Japan: the one country here where the arrow can point the other way
Every other country on this site turns a past sum into a larger present one. Japan does not always. Consumer prices fell by a cumulative 4% between 1998 and 2012, a mild decline but the most persistent in the post-war record, and across that stretch this calculator returns less than you put in.
That is not a fault in the data. It is the period Japan spent in deflation after the asset bubble of the late 1980s collapsed, and it is why advice built on the assumption that money always loses value travels badly here. Prices have since turned: the 2025 all-items index stood at 111.9 against 2020, up 3.2% on the year.
The yen has never been redenominated, despite what 1946 looks like
Japan's index begins in August 1946, and people who know that a currency event happened around then often assume the yen lost zeros the way the Finnish markka or the German mark did. It did not.
Under the emergency financial measures of early 1946, deposits were frozen and old notes withdrawn, with certificate paper affixed to banknotes as a stand-in until new series notes appeared between 1946 and 1948. The exchange was at face value, one for one. It was a confiscatory deposit freeze, not a cutting of zeros, so a yen figure from 1950 is in the same unit as a yen figure from today and needs no conversion before you enter it.
Two caveats on the early years: nothing usable crosses 1945, because the wartime price-control system, the basket and the price sample all broke at once; and values before 1970 exclude imputed rent and rest on a narrower index than the modern one.
"Core" means something different in Japan
If you compare this page against a Japanese inflation headline, check the definition. Japan's core index excludes fresh food only and still contains energy. The measure that matches the American convention, stripping both food and energy, is called core-core.
The index used here is all items, which is also the measure named in the Bank of Japan's formal 2% price stability target, set in January 2013. The Bank's forecasts are usually quoted on the fresh-food-excluded measure, which is the source of most of the confusion.