Inflation Rate Calculator
The two prices and the time between them
Your result will appear here
Fill in the fields on the left and this updates as you type.
Know what this estimate is based on
- Jurisdiction
- General mathematical model
- Scope and limitations
- Educational estimate only. Confirm the assumptions, current rules, fees, and rounding that apply to your situation before making a decision.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Enter the earlier price of something you buy or pay, such as a rent, a bill or a product.
- 02
Enter the later price of the same thing, with the same size, features and terms.
- 03
Enter the whole years between the two prices, then any extra months, from 0 to 11.
- 04
Read the compound yearly rate, then compare it with the simple average and the doubling time.
- 05
If the later price is a current one, check the headline CPI-U rate over the same span to see whether this price rose faster or slower than inflation overall.
Formula
Years t = whole years + extra months ÷ 12. Total change = later price ÷ earlier price − 1. Yearly rate = (later price ÷ earlier price) raised to the power 1 ÷ t, minus 1. Simple average = total change ÷ t. Doubling time = ln 2 ÷ ln(1 + yearly rate), and the rule of 72 approximates it as 72 ÷ the rate in percent. The CPI comparison runs the same formula on the CPI-U from the start of the span to August 2026, and the real change = (later ÷ earlier) ÷ (CPI-U ratio over the span) − 1.
Example
A rent of $1,200 is now $1,650, 6 years and 4 months later. The total rise is $450, or 37.5%. Over 6.33 years that is a compound rate of 5.16% a year. Dividing 37.5% by the years gives a simple average of 5.92%, but used as a yearly rate it would have predicted $1,727, $77.44 too much; after one year the compound path is at $1,262 and the simple-average path already at $1,271. At 5.16% a year the rent doubles in 13.8 years, against 14 years by the rule of 72. Over the same span, from the 2020 average to August 2026, the CPI-U rose 4.16% a year, so the rent rose 1.00 point a year faster than inflation, a real increase of 6.2%.
Definitions
- Compound yearly rate
- The constant rate that, applied to the price each year on top of the previous year's increase, turns the earlier price into the later one.
- Simple average rate
- The total percentage change divided by the number of years. It overstates the yearly pace of a rising price because it ignores compounding.
- Doubling time
- How many years a price takes to double at a given yearly rate: the natural log of 2 divided by the natural log of one plus the rate.
- Rule of 72
- A shortcut for doubling time: 72 divided by the yearly rate in percent. It is close at ordinary rates and drifts at high ones.
- Real price change
- A price change after removing general inflation over the same period, showing whether the item became dearer or cheaper relative to prices overall.
Good to know
Compound versus simple: why the average misleads
When a price rises over several years, the most natural calculation is to take the total increase and divide it by the years. It is also the wrong one for a yearly rate. Prices compound: a rent that rises 5% this year and 5% next year ends up 10.25% higher, not 10%, because the second increase applies to a rent already raised by the first. Dividing the total by the years ignores that, and so it overstates the yearly pace of anything that rose. This page's example makes the gap concrete. A rent went from $1,200 to $1,650 over 6 years and 4 months. The total rise is $450, or 37.5%. Dividing 37.5% by 6.33 years gives a simple average of 5.92% a year. The compound rate that actually connects the two prices is 5.16% a year. The difference looks small, but it matters the moment someone uses the average to project forward. Compounding 5.92% a year from $1,200 over the same period would have predicted $1,727, which is $77.44 more than the actual rent. After just one year the two paths already differ: $1,262 at the true compound rate against $1,271 at the simple average, and the table on the page shows the error building year after year until the end of the span. The longer the period and the larger the total change, the bigger the gap. A price that doubles over ten years has a simple average of 10% a year, but its compound rate is about 7.2%. That is why official statistics, interest rates and investment returns are quoted as compound annual rates, and why the compound rate is the one to compare with the CPI. The simple average is still a fair description of the total change spread evenly over time, as long as nobody treats it as a rate to compound.
Doubling time and the rule of 72
A yearly rate is easier to feel when it is turned into a doubling time: the number of years a price takes to double if it keeps rising at that pace. The exact figure is the natural logarithm of 2 divided by the natural logarithm of one plus the rate. For the example's rent, rising 5.16% a year, that is 13.8 years. The rule of 72 is the mental shortcut: divide 72 by the rate in percent. It gives about 14 years for the rent, close enough for most purposes. The shortcut works because the natural log of 2 is about 0.693, and 72 is a convenient nearby number with many divisors. It is most accurate around 8%, and it drifts at the extremes. At 2% a year the exact doubling time is 35.0 years against 36 by the rule, and at 25% a year it is 3.1 years against 2.9. Doubling time is a useful way to read inflation figures themselves. Headline CPI-U rose 3.4% in the 12 months to August 2026. If prices kept rising at that pace, the general price level would double in about 20.8 years, well within a working life or a retirement. At the 2.30% a year the CPI-U averaged from 1990 to 2020, doubling takes about 30 years. The same arithmetic runs in reverse for falling prices. A price dropping at a steady rate never reaches zero but halves over a fixed span, which this page reports instead of a doubling time when the later price is lower. Keep in mind that a doubling time is a projection of a constant rate, not a forecast. Rents, fuel and food prices rarely rise evenly, and a few years at a high rate can be followed by years at a low one. Use the figure to compare paces, not to predict a date.
Setting one price against the CPI
A yearly rate for one price becomes more informative when you compare it with inflation overall. If the later price is current, this page measures the CPI-U over the same span, ending with the August 2026 index, and reports the gap. For the example's 6 years and 4 months, the span begins in 2020. The page holds monthly index values only from January 2024, so it starts from the 2020 annual average, a close but not exact match for a rent quoted in the spring of that year. Over that stretch the CPI-U rose 4.16% a year. The rent's 5.16% a year was therefore 1.00 point a year faster than prices overall. In real terms, after removing general inflation, the rent rose 6.2%. The real change is the more telling number for a household, because it says whether this cost took a bigger or smaller share of a budget whose other prices were also rising. There are good reasons a single price can drift from the index. The CPI is an average across many items and places, and the Bureau of Labor Statistics adjusts its prices for changes in quality and quantity, so a product that got better, or smaller, is not counted as the same thing at a different price. Local conditions matter too: rent in a fast-growing city can outrun the national figure for years. For reference, the CPI index for rent of primary residence rose 2.7% in the 12 months to August 2026, and shelter as a whole 3.0%, against 3.4% for all items. A price that rises faster than the CPI is not proof that the index is wrong; it usually means that item's market differs from the average. If you want the effect of all your prices at once rather than one of them, the personal inflation rate calculator weights each category by your own spending.
Getting the inputs right: same item, exact dates
The formula is simple, which puts all the weight on the inputs. Three things go wrong most often. The first is the time between the prices. Rates are sensitive to the span, especially over short periods, so count months as well as years. For the example rent, treating 6 years and 4 months as exactly 6 years would give 5.45% a year instead of 5.16%, overstating the pace by about 0.3 point. If you know the dates, count the whole years and enter the remaining months in their own field. The second is comparing different things. A lease that now includes parking, a phone plan with more data, a car with more equipment or a cereal box that shrank are not the same purchase at two prices. Before calculating, make sure the size, features and terms match, or adjust one price so they do. For a package that got smaller, the shrinkflation calculator works out the price per unit, which is the comparison that holds. The third is mixing list prices with what you actually paid. Sale prices, discounts, delivery fees and taxes can each move a price by more than a year of inflation. Use the same kind of price at both ends: the shelf price both times, or the total paid both times. It also helps to know what the answer is for. For judging whether a cost has outgrown your income, compare its rate with how fast your pay grew over the same years. For judging whether a price rose unusually fast, compare it with the CPI figure on this page. And for converting an amount between years when you have only one price, use the CPI inflation calculator, which applies the official index rather than your own two prices.
Frequently asked questions
How do I calculate the inflation rate between two prices?
Divide the later price by the earlier one, raise the result to the power of one divided by the years between them, and subtract one. A rent that went from $1,200 to $1,650 over 6 years and 4 months, which is 6.33 years, rose 37.5% in total, or 5.16% a year compounded.
Why not just divide the total increase by the number of years?
Because that simple average overstates how fast a rising price grew. Each year's increase builds on a price already raised by the years before, so a smaller yearly rate is enough to reach the total. In the example, 37.5% divided by 6.33 years is 5.92% a year, but compounding 5.92% from $1,200 would have predicted $1,727, which is $77.44 more than the actual $1,650. The correct compound rate is 5.16%.
How long does it take for a price to double?
At 5.16% a year, 13.8 years. The rule of 72, which divides 72 by the rate, gives about 14 years and is a good shortcut at ordinary rates. At the 3.4% headline CPI-U rate for the 12 months to August 2026, the general price level would double in about 20.8 years.
Did my price rise faster than inflation?
If the later price is current, the page compares it with the CPI-U over the same span, ending in August 2026. For the example's 6 years and 4 months, the span starts in 2020. Because this page holds monthly index values only from January 2024, it starts from the 2020 annual average, a close but not exact match. The CPI-U rose 4.16% a year from there, so the rent rose 1.00 point a year faster than prices overall, a real increase of 6.2% after inflation.
What if the price went down?
The rate comes out negative, and the page shows how long the price would take to halve at that pace instead of how long it would take to double. Falling prices are common for electronics and some used goods even while the overall price level rises.
What if the product changed between the two prices?
Then the two prices are not for the same thing, and the rate mixes a price change with a change in what you get. If a package got smaller, use the shrinkflation calculator to find the price per unit. Official price indexes make these adjustments, which is one reason a single price can drift away from the CPI.