How to calculate cumulative inflation without adding percentages
Editorial draft awaiting owner review. Excluded from indexing and the sitemap.
Compare general price levels across months using the ratio of their indices. Both must belong to a compatible series.
The ratio captures the change
Divide the final index by the initial index. Subtract one and multiply by 100 for the cumulative percentage change. Fictional mathematical example: 125 ÷ 100 = 1.25, a 25% change. An initial €1,000 would be equivalent to €1,250. These are not published CPI figures.
Do not add rates from successive years
A 10% increase followed by another 10% multiplies by 1.1 twice: the total increase is 21%, not 20%. An index below its initial value gives a negative change. The calculator handles deflation with the same formula.
Nominal and real are different
Nominal change compares income in euros. Real change adjusts this ratio by the CPI factor. Income rising from €1,000 to €1,100 with prices multiplied by 1.25 gives 1.1 ÷ 1.25 = 0.88: a real change of −12%.
The basket is not personal
We use national all-items CPI, base 2025, including official historical values. We neither mix bases nor fill missing months. The result does not predict individual prices, replace a personal basket or provide a legally valid rental adjustment.
Check the calculation
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Open calculatorProject sources and methodology. This guide uses mathematical examples, not population statistics.