"A Seoul apartment cost 50M won in 1990 and 1B won now." You've heard a line like this. Twentyfold! Take it at face value, though, and you'll almost certainly misread it. The two figures aren't even in the same unit. Until you align inflation, FX, and the qualitative gaps (size, neighborhood, new vs. old build), 50M and 1B are numbers written in two different languages.

Strip the inflation off first

Step one is turning nominal into real. Pick a base year, drag both amounts into that year's purchasing power. The formula is almost anticlimactic.

Real amount (base year) = nominal × (base-year CPI / original-year CPI)

Statistics Korea publishes CPI; so does the U.S. BLS. But the basket differs country to country, so borrow one series for a cross-border comparison and you'll cause a wreck. More on that later.

Second, drag the currency to the same moment

When you compare foreign currencies, convert at the exchange rate of their own moment. This is where people slip most. Convert a historical dollar figure at today's rate and the past suddenly looks absurdly large, or absurdly small.

And over long runs, don't forget the exchange rate itself has drifted. USD/KRW in 1995 and USD/KRW in 2026 are just different worlds.

Third — is it actually the "same thing"?

A 1990 apartment and a 2026 apartment are not the same apartment. Insulation, soundproofing, parking, community facilities — all in another league. Same for cars, phones, medical care. A pure price comparison loses more than half its meaning the moment the "comparing the same thing" premise breaks.

Fourth — is the tax inside the number or outside it?

"Consumer price" covers different things in different countries. U.S. tags often quote pre-tax; in Europe, VAT-included is the default. Salaries, too, snarl instantly once pre-tax and post-tax get mixed.

Fifth — dress it in the era's wallet

Convert the price into a share of that era's average income and it finally gets fair.

• "50M won in 1990 / urban household average income of the time"
• "1B won in 2026 / current urban household average income"

Compare those multiples of annual income and you get a completely different picture from the raw ratio. A far more honest one.

Where everyone stumbles at least once

1. Today's FX on a historical foreign-currency amount — both sides skew at once.
2. "It went up N-fold" with no CPI adjustment — nine times out of ten, exaggerated.
3. Mixing pre-tax and post-tax — a regular at the international-salary table.
4. Ignoring that the product changed wholesale even within the same category — true of nearly anything tech has touched.

PriceGuess's Time Machine mode

Time Machine replays, hypothetically, what an amount from some past date would have grown into by now. Fees, taxes, and dividend reinvestment are simplified. The whole point is one thing: to make you feel — through your fingertips, not your head — that past returns don't guarantee future ones.

Let's thread it with actual numbers

Words don't land, so here's the logic strung through figures. A hypothetical: a record of a 20M-won appliance bought in 1995, against a 6M-won appliance in 2026, both nominal. On face value the older one looks more than three times pricier. That impression? Almost always padded.

• Inflation adjustment: assume prices roughly doubled over the 31 years, and the 1995 figure of 20M won maps to about 40M won in 2026 purchasing power. The real gap actually widens.
• Quality match: the 2026 unit is a different machine in energy efficiency, features, durability. Not the "same appliance," so the raw multiple goes limp.
• Relative to income: as a share of the average household income of the time, the older item turns out to have sat far heavier on a budget.

Run all three axes and the sentence tightens from "the past was pricier" to "the past took a larger slice of income for the same usefulness." The key is never letting a single nominal number be the conclusion.

Finally, three misconceptions

• "Real terms always shrink the number" — no. Adjustment aligns units, not direction. For some periods and items, the gap grows once you adjust.
• "One adjustment is enough" — inflation, FX, quality, and tax scope are separate axes. Fix one, mix the rest, and the result blurs right back.
• "If it's the average, it fits my case" — CPI is an average basket. The inflation you actually feel shifts with your own spending mix.

If I had to underline one thing: comparison isn't a verdict tool. It's the tidy work of setting two moments on one shared floor. And even the numbers that come out of it are approximations leaning on public records at the time of writing.