CPI Adjustment Calculator

What the CPI adjustment calculator does

Many payments are tied to the price level. A lease may say the rent rises each year by the change in the consumer price index. A service contract, a ground rent, a licence fee or a private pension arrangement may say the same about its amount. Some clauses pass on the full change, others only a share of it, and some never let the amount fall. This calculator does the arithmetic behind such a clause: it takes an amount, a base month and a target month, and returns the amount after adjusting it by the change in an official consumer price index.

By default it uses the Consumer Price Index for All Urban Consumers (CPI-U) of the U.S. Bureau of Labor Statistics (BLS), U.S. city average, not seasonally adjusted, with monthly data back to January 1913. You can also pick Germany, France, Spain, Italy or Ukraine, and the calculator switches to that country's national consumer price index and currency. The data are bundled with the page, so the calculation runs in your browser and nothing you enter is sent anywhere.

The result is a number, not a legal opinion. The calculator knows the index values and the formula that most index clauses use. It does not know your agreement, its caps, floors or rounding, or any local law that limits indexation. Those you apply yourself, and this guide shows how.

In this guide:

• How to fill in the fields and read the results
• The formula, and why the base month matters more than it seems
• How index clauses work in practice, with the BLS checklist for writing one
• Partial indexation, caps and what happens when prices fall
• Which index your agreement names, and why the result can differ from it
• Worked examples, common mistakes and limits

If inflation itself is new to you, start with the basics article on inflation and why money loses value.

How to use it

The Your numbers panel has five rows.

1. Country. The United States is selected by default. Switching the country changes the index, the available months and the currency, and clears any pinned scenario, because dollars and euros cannot be compared directly.
2. Amount. The amount to adjust: a monthly rent, a yearly fee, a contract price. It is $1,000 by default. The slider runs to 100,000, and you can type up to a trillion. On a phone the field has − and + buttons.
3. Base month. The month your agreement takes as its starting point, picked from a month list and a year list. The hint names the earliest available month, and for the United States it adds that October 2025 is not available.
4. Target month. The month you index the amount to. It cannot be earlier than the base month. The hint names the latest available month.
5. Share of change applied. The part of the index change your agreement passes on. Type a value from 0% to 100%, drag the slider, or use the 100% and 75% buttons.

When you open the calculator, it shows the last 12 months of data. With data up to September 2026 that is $1,000 from September 2025, indexed to September 2026: $1,037.38.

Reading the results

The headline repeats what was calculated, such as "$1,000.00 from September 2025, indexed to September 2026", followed by the indexed amount. Four figures sit below it:

• Index change: how much the index rose or fell between the two months, +3.7% in the default view;
• Change applied: the index change multiplied by your share. At 100% it equals the index change; at 75% it is +2.8%;
• Change in amount: the difference between the indexed and the original amount, +$37.38;
• Months in the period: the number of months between the base and the target month, 12.

Two tabs follow. Chart draws the indexed amount month by month, with a dotted line for the original amount, so you can see when the adjustment would have been higher or lower along the way. By year splits the period into calendar years: for each one it shows the months covered, the index change in that stretch, the change since the base month and the indexed amount at the end of the stretch. In the default view the first row, September to December 2025, shows −0.2%, and the second, December 2025 to September 2026, shows +4.0%, for +3.7% since the base month.

Notices appear under the figures when they matter: when prices fell and the amount went down, when the base month is before a currency change, and always a reminder that your agreement may name another index. The line under the calculator names the last month of data and the source. Pin to compare keeps the current result while you change the inputs, so the headline shows the difference against it. PDF saves the inputs, figures and the by-year table, and Reset restores the defaults. On phones and tablets the results live in a panel at the bottom of the screen, and Details opens the figures and tabs.

The formula

An index clause compares two values of the same price index. The ratio between them is the price change; the share decides how much of it reaches the amount.

Index change = CPI(target month) ÷ CPI(base month) − 1
Change applied = Index change × Share
Indexed amount = Amount × (1 + Change applied)

• the base year of the index cancels out in the ratio, so it does not matter that the CPI-U equals 100 on average over 1982–1984;
• at a share of 100% the indexed amount is simply the amount multiplied by the ratio of the two index values;
• a falling index gives a negative change, and the calculator applies it as it is.

The BLS fact sheet on using the CPI for escalation describes the same steps: first the change in index points between the two periods, then that change divided by the earlier index. Its own example takes an index of 229.815 and a later one of 232.945: the difference is 3.130 points, which is 1.4% of the earlier value. Index points on their own say nothing about the size of a change, because a 3-point rise meant far more when the index stood at 30 than it does at 330. Always work with the percentage.

In the default view the CPI-U stood at 324.800 in September 2025 and 336.940 in September 2026. The ratio is 1.0374, so prices rose by 3.7% and $1,000 becomes $1,037.38. With a share of 75%, the change applied is 2.8% and the amount becomes $1,028.03.

The calculator keeps every decimal of the index ratio and rounds only for display: amounts to the cent, percentages to one decimal. Some agreements round the change before applying it. If yours does, round the index change the same way and apply it by hand, or compare the result with the calculator's figure, which will differ by a few cents or dollars.

Choosing the base and target months

The months decide the result more than anything else, and most disputes about index clauses are about the months rather than the arithmetic.

The base month is the month of the index value the amount refers to. Usually that is the month the contract was signed or the amount was last adjusted. It is not necessarily the month of the first payment: a lease signed in August with rent from October may still take August as its base. If the clause names a specific index value, find the month that value belongs to.

The target month is the month of the index value the new amount refers to. For a yearly adjustment it is usually the same calendar month a year later. Many clauses add a lag: the index for "the month two months before the adjustment date", because the latest index is published a few weeks after the month ends. The BLS releases a month's CPI around the middle of the following month, so an adjustment due on 1 October can rely on the August index, not the September one. Move both months back by the lag your clause names.

Shifting both months by one changes the answer. With $2,400 a month, September 2025 to September 2026 gives +3.7% and $2,489.70, while August 2025 to August 2026 gives +3.4% and $2,481.52. Neither is wrong; they answer different clauses.

Annual averages are a different measure. Some clauses compare yearly averages of the index instead of single months, and Social Security's cost-of-living adjustment, according to the BLS, is defined from third-quarter averages of the CPI-W. The calculator compares single months only. For an average-based clause, average the monthly index values yourself and apply the ratio of the averages.

The missing October 2025

The US series has one gap. During the 2025 lapse in federal appropriations the BLS did not collect October 2025 price data, and for most CPI series it published no October 2025 index; its data show a dash for that month. The calculator does not let you choose October 2025 as a base or target month for the United States. Periods that merely pass through it work as usual, because the change needs only the index values of the two chosen months.

For agreements tied to October, the BLS describes an approximation that applies only if all parties agree: the geometric mean of the index values for the months immediately before and after. For September 2025 (324.800) and November 2025 (324.122) that is about 324.461. The BLS adds that values calculated this way are not official CPI data and that percent changes based on them are unofficial estimates. It has also said it will not publish 12-month percent changes for October 2026 for indexes where October 2025 data are not available. Check your agreement for a fallback rule before choosing a method.

Index clauses in practice: rent, contracts and payments

Index clauses are common wherever a payment runs for years: commercial and residential leases, ground rents, maintenance and service contracts, licence fees, some pensions and annuities, and collective agreements with cost-of-living adjustments. The BLS notes that the CPI is often used to adjust income payments, such as Social Security, and that the CPI-U and CPI-W are commonly used in escalation agreements and to adjust pensions and tax brackets. It also stresses that the decision to use an escalation mechanism, and the choice of index, is up to the user.

The BLS fact sheet on escalation lists what a well-written clause should pin down. It is a useful checklist both for drafting a clause and for reading one:

• the base payment: the amount to be adjusted;
• the exact index series: population coverage (CPI-U or CPI-W), area (U.S. city average, a region or a metropolitan area), the series title and the reference base, such as 1982–84=100;
• the reference period: a single month or an annual average;
• how often the payment is adjusted, usually once a year;
• the formula, including any caps or floors;
• what happens after a major CPI revision or a change in the index base.

The BLS recommends the U.S. city average for escalator clauses. Metropolitan area indexes have relatively small samples and larger sampling errors, and most of them are published every two months rather than monthly. It also says it neither encourages nor discourages escalation and cannot write contract language or settle disputes.

For other countries the logic is the same: the clause names an index, a base month, a target month and a formula, and the calculator applies the national consumer price index of Germany, France, Spain, Italy or Ukraine. What differs is the law around the clause. Some countries fix their own rules for indexing rents, wages or court-ordered payments, such as which index may be used, how often the amount may change or how far it may rise. The calculator does not apply any of these rules, so check them before relying on the result.

Partial indexation, caps and falling prices

Partial indexation. A share below 100% passes on only part of the price change. It is a compromise between a fixed amount and full indexation: the payer carries part of the inflation risk and the payee the rest. Enter the share in the last field. At 75%, a 3.7% index change becomes a 2.8% adjustment.

Yearly steps compound. A clause that adjusts every year starts each step from the amount after the previous one. At 100% the steps telescope: adjusting $2,000 from September 2023 in three yearly steps gives exactly the same $2,189.42 as one adjustment from September 2023 to September 2026, because the intermediate index values cancel out. With a share or a cap that is no longer true. The table shows three yearly steps on the CPI-U, each calculated with this tool from the previous year's amount; the cap column applies the cap by hand to the tool's yearly change.

One adjustment at 75% from September 2023 straight to September 2026 gives $2,142.07, a little more than the $2,141.01 from three yearly steps. The cap mattered in the last two years, when the index rose by more than 3%, and kept the rent $15.83 below the fully indexed amount. To model a cap or a floor, run the calculator for each year and replace the change applied with the cap whenever it is higher.

Falling prices. When the index of the target month is lower than that of the base month, the calculator shows a lower amount and a notice. Prices do fall from time to time: in the United States the CPI-U went from 219.964 in July 2008 to 215.351 in July 2009, a change of −2.1%. Whether the amount should fall depends on the clause. Many leases and contracts only allow increases, and some hold the amount until the index climbs back above its base level. If your clause works like that, keep the original amount and read the result as how far the index is below its base.

Which index your agreement names

An index clause matches the calculator only when it names the same index. The table compares the series you are likely to see in an agreement with what this calculator uses.

The CPI-W population is a subset of the CPI-U population and excludes households of professional and salaried workers. The two are separate series with their own monthly values, so a clause that names the CPI-W needs the CPI-W itself; the calculator's CPI-U figure is an estimate in that case. The same goes for a regional index: the BLS notes that regional and metropolitan indexes often move more erratically than the national one.

Seasonal adjustment. The calculator uses indices that are not seasonally adjusted, and so should an index clause. The BLS strongly recommends unadjusted indexes for escalation: seasonal factors are recalculated every year and seasonally adjusted data can be revised for up to five years, while the unadjusted CPI-U and CPI-W are final when issued. A figure you calculate today will therefore stay the same.

Euro countries and Ukraine. For Germany the calculator uses the consumer price index (VPI) from Destatis, taken from the Deutsche Bundesbank; for France and Spain the IPC of Insee and INE; for Italy Istat's FOI index excluding tobacco; for Ukraine the consumer price indices of the State Statistics Service. Each of the euro countries also publishes a harmonised index (HICP), which is built for comparisons between countries and is not what the calculator uses.

Where the index data come from

All series are taken from the producers' own public data services, or from the Deutsche Bundesbank in the case of Germany, and bundled with the page. The line under the calculator names the producer, the series and its licence; US data are in the public domain. The data are refreshed after new releases, so the latest available month can trail the calendar by one or two months. With the current data the latest month for every country is September 2026.

The same 12 months, September 2025 to September 2026, show how different the national indices can be:

For Ukraine the same period gives +10.0%: 10,000 hryvnias become 11,000.69. Use the country whose index your agreement names, which is usually the country of the currency the payment is made in.

For older base months, two reminders appear. In Germany, France, Spain and Italy prices were quoted in national currencies before euro cash arrived in 2002, so an amount from before then has to be entered in euros at the fixed conversion rate; the calculator does not convert it. For Ukraine, amounts from before September 1996 were in karbovanets.

Worked examples

The figures below are calculator results on data up to September 2026. Once newer data arrive, the latest months and amounts will change.

Indexation vs inflation calculator

KrokFin has several tools around inflation, and each answers a different question.

The CPI inflation calculator uses the same bundled indices but looks at the price change itself: two ways of counting months, both directions of conversion, average annual inflation and a month-by-month table. Use it to check a year-to-date figure or to see what a salary from ten years ago is worth today.

The inflation impact calculator looks ahead with a rate you choose, and the percentage calculator handles any percentage change, for example when an agreement gives a fixed yearly increase instead of an index. If an index clause raises one of your regular costs, the budget planner shows how much room the new amount leaves.

Common mistakes

Using the wrong months. The base month is the month of the index value the amount refers to, not the month of the first payment, and the target month may carry a lag. One month either way can change the result noticeably.

Adding up yearly percentages. Changes compound. Three years of 2.4%, 3.0% and 3.7% are not 9.1% but 9.5%, as the ratio of the two index values shows directly.

Using index points as a percentage. A rise of 12 points is a different percentage at an index of 120 than at 330. Divide by the earlier index value.

Mixing up CPI-U and CPI-W, or a national and a regional index. The result matches your agreement only when the series is the same. Check the series title, area and population coverage in the clause.

Using seasonally adjusted figures. Monthly figures in news reports are often seasonally adjusted. For escalation, use the unadjusted index, which is what the calculator uses.

Applying a falling index the clause does not allow. If prices fell and the agreement allows only increases, the amount stays where it was.

Indexing from the original amount after earlier adjustments. If a clause adjusts each year from the latest amount, start each year from the amount after the previous adjustment. With a share or a cap, the two methods give different results.

Forgetting the currency. For euro countries, an amount from before 2002 must be converted to euros at the fixed rate before you enter it.

Limitations and data updates

The calculator applies one formula: the ratio of two monthly index values, multiplied by your share. It does not apply caps, floors, thresholds, rounding rules, annual averages or lags; it does not know local laws that limit indexation; and it does not forecast index values beyond the latest published month. For the United States it offers the CPI-U only, not the CPI-W or regional indexes. How a steady price rise compounds over many years is explained in the basics article on compound interest and the time value of money.

The data end with the latest published month and are refreshed after new releases. Old values seldom change, and the unadjusted CPI-U is final when issued, but a producer can rebase or revise a series, and the next data refresh picks that up. Results are rounded only for display.

The result is not legal, tax or financial advice and does not replace a calculation that a law, a court or your agreement requires. If indexation leaves your savings behind rising prices, the articles on whether to keep money in a savings account and on risk and return are good next reads.