Historical Currency Converter

For US dollars, yes: it uses the Bureau of Labor Statistics CPI-U annual average for every year from 1913 through the latest full year, so a conversion reflects the actual price level in both years rather than a smoothed average. British pounds and euros still use one constant average rate (about 3.8% for GBP and 2.5% for EUR), so those results are long-run approximations.

USD data starts in 1913, GBP's flat rate is treated as reliable from 1900, and EUR starts in 1999 (the euro's founding year). Before those dates the calculator extrapolates a 20th-century rate backward, and US and UK prices were roughly flat or falling through most of the 1800s, so that can overstate real historical inflation by a large factor. Reliability Note appears whenever Original Year or Target Year falls before that starting year. For USD it also appears when a year is later than the latest published CPI-U annual average, because those years are projected at 3.1% a year until the official figure is out.

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.