RPI Calculator. Retail Price Index, abbreviated as RPI is the measure of the inflation that is published monthly by the Office of National Statistics. It is the percentage change in the price of the sample products of the retail goods and services. RPI was once, the principal official measurement of inflation. Consumer Price Index (CPI) is a statistic used to measure average price of a basket of commonly-used goods and services in a period relative to some base period. The base period price of the basket is marked to 100 and CPI value hovers above or below 100 to reflect whether the average price has increased or decreased over the period.