Update: The first version of our data-driven hop substitute tool is here!
There are well over five hundred hop varieties available for use in brewing beer. If a beer recipe calls for a particular variety of hop that it is unavailable, there is a good chance that a substitution can be made without much of a difference.
Hop substitutions need to be chosen carefully. The alpha and beta acid contents should be close to ensure the bitterness and stability is similar. Hops also have different essential oil percentages, which will change the flavor and aromas. These oils should also be close when swapping one hop for another, but these are only essential to keep aromas similar.