Two ways to measure the loss
Purchasing power is what a sum of money can buy once inflation is taken into account. With a rate you choose, the calculator divides by the growth in prices. Since a past year, it uses actual prices instead: amount × CPI(start) ÷ CPI(end), from the BLS CPI-U annual averages.
Buying power after n years = amount ÷ (1 + i)^ni = yearly inflation rate. "Prices double in" = ln 2 ÷ ln(1 + i) years.
The coins shrink in area with what the money buys. Once prices have doubled, the same notes buy half as much.
Example: at 3% a year, after 20 years $1,000 buys what $553.68 buys today (1,000 ÷ 1.03^20), a loss of 44.6%. Kept as cash from 1980 to 2025, $1,000 bought only what $255.95 bought in 1980.