Rule of 70 is magic. Divide 70 by the inflation rate and you get the years your purchasing power is halved.
For example
-2% is approx 35 years (70/2)
-10% is approx 7 years (70/10)
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Assume 1 = 100% purchasing power today.
Future purchasing power in percent =
1 * (100% - inflation rate in percent ) ^ years.
Solve for years or purchasing power.