Historical Currency Converter

No -- it applies one constant average annual rate per currency (about 3.1% for USD, 3.8% for GBP, 2.5% for EUR) compounded across the full span of years, rather than looking up each individual year's real inflation rate. This smooths over real-world volatility like the high-inflation 1970s, so results are a long-run approximation rather than a precise historical-price-index figure -- and only within the era each rate is actually calibrated for (see the next question).

Each currency's flat rate is calibrated for a specific era: USD from 1913, GBP from 1900, and EUR from 1999 (the euro's actual founding year). Before those dates, prices did not move at anything close to the flat rate used here -- US and UK prices were roughly flat or falling through most of the 1800s, so compounding a 20th-century rate backward into that period overstates real historical inflation by a large factor (a flat $100 in 1800 compounded to 2024 at 3.1%/year overstates the real change by roughly 40x). Reliability Note appears whenever Original Year or Target Year falls before the selected currency's calibrated starting year, so you know to treat that particular result with real caution.

The calculator clamps the year to 1999 instead of extrapolating, and Reliability Note explains that the adjustment happened. The euro didn't exist before 1999 (it replaced national currencies like the French franc and German mark), so there is no meaningful "EUR value" for an earlier year to compute in the first place -- unlike USD or GBP, which existed continuously and simply have a less reliable rate before their calibrated era.

The calculator computes a genuinely reversed result: rather than projecting a sum forward in time, it discounts it backward, showing what a modern amount would have been worth in an earlier year. Inflation Multiplier and Purchasing Power Change both flip to describe that backward direction too -- Inflation Multiplier drops below 1x and Purchasing Power Change turns positive, since money buys progressively more the further back in time you go.

Yes, exactly. Modern Equivalent is Original Amount multiplied by the inflation multiplier for the chosen years and currency, so doubling Original Amount always exactly doubles Modern Equivalent, and the Inflation Multiplier and Avg Annual Inflation figures stay completely unaffected by how large or small the original sum was.