An update on this thread. I'm now more familiar with the legislation, read the RTB Rent Calculator's own source
code (it was listed in the script files), and tested it. The calculator departs from the Act in three ways. Two of them cut the landlord's increase every time; the third makes the answer depend on the day you run it. There is a fourth problem that is in the Act itself.
Everything below can be checked. The calculator is at
rtb.ie/rtb-rent-calculator, and its code is public at
rtb.ie/.../rent-calc.js. All screenshots were taken on Sunday 4 October 2026, with the time shown in the blue banner on each. The highlights are mine: in the CPI tables, yellow is the index the calculator used and green is the index the Act requires.
1. The calculator uses the wrong base month for every rent set from 1 March 2026
For a previous setting on or after 1 March 2026,
s.19(4)(b) of the Residential Tenancies Act 2004 defines the base index (the
"previous CPI number") as:
(i) the CPI number published by the Board in accordance with subsection (4C) relating to the month immediately preceding the month in which the previous setting took place, or
(ii) in circumstances where the CPI number referred to in subparagraph (i) was not so published by the time the previous setting took place, the CPI number so published relating to the month immediately preceding the month first-mentioned in that subparagraph
So the base is the month
before the setting, or the month before that. Never the month of the setting itself.
Here is the code. The function that picks the index takes the year and month of the date you enter and returns that month's figure. There is no 1 March 2026 rule, and nothing about "the month immediately preceding":
And here it is in practice. Rent last set 15 March 2026: the calculator uses
105.9, the March 2026 index. The Act requires February (104.2), or January (103.3) if February's figure wasn't yet out.
Rent last set 20 April 2026: the calculator uses
106.4, April. The Act requires March (105.9).
At the next review the new index can only be the latest one published, the month before or two months before. So for every rent set from 1 March 2026, the calculator measures
10 or 11 months of inflation for every 12-month review, every year, permanently. The Act gives the full 12 months.
Today, with inflation at 3.7%, the 2% cap applies and this costs nothing. But annual inflation was below 2% from August 2024 to August 2025. In a year like that, losing one or two months of inflation is roughly 0.15 to 0.3 percentage points off every review, and every review after that is calculated on the lower rent.
And for some landlords it costs money now. The 12-month rule in s.20(1) restricts reviews within a tenancy; you don't have to wait 12 months to set rent under a new tenancy, which is capped by reference to the last rent set under the previous one (s.19(4)(b) and (c)). And apartments and student accommodation in developments commenced from 10 June 2025 are capped by inflation only, with no 2% limit (s.19(4)(aa)). Take a previous tenancy with rent set on 20 June 2026 and a new tenancy from 20 September 2026, on EUR 1,000. The calculator takes June 2026 (106.6) as the base and allows
0.75%: EUR 7.50. The Act takes May 2026 (106.3) and allows
1.03%: EUR 10.35 (or April, 106.4, if May's figure wasn't out by 20 June: 0.94%, EUR 9.40). Either way the calculator gives the landlord between a fifth and a quarter less than the law allows, today.
2. The calculator rounds the 2% cap down to whole months
For a part-year, the Act gives:
such percentage as bears to 2 per cent the same proportion that that period bears to a year
Nothing there rounds to whole months. The code counts whole calendar months, and takes one off if the day of the month hasn't been reached (
if (b.d < a.d) diff -= 1):
The 2% is then simply 2 x months / 12:
On EUR 1,000, 1 February 2025 to 28 March 2026: the calculator credits 13 months,
EUR 21.67. Under the Act it is 2% for the year plus 55/365 of 2%:
EUR 23.01.
It is worst at month ends. Rent last set 31 January 2025, reviewed 28 February 2026: a year and 28 days. Because 28 is less than 31, the calculator counts 12 months:
EUR 20.00. Under the Act:
EUR 21.53.
The same thing shows in the screenshots in point 1: 15 March to 1 October is 200 days, but 6 months are credited (EUR 10.00, the Act gives EUR 10.96). And 20 April to 1 October is 164 days, but 5 months are credited (EUR 8.33, the Act gives EUR 8.99).
3. The same review gives a different answer depending on the day you run it
For the new index, the Act says the month before the setting, or the month before that if it
"is not so published on or before the next setting". The calculator uses the same function as above: the month of the setting if that figure exists
today, otherwise the latest earlier month.
So on the day you serve your notice, the current month's figure doesn't exist yet and it falls back to the latest one. Run the same dates later and it uses an index that wasn't published at the time. Here is 1 March 2025 to 1 March 2026, run today. "CPI (New Set)" is
105.9, the March 2026 index, which the CSO published in April:
On 1 March 2026, the latest index available was January's (103.3). So the printout a landlord attached to a notice that day won't match what the calculator now shows for the same dates. Usually the re-run figure is higher. But the index doesn't only go up: January 2026 (103.3) was lower than December 2025 (104.2), and May 2026 (106.3) was lower than April (106.4).
If an adjudicator re-runs your review months later and gets a lower figure than the one on your notice, could the notice be found invalid and the whole increase disallowed? A Tribunal has already ruled against a landlord that had "probably followed the RTB's own published guidance" (TR0001481), striking down both of its notices of termination on Argutinski.
The "**" you sometimes see next to a CPI figure is the code's fallback flag (
usedFallback): the setting month's figure wasn't available, so an earlier month was used. The page doesn't explain that anywhere, on a printout landlords have to attach to a statutory notice.
Since 1 March 2026 the Board has had a statutory duty under s.19(4C)(aa) to maintain a calculator that calculates increases
"in accordance with subsection (4)", and the Act envisages it operating
"to accurately calculate any increase in rent" (s.19(4D)(aa)). Both were inserted by s.8 of the Residential Tenancies (Miscellaneous Provisions) Act 2026, the same Act that brought in the new rules.
On points 1, 2 and 3, it doesn't.
And note who carries the risk. Under s.19(6A), failing to comply with s.19(4) is a criminal offence, and
it is the landlord's offence, not the RTB's. A landlord who applies the Act correctly and sets a higher rent than the calculator shows is relying on his own reading of the law against the regulator's official tool, with a prosecution on the line.
The prescribed Notice of Rent Review (Part 4 of the Schedule to
S.I. No. 68 of 2026, as substituted by https://www.irishstatutebook.ie/eli/2026/si/470/made/en/print (S.I. No. 470 of 2026)) asks the landlord to confirm they
"used the RTB Rent Increase Calculator to calculate the maximum rent increase allowed" and to attach a
"copy of calculation". So the
statutory form directs every landlord to the RTB's calculator and attaches its output to the notice. Yet I can find no regulation made under s.19(4D)(aa) setting out how that calculator must work. Neither instrument says anything about months, days or which index to use.
4. The first review under the new regime: this one is the Act itself
I said earlier the RTB had built the 11-month problem into the calculator. For the first review after 1 March 2026, it hadn't. For a previous setting before 1 March 2026, the Act defines the base as:
the CPI number published by the Board in accordance with subsection (4C) relating to the month in which the previous setting took place
Until this year, that rule only applied to settings before 11 December 2021, when s.3 of the Residential Tenancies (Amendment) Act 2021 commenced. Section 8(b)(iii)(II) of the Residential Tenancies (Miscellaneous Provisions) Act 2026 (No. 3 of 2026), commenced on 1 March 2026 by S.I. No. 67 of 2026, replaced that date with 1 March 2026. Every rent set between 11 December 2021 and 28 February 2026 was moved back onto the worse rule.
A landlord who reviews every year: rent set 1 March 2025, reviewed 1 March 2026. Under the old rule, January 2025 (100.6) to January 2026 (103.3) is 2.68%, so the 2% cap applies:
2.00%. Under the rule now in the Act, March 2025 (102.2) to January 2026 (103.3) is
1.08%. Ten months of inflation for a twelve-month review.
On a EUR 1,500 rent, EUR 30 a month cut to EUR 16: about EUR 166 a year, on one tenancy.
A landlord who hasn't reviewed in nearly three years: rent set 1 June 2023, reviewed 1 March 2026. Under the old rule, April 2023 (97.9) to January 2026 (103.3) is 5.52%; the 2% a year cap over 33 months is
5.50%. Under the rule now in the Act, June 2023 (99.1) to January 2026 is
4.24%. Two months of inflation gone.
On a EUR 1,500 rent, EUR 82.50 a month cut to EUR 63.60: about EUR 227 a year, on one tenancy.
This is the landlord who held the rent down for nearly three years through a cost-of-living crisis, now penalised for it.
(You can't reproduce these on today's calculator, for two reasons. Run today with these dates, it picks up the March 2026 index, which didn't exist on 1 March 2026 (point 3), and shows the full 2% and 5.50%. And for a review set today, inflation is high enough that the 2% cap bites whichever month you start from. The cost falls on reviews in the months after 1 March 2026, and it will fall again in any year when inflation drops below 2%. Here is the June 2023 one as the calculator shows it now.)
These losses are permanent. The next review is calculated on the lower rent, and the one after that. The Oireachtas tightened the rent rules more than was publicly flagged, retrospectively, in a clause about which month's index you use.
Why is the Government doing this?
Why would legislators, in the middle of a housing crisis, take one or two months of inflation off every landlord's next review, with a change to a date in a definitions subsection that nobody would notice?
Why is the RTB doing this?
And why is the RTB, on top of that, running a calculator that departs from the Act? If it had to simplify, it could have rounded to the nearest month. It rounds down. It could have used the base month the Act specifies. It uses a later one. Both of those departures cut the landlord's increase, every time.
This is just another example of the bias we are seeing elsewhere at the RTB. At disputes, accommodation providers are often ordered to pay on far quicker terms than tenants: in TR0001427 a landlord was given 14 days to pay, inside the 21-day appeal window, while in TR0001417 a tenant was given 80 months to repay EUR 16,000 at EUR 200 a month. Tenants are awarded compensation for stress and anxiety (EUR 2,500 in TR0001469 alone). I have yet to see a landlord awarded a cent for the stress of a dispute.
And now, in the calculator: round down, measure short, always against the accommodation provider.
Please let me know if you see any mistakes or errors in my analysis.
Sources:
Residential Tenancies Act 2004 (Revised), s.19(4), (4C), (4D) and (6A), and s.20(1);
S.I. No. 68 of 2026; https://www.irishstatutebook.ie/eli/2026/si/470/made/en/print (S.I. No. 470 of 2026);
RTB Rent Calculator;
RTB Rent Calculator source code;
CSO Consumer Price Index, Table 1.
