Purchasing Power Calculator — Money's Value Over Time

The Long-Term Cost of Inflation

Over 30 years at 3% average inflation, 100,000 keeps the buying power of 41,199 in today's terms. That is the time value of money working in reverse: instead of asking what a sum grows into, it asks what a future sum is worth now. Both questions use the same discount factor, and only the direction differs.

What People Often Overlook

The balance in an account is a nominal figure, and nominal figures are the ones that feel real. What that balance buys is a separate quantity, and it moves even when the balance does not. A rate of 2 or 3 per cent barely registers in any single year, which is precisely how it accumulates: nothing about it is alarming until a decade has passed. Any goal measured in years rather than months runs into this, whether it is retirement, a property purchase, or simply keeping a standard of living where it is.

How This Calculator Can Help

Running the same amount at two or three different rates is more informative than picking one, because the spread widens with time in a way percentages alone do not convey. Over 30 years the difference between 2 and 4 per cent is not double: it is 55,207.09 against 30,831.87, a gap of 24,375.22 on the same starting sum. The outputs here are illustrative estimates from the inputs given, not forecasts.

Run it with sensible defaults

A starting amount of 100,000, an annual rate of 3 per cent and a 30-year horizon give 41,198.68. The rows beneath fill in the rest: 58,801.32 of buying power lost, 242,726.25 needed at the end to match 100,000 at the start, and cumulative inflation of 142.73 per cent.

The levers in this calculation

Starting Amount is a straight multiplier, so a 1 per cent change moves the result by exactly 1 per cent. Annual Inflation Rate pushes the other way and slightly less hard, a 1 per cent relative change costing about 0.87 per cent of the result at these settings. Years is the one that compounds: each decade at 3 per cent removes about a quarter of whatever buying power was left at the start of it, so the absolute losses shrink as they go while the rate of erosion never changes.

How the math works

The amount is divided by one plus the annual rate, raised to the power of the years. At 3 per cent over 30 years that divisor is 2.4273, which is how 100,000 becomes 41,198.68. The model holds the rate constant across every year and applies it uniformly to everything bought.

Choosing an inflation assumption

No single figure is correct here. Inflation is measured after the fact and varies by country, by decade, and by what a particular household actually buys, so any forward assumption is a choice rather than a reading. What the calculator can show is how much the choice matters: where two plausible rates give answers close together the assumption is not load-bearing, and where they diverge sharply the plan depends on something nobody can supply.