If one's monetary income stays the same, but the price level increases, the purchasing power of that income falls. Inflation does not always imply falling purchasing power of one's money income since it may rise faster than the price level. A higher real income means a higher purchasing power since real income refers to the income adjusted for inflation.

For a price index, its value in the base year is usually normalized to a value of 100. The purchasing power of a unit of currency, say a dollar, in a given year, expressed in dollars of the base year, is 100/P, where P is the price index in that year. So, by definition the purchasing power of a dollar decreases as the price level rises. The purchasing power in today's money of an amount C of money, t years into the future, can be computed with the formula for the present value: where in this case i is an assumed future annual inflation rate.

Adam Smith used an hour's labour as the purchasing power unit, so value would be measured in hours of labour required to produce a given quantity (or to produce some other good worth an amount sufficient to purchase the same).