calculation uses.
- Step 2
Wait for the figure to be published
INSEE releases each quarter about two weeks after it closes. A revision worked on an unpublished quarter is a revision you will have to redo.
- Step 3
Check the rating before you write
An F or G rating refuses the revision entirely. Checking takes a moment and saves sending a tenant an increase you have to withdraw.
- Step 4
Tell the tenant, in writing, inside the window
Name the two index figures and the quarters they come from, so the tenant can reproduce the number. Then keep the letter, because the claim date is what the twelve months are counted against.
The questions that follow.
How is a French rent revision calculated?
Multiply the rent currently being paid by the IRL for the reference quarter named in the lease, divided by the IRL for that same quarter a year earlier. The result is the new rent. It is the rent in payment that is revised, not the original rent in the lease.
Which IRL quarter do I use?
The reference quarter written into the lease, not the most recently published one. Where the lease names none, it is the quarter the lease was signed in. INSEE publishes each quarter about two weeks after it closes.
What happens if I revise late?
The year is forfeited outright. A revision must be claimed within twelve months of the date it fell due, and there is no back claiming in France: the new rent runs forward from the date you claim it, never from the anniversary you missed.
Can I revise the rent on a dwelling rated F or G?
No. Since 24 August 2022 the rent of a dwelling rated F or G on the DPE cannot be revised at all in metropolitan France, whatever the lease says. Renovating the dwelling to a better rating is the only route to a revision.
Does the lease need a revision clause?
Yes. Without a clause providing for annual revision there is no right to revise at all, and adding one later requires the tenant to agree. The clause names the reference quarter the calculation runs on.
What if the index has fallen?
The same formula gives a lower figure, which would be a decrease. Check the reference quarter before acting on it, because applying a fall you were not obliged to apply is difficult to reverse.
The calculation takes a minute. The date is what costs you.
Nobody forfeits a year because the arithmetic defeated them. They forfeit it because the anniversary fell in a quiet month, the index was not out yet, and by the time it was, there was nothing to remind them.
- It watches the anniversary for youDardaris holds the lease, its reference quarter and its anniversary, and tells you a revision is due rather than waiting to be asked.
- And works it out on the right two figuresThe reference quarter the lease names, against the same quarter a year earlier, applied to the rent in payment. The DPE block and the twelve month window are part of the same answer.
- But it does not send itA letter that raises somebody's rent waits for you. Dardaris prepares it and says what it is based on, and you are the one who decides it goes.
The number is the easy part.
Dardaris keeps the lease, raises the invoice, matches the payment and writes down what it did. Bring one building and watch a month before you decide anything.
14 days before the first charge · Nothing is switched on until you say so