Which CPI month should I use?

The month the rent was last set, and the most recently published month at the date you serve the notice. The CSO publishes roughly ten days after month end, so in mid-September the latest available figure is August.

Do I round up or down?

Down, to the nearest cent. Rounding up risks exceeding the cap by a fraction, which is still exceeding the cap.

How to calculate the maximum rent increase, step by step

Updated 2026-09-20 · Ireland · applies from 1 March 2026

Short answer

Take the number of days from the date the rent was last set to the date the new rent will take effect. Work out the CPI change between those two months, and separately 2% × days ÷ 365. The maximum increase is whichever is smaller, applied to the current rent and rounded down.

A worked example using real CSO Consumer Price Index figures: reference period, ΔCPI, the annualised 2% cap and which one binds.

The four steps

Every figure below comes from the worked example that follows: a rent of €1,800 set on 1 August 2025, with notice served on 19 September 2026.

1. Fix the reference period. The period runs from the date the current rent took effect to the date the new rent will take effect. Rent set on 1 August 2025, notice served 19 September 2026: the earliest effective date is 18 December 2026, which is 90 days later. That is a reference period of 504 days.
2. Look up the two CPI figures. Use the index for the month the rent was set and the most recently published month. The CSO publishes about ten days after month end, so on 19 September 2026 the latest available figure is August 2026. August 2025 stood at 103.6 and August 2026 at 107.4, so ΔCPI is 107.4 ÷ 103.6 − 1 = 3.668%.
3. Work out the annualised cap. 2% × 504 ÷ 365 = 2.762%. This is lower than the CPI change of 3.668%, so the 2% cap is the binding limit.
4. Apply it and round down. €1,800 × 1.02762 = €1,849.71, rounded down to €1,849.70. That is the maximum lawful rent from 18 December 2026 — an increase of €49.70 per month.

Print the workings on the notice

If the review is ever disputed, the arithmetic is your defence. State both CPI months and their index values, the number of days, both candidate rates, and which one applied. Cite the source: CSO PxStat dataset CPM24, statistic CPM24C01, sub-index CP00 (All Items), base December 2023 = 100.

Where the official figures come from

The Consumer Price Index is published monthly by the Central Statistics Office. There is no separate 'RTB rent calculator' index — the RTB's own guidance points at the same CSO series. This site refreshes its cached copy weekly and prints the exact months used in every document it produces.

Questions

Which CPI month should I use?: The month the rent was last set, and the most recently published month at the date you serve the notice. The CSO publishes roughly ten days after month end, so in mid-September the latest available figure is August.
Do I round up or down?: Down, to the nearest cent. Rounding up risks exceeding the cap by a fraction, which is still exceeding the cap.

Check your own figures

The calculator applies these rules to your dates and produces the notice for you. Free, no sign-up, and you can read the finished document before deciding to buy it.

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