The metal content of a US Dime (for example), is based on year. Pre-1965 US coins are about 90% silver. It's fairly easy to convert their value into Fed Notes, based on the coinage, quantity, year, and current spot silver price, at a point of sale or transaction.
Yes, by silver weight. A pre-1965 dime is of far more value than a 2021 dime because of its metallic composition.