AmericaCumulative inflation is the percentage change between two price-index levels. Divide the later index by the earlier one, subtract one, and multiply by 100. It is not the sum of the annual inflation rates.
Your numbers. A transparent calculation.
What changed between two dates?
Use index levels, not inflation percentages. Enter values from the same CPI series, geography, adjustment and reference base. No account, data upload or live-data claim.
Find national all-items CPI-U index levels at BLS ↗. This series is not seasonally adjusted. If another chart says “3% inflation,” do not enter 3 as its CPI index level.
From the values you entered
- Total price change
- Same basket, later cost
- Fixed budget’s purchasing-power change
For current annual comparisons, open the official BLS calculator ↗. This page does not automatically download CPI data.
Three checks before you divide
- Match the series. Do not combine CPI-U with CPI-W, a city series with a national series, or seasonally adjusted with unadjusted values.
- Match the basis. Both index levels must use the same reference base. Use two monthly levels for a monthly-date comparison or two annual averages for an annual-average comparison—not one of each.
- Describe the interval. December to the next December is 12 months. January to December in the same year is 11 months. Name both periods when you share the result.
Price change = (later index ÷ earlier index − 1) × 100
Same-basket later cost = earlier cost × later index ÷ earlier index
Fixed-budget purchasing-power change = (earlier index ÷ later index − 1) × 100
A rise from 100 to 120 means prices increased 20%. A fixed dollar budget then buys about 16.67% less of that basket. Those percentages answer different questions.
A historical example you can verify
BLS’s worked example uses national all-items CPI-U, not seasonally adjusted: 278.802 in December 2021 and 296.797 in December 2022. These are historical values, not the latest inflation reading.
| Question | Result, rounded |
|---|---|
| How much did the price level rise? | 6.45% |
| What would a $500 earlier basket cost later? | $532.27 |
| How did a fixed $500 budget’s purchasing power change? | −6.06% |
BLS rounds the index change to 6.5% at one decimal place. Our two-decimal display uses the same inputs. An average national basket is not a quote for your rent, groceries or medical bill.
Common questions
Why can inflation fall while prices stay high?
A lower positive inflation rate means prices are rising more slowly, not returning to their earlier level. A fall in the price level is a different change.
Can I add annual inflation rates?
No. Price changes compound. Two consecutive 10% increases produce a 21% total increase, not 20%. Compatible start and end CPI levels give the cumulative change directly.
Does this show my personal cost of living?
No. CPI reflects a defined population and basket. Your household’s purchases, location and circumstances can differ. This tool performs the arithmetic on the values you enter; it does not fetch current data.
Sources and method
Official sources consulted November 6, 2026. Explanations, checklists and tool interfaces are America.co guidance, not government certification or legal, financial or compliance advice.
- BLS: calculating percent changes ↗Index-ratio method, compatible reference bases and matched periods
- BLS: purchasing power and constant dollars ↗Converting a dollar amount using the ratio of price indexes
- BLS official inflation calculator ↗Annual comparisons using the national all-items, not-seasonally-adjusted CPI-U
AI-assisted drafting with source checks; not represented as expert-reviewed advice. Who publishes this work · Our editorial approach · Send a source-backed correction.