There are many different ways to calculate the "strength" of a currency, for example:
If you want to analyze the EUR / USD pair you can calculate the "strength" of the EUR in relation to the other 8 major currencies (or all 28), for
example EUR / JPY in the timeframe you took into consideration (for example the daily ) rose 2.1% (+2,1%) compared to the opening price (green
candle on daily), EUR / CAD fell 1.2% today so EUR lost 1.2% (- 1.2%) .... Continue with the others, remembering that the first currency of the
pair gains if the price goes up (example EUR / CAD, EUR), the second currency gains if the price goes down (CAD). In the end, sum up the results: +
2.1% -1.2% .... = (for example) 13.5% (that is the "strength" of the EUR today). Do the same thing with the USD and then compare them for example
if the "strength" of the USD is 5.6% then it means that the EUR today is stronger than the USD ("buy" on EUR / USD, of course with other
confirmations). This is just one example of the calculation that can be done to calculate "strength", there are many different ones.