Guide · 6 min read
Inflation Works on Both Ends of the Dollar
Every inflation number is two numbers: what the expense becomes and what the money becomes. They are the same loss seen from opposite ends.
At 3% inflation, a $1,000 expense costs $1,344 in ten years — and $1,000 held in cash buys what $744 buys today. The same 3%, applied in both directions.
A $1,000 expense at 3% inflation costs $1,344 in ten years. The same $1,000 held in cash for those ten years has the purchasing power of $744 today. Both figures come from the identical compounding — applied forward to prices, backward to money.
Reading them together prevents the most common planning error: adjusting the future budget for inflation while also discounting today's savings for inflation, which counts the same loss twice.
The halving time
The rule of 72 works on inflation as well as on investments: divide 72 by the rate. At 3%, prices double roughly every 24 years and purchasing power halves on the same schedule. At 6% the schedule compresses to twelve years — the difference between an unnoticeable drift and a decade that reorganizes household finances.
The doubling is worth feeling concretely: a $3,000 monthly budget at 6% inflation becomes a $6,000 budget in twelve years, with nothing else having changed about the life it funds.
What a fixed rate is really earning
Subtract inflation from any advertised rate and you have the real return — the only one that buys tomorrow's prices. A 4.5% CD at 3% inflation grows purchasing power by about 1.5% a year; at 6% inflation the same CD loses ground while paying interest on schedule.
This is the quiet function of every savings decision: not the rate on the statement, but the rate minus the thing the money will have to buy.
Frequently asked questions
What inflation rate should I plan with?
The US long-run average of about 3% is the standard anchor for long horizons. Test the plan at 5–6% too: a plan that survives the high case at a worse outcome is the only kind worth having.
Why do future cost and purchasing power differ?
They are the same loss from opposite ends — the expense grows by the factor, the cash shrinks by it. Averaging or adding them double-counts; each reads against its own baseline.
Does inflation itself get taxed?
No — but the interest earned while beating it is, which is why the real, after-tax return is smaller than the rate gap suggests. A 4.5% CD in a 25% bracket nets about 3.4% before inflation: barely the historical average.