Fiat currencies are measured against one another to determine which are rising and which falling. This is only a relative measure of strength. In reality, they are all falling but at different rates.
As expressed by Charles Vollum at Casey Research:
Fiat currencies the world over are being manipulated by central banks, which is distorting asset and commodity prices. Successful investing requires that investors have a good idea of what things cost and what they are really worth – and using the world’s oldest and most stable form of money, gold, to compare prices is one way to get that insight.
As an example, here are four currencies and how they have performed recently versus gold:
To see how the costs of various other items have fared in terms of gold, see Mr. Vollum’s article.