How Inflation Erodes Your Savings: The Rule of 72 and Real Returns

The Invisible Withdrawal: Inflation as a Tax on Cash

The balance in your savings account never shrinks on its own — but what it can buy does. The SEC's investor education site, investor.gov, defines inflation as a rise in the prices of goods and services over time and reminds investors to account for it when judging any savings or investment outcome. Historically, US inflation from 1990 to 2024 annualized at about 2.6% — modest-sounding, yet enough to cut the purchasing power of untouched cash by more than half over those 34 years.

The Rule of 72: Mental Math for Halving Purchasing Power

The Rule of 72 is a classic shortcut: divide 72 by an annual growth rate to estimate the years needed for a doubling. It works just as well in reverse, for purchasing power decay:

• 2% inflation → 72 ÷ 2 = prices double in about 36 years; cash loses half its purchasing power.
• 3% inflation → 72 ÷ 3 = about 24 years to halve.
• 6% inflation → 72 ÷ 6 = only about 12 years.

Flip it around and the same rule estimates compound growth: an asset earning 6% a year doubles in roughly 12 years. That symmetry is the whole story of money that works versus money that sits.

Nominal vs Real Returns

The return printed on your statement is the nominal return; what is left after subtracting inflation — the actual growth in purchasing power — is the real return. A quick estimate is simple subtraction: 5% nominal minus 3% inflation is roughly 2% real. The precise formula is (1 + nominal) ÷ (1 + inflation) − 1. Two traps worth knowing:

• A savings account paying 1.5% during 2.5% inflation is gaining on paper and losing in reality — about 1% of purchasing power per year.
• Returns from different eras cannot be compared at face value: double-digit deposit rates in the early 1980s coexisted with double-digit US inflation, so the real return was far less impressive than it looks.

Building Inflation into Your Savings Plan

Two habits make long-term planning more honest. First, state every goal in real purchasing power — "$1 million in 20 years" is not today's $1 million. Second, ask of every savings vehicle: what is left after inflation? The fastest way to build intuition is to look at history: open the inflation calculator, enter your birth year and this year, and see what happened to a fixed sum of money — then check it against the Rule of 72.