The formula
equivalent salary = old salary × (1 + inflation₁) × (1 + inflation₂) × …
Where it comes from
Inflation across several years does not add up, it compounds: two years at 10 % is not 20 % but 21 %, because the second year rises on prices that already rose. That is why a rise that looks as though it covers cumulative inflation often does not.
How to work it out by hand
- Type the inflation rate for each year of the period
- Multiply the (1 + inflation) factors for every year
- Apply that factor to your old salary
- Compare the result with what you earn now
What is worth knowing
There is an asymmetry that trips people up: if prices rise 10 %, keeping your purchasing power needs a 10 % rise, but a frozen salary does not lose 10 %, it loses 9.09 %. They are different percentages because they are worked out on different bases. The honest figure to negotiate with is the first one: what the payslip has to rise by. Official inflation data comes from each country's statistics office.