The Netherlands: the cleanest long series in this calculator
Most of the long runs here are stitched from indices that measured different things. The Dutch one is not. It is genuine consumer prices from 1900 to today, published by CBS itself, with no proxy segment and no wholesale or retail substitute along the way.
The start date has a plain explanation: CBS was founded in 1899, so 1900 is simply the first full year it existed to measure anything. Nothing statistical happens at that boundary.
Before it, the honest answer is that the numbers get much weaker, and the specialists agree. The International Institute of Social History withdrew its own guilder converter on the grounds that it "presented an oversimplified picture that does not do justice to the fragmentary nature of historical data", and now points people to CBS's series instead. If a nineteenth-century Dutch conversion matters to you, it needs a historian rather than a calculator.
The war years measure the wrong prices
One caveat sits inside the series rather than before it. The index numbers for 1940 to 1945 record controlled, rationed prices, which is what an official index is able to collect under occupation. They do not reflect the black market, and for the Hongerwinter of 1944-45 the gap between the two was enormous.
A conversion spanning those years is arithmetically fine and historically misleading. The guilder itself was then invalidated wholesale in the currency purge of 26 September 1945, when every Dutch note ceased to be money and each person received ten guilders to live on.
Two Dutch indices, and they can point opposite ways
CBS publishes the CPI used here and a derived CPI that strips out changes in VAT, excise and subsidies, answering what prices would have done had tax rates held still. It is the one commonly written into wage agreements, and the difference is not small in either direction.
In 2019 the CPI rose 2.6% against the derived index's 1.6%, because VAT went from 6% to 9%. In 2022 the relationship inverted: CPI 10.0% against derived 11.8%, because temporary cuts to energy VAT and fuel excise flattered the headline. That 10.0% remains the official figure and is the highest since 1975, when this series gives 10.23%, so 1975 keeps the record, narrowly.