Inflation is the rate at which the general level of prices for goods and services rises, causing purchasing power to fall. When prices increase, each unit of currency buys a smaller percentage of a good or service. The loss of real purchasing power is calculated using the discounting formula: Real Future Value = Nominal Amount / (1 + i)^n, where i is the annual inflation rate and n is the number of years.
Consider $100,000 held in physical cash or a non-interest-bearing account. Over a 20-year period with an average annual Consumer Price Index (CPI) inflation rate of 3.5%, the nominal value remains $100,000, but its real purchasing power drops to $100,000 / (1.035)^20 = $50,256.59. Cash stored under a mattress loses nearly 50% of its real purchasing capacity over two decades.
The Rule of 72 and purchasing power halving times
The Rule of 72 provides a rapid mental math formula to calculate how long it takes for inflation to cut purchasing power in half: Halving Time in Years ≈ 72 / Inflation Rate. At a moderate 3% inflation rate, purchasing power halves in 72 / 3 = 24 years. At a higher 6% inflation rate, purchasing power halves in just 72 / 6 = 12 years.
This compounding loss poses a significant challenge for fixed-income retirees. A fixed monthly pension payout of $3,500 that receives no annual cost-of-living adjustment (COLA) will see its real purchasing power fall to $2,425/month after 10 years at 3.7% annual inflation, representing a 30.7% reduction in real standard of living.
The Fisher equation and real rate of return math
To determine if your savings or investments are growing after inflation, you must calculate the real rate of return using the Fisher equation: (1 + nominal rate) = (1 + real rate) × (1 + inflation rate). This can be approximated as: Real Rate ≈ Nominal Interest Rate - Inflation Rate.
If a high-yield savings account pays a 4.5% nominal interest rate while annual inflation is 3.2%, your gross real return is approximately 4.5% - 3.2% = 1.3%. However, after factoring in a 24% marginal tax rate on interest income (reducing net nominal yield to 4.5% × (1 - 0.24) = 3.42%), your net real return drops to 3.42% - 3.20% = +0.22%. Utilizing an inflation calculator helps measure exact historical and future purchasing power changes.