October 27, 2026 · Home Finance · 5 min read

How Inflation Eats Your Money: Future Costs and Purchasing Power

Inflation is the quietest expense there is. Nothing appears on a statement, no fee is charged — and yet a fixed pile of money buys less every year. Understanding the math behind it takes one formula and one habit: always asking "in today's money?"

One formula, two directions

Inflation compounds: each year's rise applies to a price that has already risen. With an average annual rate r over n years and an amount A:

Future cost = A × (1 + r)^n Purchasing power = A ÷ (1 + r)^n

Read it forwards and a 100 grocery bill at 3% inflation costs 134.39 in ten years. Read it backwards — divide instead of multiply — and 100 of future cash buys what 74.41 buys today. Same formula, opposite sides of the coin. Our Inflation Calculator shows both at once.

Why 3% for 10 years is not 30%

The intuitive answer multiplies rate by time: 3 × 10 = 30%. Compounding says otherwise: 1.03^10 ≈ 1.344, a 34.4% rise. The gap widens with both rate and time. At 5% for 20 years the naive answer is 100%; the true rise is about 165%. If you remember nothing else, remember that inflation grows like a snowball, not like a ruler.

The rule of 72

For mental math, divide 72 by the annual rate to get the doubling time. At 3%, prices double roughly every 72 ÷ 3 = 24 years; at 6%, every 12; at 12%, every 6. It works in reverse too — at 3% inflation, the purchasing power of idle cash halves in about 24 years. The rule is approximate but remarkably good for rates between 2% and 15%.

Planning in real terms

Salaries: a flat salary loses value even if the number never changes. At 4% inflation, holding pay still for five years is a real cut of about 18%.
Savings: what matters is not the interest rate but the gap above inflation. 2% interest with 3% inflation is a real loss of about 1% a year.
Long-term goals: price a future goal in future money, or discount a future pot into today's money — either way, use one consistent rate.
Fixed pensions: a payment fixed at today's level loses purchasing power at (1 + r)^n each year it isn't indexed.

Which rate should you use?

Headline CPI is the honest default, but your personal inflation depends on your basket: rent-heavy budgets often inflate faster than the average, tech-heavy ones slower. For long plans, a long-run average (historically around 2–4% in most developed economies) beats reacting to last year's spike — just rerun the numbers when the environment genuinely changes.

Tip: the free Inflation Calculator projects future costs and purchasing power for any amount, rate and horizon — and doubles as a "what did it cost back then?" tool, entirely in your browser.

Once you know what money will be worth, make sure you know how much of your salary you actually keep. See Take-Home Pay: From Gross Salary to What Actually Lands in the Bank.