Britain: the series reaches 1661, and for most of it prices went nowhere
The Bank of England's spliced historical series carries British consumer prices back to 1661, and this page will convert from any point in it. What surprises people is how little happened early. Prices in 1900 were about 4.4% lower than they had been in 1661: 239 years averaging fractionally below zero. From 1900 to 2025 they rose at 3.92% a year.
Two consequences follow. A sum from the seventeenth century converts to far less than most people expect, and two amounts a century apart in the 1700s convert to almost the same figure. British inflation is very nearly a twentieth-century invention.
Accuracy falls away with distance. The Bank calls its own long series a rough guide, and the Office for National Statistics treats its pre-1988 CPI estimates as indicative rather than National Statistics. A conversion from 1750 is an order of magnitude, not a number to quote in an argument.
Decimalisation did not change the pound
On 15 February 1971 the pound was subdivided differently: 240 old pence became 100 new pence. The pound itself was untouched. Unlike the euro changeovers elsewhere in this calculator there is no conversion factor to apply, and the index runs straight through the date.
The trap is notation rather than arithmetic. "Three and six" means three shillings and sixpence, which is £0.175 and not £3.60. Convert old money to decimal pounds before you type it in, or the answer will be out by a factor of twenty.
The famous 1975 figure is not the one this page uses
British inflation in 1975 is remembered as 24.2%, with a monthly peak of 26.9% that August. Both of those are RPI. This page is built on the CPI basis, which puts 1975 at 22.7%.
Neither is wrong. The RPI includes mortgage interest, covers a different population and uses a formula the ONS has criticised for years; it lost its National Statistic designation in 2013, and from February 2030 its methodology aligns with CPIH. But almost all British inflation history was reported on RPI at the time, so a figure you remember from a newspaper, a pay negotiation or an index-linked contract will usually be the RPI one. Check which basis you are comparing before concluding this page is wrong.