Or if you're looking for a core extension that adds this to the Array class, I'd recommend the facets gem (require 'facets/array/average'). Then you can just do array.average. And, from looking at the source, it turns out they do the exact same thing as the instance_eval approach above. The only difference is that it's implemented as a method—which of course already has self pointing to itself—instead of a block): def average; return nil if empty?; reduce(:+) / length.to_f; end Main advantage of this is that it's even more concise/readable and it handles the empty? case.