One rate, every year
The future dollars needed to match a sum of today's dollars are the sum times (1 + rate) raised to the number of years. The purchasing power of that same sum, after the increase, is the sum divided by the same factor. At 2 percent for 10 years, $100.00 needs $121.90 later to buy what $100.00 buys now, and the $100.00 itself has $82.03 of today's buying power left. Those cents are half-up rounding of 100 times 1.02 to the 10th, and of 100 divided by that power. Two percent is in the form because the power is easy to repeat. It is not a reading from a price index, and it is not the Federal Reserve's target stated as a fact about the next ten years. The page does not store a target.
The Federal Reserve's FAQ says inflation is the increase in the prices of goods and services over time, and that it cannot be measured by one product or even several. It is a general increase in the overall price level. Policymakers watch several indexes because the indexes track different baskets and are built differently, so they can send different signals. The FAQ says the Fed follows consumer price indexes and producer price indexes issued by the Department of Labor, among other measures. This page cites that description and then refuses to pretend it has downloaded an index. A constant rate is a teaching model. A history of the CPI is a table of published numbers between two months. They answer different questions.
What this build could not read
The Bureau of Labor Statistics publishes the consumer price index and an inflation calculator that uses it. Requests to those BLS pages from the network that built this site were refused. Because the pages did not resolve here, they are not in the source list, and no CPI level is printed as if it had been read. If you want the change in the CPI between two dates, use the Bureau's calculator yourself. Do not treat $82.03 as that answer. It is the answer only for a flat 2 percent path.
A rate that changes every year needs a list of rates. This form has one. Compounding a different rate each year is a product of factors, not a single power. The compound-interest page grows money at a rate of return. This page shrinks buying power at a rate of price increase. Using the same percent in both places is a coincidence you would have to intend. A 5 percent return and 2 percent inflation are not "3 percent" unless you choose that approximation. This site does not subtract them for you, because a real return and a price index are not required to use the same year or the same basket.
How the other calculators stay in nominal dollars
The savings-goal page solves for a deposit that reaches a balance in the dollars you type. It does not inflate the goal. If you want the goal to be "what $100.00 of today's groceries costs in ten years at 2 percent," the dollars-later figure on this page is the number to carry over. The savings page will then treat that number as a nominal target. The rent-versus-buy page has its own appreciation box, defaulting to zero, for the same reason: a house is not the CPI, and a rent increase is not the CPI. The Federal Reserve's sentence that inflation is not the change in one product is why those boxes are separate.
Loan payments on this site are also nominal. A fixed mortgage payment does not rise with prices, which is a different economic fact from the purchasing power of the dollars you use to make the payment. This page will not adjust a mortgage payment. The mortgage calculator already shows the contractual payment. If you want to know what that payment is "in today's dollars" after ten years of 2 percent inflation, divide it by the same factor this page uses. The page will do that division if you type the payment as the amount. It still will not pick the inflation rate.
Negative rates and long horizons
A negative rate is allowed down to minus 100 percent, which is a price level that falls. Purchasing power then rises. The form does not suggest deflation. It only refuses to crash if someone types a minus sign. A horizon of zero years returns the amount unchanged. Eighty years is the same cap as the compound page. Past that, the page stops. The cap is a bound on the form, not a view about the next eighty years of prices.
Giuseppe Ilarda, the site owner, writes and checks the powers. The result is an educational estimate under a constant rate. It is not financial advice and not a forecast. The disclaimer is the right place for that limit. If a source later publishes a CPI change you want to use, type that percent yourself. The page will not be updated to embed an index it could not read.