Index-linked rent increase: formula and examples

An index-linked rent increase adjusts the rent in line with a price index, usually a consumer price index (CPI), using one formula: new rent = old rent × new index ÷ old index. The old index is the value that applied when the rent was last set or adjusted; the new index is the latest value your contract refers to. If the index has risen by 4%, the rent rises by 4%, subject to any cap, floor, threshold or rounding rule in the agreement.

Whether index clauses are allowed, which indices may be used and how often rent may be adjusted differ between countries. This guide explains the maths and common contract mechanics in general terms. All index values below are hypothetical, chosen only to illustrate the calculation, and the maths works the same in any currency. Check local law, or ask a lawyer, before drafting or applying an index clause.

How the index-linked rent increase formula works

A price index measures how the cost of a basket of goods and services changes over time relative to a base period, which is set to 100. If the index stands at 112.4, prices are on average 12.4% higher than in the base period.

The formula for an index-linked rent increase is:

New rent = old rent × (new index ÷ old index)

The percentage change is:

Change in % = (new index ÷ old index − 1) × 100

Worked example 1: a simple increase

• Current rent: €800 per month
• Index value when the rent was last set: 100.0 (hypothetical)
• Latest index value: 104.0 (hypothetical)

New rent = €800 × 104.0 ÷ 100.0 = €800 × 1.04 = €832.00

The rent rises by €32 per month, or €384 per year, a 4% increase.

Worked example 2: realistic-looking values

• Current rent: €1,000 per month
• Old index: 112.4 (hypothetical)
• New index: 118.6 (hypothetical)

New rent = €1,000 × 118.6 ÷ 112.4 = €1,000 × 1.055160… = €1,055.16

The increase is €55.16 per month, or €661.92 per year, which is about 5.52%.

The rent increase calculator does this for you: enter the old rent and both index values and it shows the new rent and the monthly and yearly difference.

Points versus percent: a common error

The index rose from 112.4 to 118.6, which is 6.2 points. It is tempting to read that as a 6.2% increase, but it is not. A change in points equals a change in percent only when the old value is exactly 100.

Approach	Calculation	New rent	Correct?
Percentage change (formula)	€1,000 × 118.6 ÷ 112.4	€1,055.16	Yes
Point difference read as percent	€1,000 × 1.062	€1,062.00	No

The wrong method overcharges by €6.84 per month in this example, and the error grows as the index moves further from 100. Some contracts do refer to points, for example "when the index has changed by 5 points"; that wording usually defines when an adjustment is triggered, while the increase itself is still calculated as a percentage. Read your clause carefully.

Which index values to use

Index clauses depend on the right inputs. Before you calculate, check:

• Which index. Use exactly the index named in the agreement, for example the national all-items consumer price index. Regional, core or harmonised indices give different results.
• Which month. The clause often specifies a reference month, such as the latest published monthly value at the time of the adjustment, or the value for a particular month.
• Which base year. Indices are periodically rebased; see the section below.
• Preliminary versus final values. Some statistics offices publish provisional values that are revised later. Your contract may say which to use. If not, use the published final value where available and note the publication date.
• The old value. This is the index value at the last adjustment or at the start of the tenancy, not the value from a year ago if you skipped a year.

Keep a record of each value used and where you found it. You will need it if the tenant asks, and for the next adjustment.

Thresholds: when an adjustment is triggered

Many clauses do not adjust the rent every time the index moves. Instead, they allow or require an adjustment only once the index has changed by a minimum amount, such as a set percentage or number of points, or only after a minimum period.

Worked example 3: a 5% threshold

The rent of €800 was set when the index stood at 100.0. The clause allows an adjustment once the index has risen by at least 5%.

• Index at 104.0: change is 4.0%, below the threshold. No adjustment yet.
• Index later at 105.5: change is 5.5%, above the threshold.

New rent = €800 × 105.5 ÷ 100.0 = €844.00

Note that the old index stays at 100.0 until an adjustment actually happens. You do not move the reference point each year just because a year has passed.

Caps and floors

Contracts and, in some countries, laws can limit how far the rent moves.

A cap limits the increase, for example to a set percentage per adjustment or per year.

A floor limits decreases, for example by stating that the rent will never fall below its current level, or by setting a minimum increase.

Worked example 4: a cap

Using example 2, the index change gives an increase of 5.52%. Suppose the contract caps any single adjustment at 3%.

• Uncapped new rent: €1,055.16
• Capped new rent: €1,000 × 1.03 = €1,030.00

How the next adjustment works after a cap depends on the clause. Some contracts take the index value at the date of the capped adjustment as the new reference point, so the excess is lost. Others allow the part above the cap to be carried forward to a later adjustment. If the clause does not say, take legal advice rather than guessing.

Worked example 5: a falling index and a floor

• Current rent: €1,200
• Old index: 120.0 (hypothetical)
• New index: 117.6 (hypothetical)

Formula result: €1,200 × 117.6 ÷ 120.0 = €1,176.00, a 2% decrease.

Clause type	Rent after adjustment
Two-way clause, no floor	€1,176.00
Floor at current rent	€1,200.00

Be aware that in some jurisdictions a clause that only ever moves upward may be restricted or unenforceable. This is a question for a local lawyer when drafting the agreement.

Base-year changes (rebasing)

Statistics offices periodically rebase an index so that a more recent period equals 100. The underlying price movements stay the same, but every value in the series changes. This catches out landlords who mix values from the old and new base.

Worked example 6: rebasing

• Current rent: €900, set when the index stood at 125.0 on the old base (hypothetical)
• The index has since been rebased. On the new base, that same month is published as 96.0 (hypothetical)
• The latest value on the new base is 108.0 (hypothetical)

Correct calculation, both values on the new base:

€900 × 108.0 ÷ 96.0 = €900 × 1.125 = €1,012.50

Wrong calculation, mixing bases:

€900 × 108.0 ÷ 125.0 = €777.60, which would wrongly suggest a large rent cut.

Method	Old value	New value	New rent
Same base (correct)	96.0 (new base)	108.0 (new base)	€1,012.50
Mixed bases (wrong)	125.0 (old base)	108.0 (new base)	€777.60

To avoid this, look up the old value in the officially rebased series, or convert it using the conversion or linking factor the statistics office publishes. If your contract names a specific base year, check whether it says how rebasing should be handled.

Rounding

Small rounding choices can change the result, and occasionally whether a threshold is reached at all.

Good practice: carry the full precision of the index ratio through the calculation and round only the final rent, usually to the cent. Some contracts instead specify that the percentage change is rounded, for example to one decimal place. Follow the contract.

Worked example 7: rounding and a threshold

• Current rent: €950
• Old index: 104.1 (hypothetical)
• New index: 109.3 (hypothetical)
• Clause: adjustment only when the index has risen by at least 5%

Exact change: 109.3 ÷ 104.1 − 1 = 0.049952…, or 4.995%. Strictly, that is below 5%.

Rounding rule	Change used	Threshold met?	New rent
No rounding of percentage	4.995%	No	€950.00 (no change)
Percentage rounded to one decimal	5.0%	Yes	€950 × 1.05 = €997.50

Here, €47.50 a month depends purely on a rounding rule. If the clause does not specify one, landlord and tenant can reasonably disagree. When drafting an agreement, state the rounding rule explicitly.

Step-by-step: applying an index-linked increase

1. Read the index clause: index name, reference month, threshold, cap, floor, frequency, rounding and notice requirements.
2. Find the old index value used at the last adjustment or at the start of the tenancy.
3. Find the latest relevant index value, on the same base.
4. Check whether any threshold or minimum period is met.
5. Calculate: new rent = old rent × new index ÷ old index.
6. Apply any cap or floor, then round as the contract specifies.
7. Check local legal limits on frequency, amount and form.
8. Send a written notice with the calculation, the index values used and their sources, and the effective date.
9. Record the new reference index value for next time.

The general process for notifying tenants, timing and handling objections is the same as for any increase; see how to raise rent.

Common mistakes with index-linked rent

• Reading points as percent when the old value is not 100.
• Mixing base years after a rebasing.
• Using the wrong index, region or series.
• Moving the reference point each year even when no adjustment was made.
• Applying the change to the wrong rent, for example including service charges that are adjusted separately.
• Missing the notice requirements, so a correctly calculated increase is still not valid.
• No record of the values used, which makes the next adjustment harder and disputes more likely.

Index clauses in your wider strategy

Indexation gives predictable, transparent rent changes that follow general prices, but it does not track your actual costs or the local rental market. In periods of high inflation, increases can be large and harder for tenants to afford; in low-inflation periods, rent may lag behind rising costs such as maintenance or financing. Check what different scenarios mean for your rental property cash flow, and see how to manage a rental property for how the yearly rent review fits into your overall routine.

Key takeaways

• The formula is new rent = old rent × new index ÷ old index.
• Use exactly the index, month and base the contract specifies, and keep a record of every value.
• A change in points is not a change in percent unless the old value is 100.
• Apply thresholds, caps and floors exactly as written, and clarify rounding in the contract.
• After a rebasing, put both values on the same base before dividing.
• Index clauses and rent increases are regulated differently across countries, so check local rules or ask a lawyer.