Inflation calculator

How the inflation calculator works

Inflation is the rate at which prices rise, which means it is also the rate at which a fixed sum of money buys less. Enter an amount, an assumed annual rate and a number of years, and this projects the cost forward or discounts a future sum back into today's money.

Two modes, because the word covers two different questions. The CPI lookup uses the index the Bureau of Labor Statistics actually published, back to 1913. The projection compounds a rate you choose, which is the only option when the period has not happened yet.

The arithmetic

Inflation compounds, exactly like interest:

Future cost = amount x (1 + i)^n

Running it backward divides instead:

Past equivalent = amount / (1 + i)^n

At 6% for ten years, something costing 100 today costs 179.08. The two directions are exact inverses, so 179.08 discounted back at the same rate returns 100.

Prices and purchasing power are not mirror images

This trips up almost everyone, and it is worth doing slowly.

Ten years of 6% inflation raises prices by 79.08%. It does not cut purchasing power by 79.08%. Money kept in cash still buys 55.84% of what it does now, because purchasing power is the reciprocal of the price rise, which is a different relationship entirely.

A 220% price rise over twenty years still leaves 31.18 of buying power. The relationship is a curve that approaches zero and never crosses it.

When money halves

Years to halve = ln(2) / ln(1 + i)

At 7%, that is 10.24 years. At 3%, 23.45 years.

The rule of 70 divides 70 by the rate and gets 10 years at 7%, which runs slightly fast. The shortcut is good enough for mental arithmetic and drifts further as the rate climbs, so the exact figure is what the tool prints.

The CPI lookup, and what it shows you

The Consumer Price Index is a measure of the average change in prices paid by urban consumers, published monthly. The lookup mode uses series CUUR0000SA0, all items, US city average, not seasonally adjusted, which runs from January 1913 and is the series the Bureau of Labor Statistics builds its own calculator on.

Converting between two months is nothing more than the ratio of two readings:

Amount x (index at the target month) / (index at the source month)

So $100 in January 1970, when the index stood at 37.8, is $883.47 in June 2026 money, when it stood at 333.952.

This page prints both readings. That sounds like a small thing and it is the whole difference: with 37.8 and 333.952 on screen you can check the arithmetic in your head, and with only the answer you are trusting a black box. It also prints the annual rate the two dates imply, 3.937% a year across those 56 years, which is the number that lets the projection mode reproduce the same result. No calculator ranking for this term shows either.

Some months have no reading, and we say so

October 2025 is returned by the BLS API with a value of -. The month exists in the series and carries no figure.

Averaging the neighbours would produce a number that looks like published data and is not. So a conversion touching that month is declined with the reason, rather than quietly filled in. It is worth checking whether any calculator you use does the same, because the interpolated answer is indistinguishable from a real one once it is on screen.

The same phrase means two things by region

In the US, inflation calculator is a historical lookup. The Bureau of Labor Statistics ranks first with its own tool, followed by usinflationcalculator (CPI from 1913 to 2026, updated monthly, June 2026 figures published on 14 July 2026), in2013dollars, and the Minneapolis Fed.

In India, the same phrase means a forward projection. Groww, ClearTax, SEBI and Scripbox all take a current cost, an assumed rate and a number of years, and return a future cost. No dataset is involved.

Both modes are here for that reason. The lookup is US-only, because it uses the US index; the projection works in any currency because it makes no claim about any particular country's prices.

Two things worth noting about the projection tools that rank. Groww prints the formula for an inflation rate from two CPI readings, ((CPI next minus CPI now) / CPI now) x 100, on a page whose calculator computes something else entirely: a compounded future value. The published equation does not describe the tool beside it. And SEBI, the securities regulator, publishes no formula or assumptions at all, disclosing only that its calculators are illustrative and that reliable rate prediction is not possible. That last point is honest and worth repeating.

What rate to enter

There is no correct answer, which is the whole difficulty. Long-run averages differ sharply by country: in2013dollars reports the rupee averaging 7.05% a year between 1958 and 2026 using World Bank and OECD data, over a 68 year span.

Running the same calculation at two or three rates tells you more than any single figure. If the answer barely moves between 4% and 7%, the assumption is not carrying much weight. If it doubles, the projection is really a statement about the rate.

Inflation against an investment return

A nominal return has to clear inflation before it adds any purchasing power, and the relationship divides:

Real return = (1 + nominal) / (1 + inflation) - 1

An 8% return against 6% inflation is a 1.887% real return, where subtraction would suggest 2%. The gap is small at these levels and grows with both numbers, which is why the shortcut is safe in a low-inflation decade and misleading in a high one.

What this calculator does not do

It doesn't predict inflation. The lookup covers the US all-items index only, not the regional or category series BLS also publishes. It also doesn't distinguish between the basket of goods you personally buy and the national average, and those can differ a great deal: someone whose main costs are rent and healthcare experiences a different rate from the headline figure.

Results are arithmetic on your inputs, not financial advice. For decisions about your money, speak to a licensed financial adviser. To grow a balance at a nominal rate instead, the compound interest calculator does that, and the present value calculator discounts a future sum at a rate you choose.

Deflation works here too, by entering a negative rate. At negative 2% over ten years a cost of 100 falls to 81.71 while 100 in cash buys 122.38 of what it buys today. Almost no inflation calculator accepts a negative rate at all, which tells you how rare sustained deflation has been in the modern record.

Frequently asked questions

How do you calculate the effect of inflation? Inflation compounds, so a future cost is the current cost times (1 + i) raised to the number of years. At 6 percent for ten years, something costing 100 today costs 179.08. Running it the other way divides, so 179.08 in ten years is worth 100 in today money at the same rate.

What is purchasing power and how is it different from the inflation rate? Purchasing power is how much a fixed sum of money actually buys, and it falls more slowly than prices rise because the two are reciprocals, which is a different relationship from mirror images. Ten years of 6 percent inflation raises prices 79.08 percent, and yet 100 kept in cash still buys 55.84 percent of what it does today. People often assume a 79 percent price rise means 79 percent less buying power, and it does not.

How long does it take inflation to halve the value of money? Divide the natural log of 2 by the natural log of one plus the rate. At 7 percent that gives 10.24 years and at 3 percent it gives 23.45 years. The rule of 70, which would say 10 years at 7 percent, is a useful shortcut that runs slightly fast, and the gap widens as the rate climbs.

Does this calculator use real CPI data? In the CPI lookup mode, yes. That mode uses the published US index, series CUUR0000SA0 from the Bureau of Labor Statistics, monthly from January 1913, and prints the two index readings it divided so the arithmetic can be checked. The projection mode does not use CPI at all: it compounds a rate you choose, which is the right tool when the period has not happened yet or the country is not the US.

Why does the calculator refuse some months? Because the Bureau of Labor Statistics has not published a figure for them. October 2025 is returned by the BLS API with a value of "-", meaning the month exists in the series and carries no reading. Filling that in by averaging the neighbours would mean inventing an index value and presenting it as published data, so any conversion touching such a month is declined and the reason given instead.

What inflation rate should I use? That is a judgement, and SEBI puts it well on its own calculator: reliable prediction of the rate is not possible. Central banks publish targets, and long-run averages differ sharply by country, with in2013dollars reporting an average of 7.05 percent a year for India between 1958 and 2026 using World Bank and OECD data. Any figure you enter is an assumption, so it is worth running the calculation at two or three rates to see how much the answer moves.

Why do inflation calculators differ between countries? They answer different questions. US inflation tools are mostly historical CPI lookups, asking what a 1970 dollar is worth now, which is why the Bureau of Labor Statistics ranks first. India inflation tools are almost entirely forward projections, asking what an expense will cost in fifteen years, which is what Groww, ClearTax and SEBI all compute. The arithmetic is the same and the intent is not.

How does inflation affect an investment return? A nominal return has to clear inflation before it adds any purchasing power, and the exact relationship divides: the real return is (1 + nominal) divided by (1 + inflation), minus 1. At an 8 percent nominal return and 6 percent inflation, the real return is 1.887 percent, where subtraction would suggest 2 percent.

Can this handle deflation? Yes, by entering a negative rate. At negative 2 percent over ten years, something costing 100 falls to 81.71, and money gains purchasing power, so 100 buys 122.38 of what it buys today. Sustained deflation is rare in the modern record, which is why almost no inflation calculator accepts a negative rate at all.

Sources

• BLS CPI inflation calculator, the authoritative source for US historical figures
• usinflationcalculator, publishing CPI data from 1913 with monthly release dates
• in2013dollars India, reporting a 7.05% average annual rate from 1958 using World Bank and OECD data
• SEBI investor inflation calculator, which states that reliable rate prediction is not possible