Rails: Is there a rails trick to adding commas to large numbers?

You want the number_with_delimiter method. For example:

<%= number_with_delimiter(@number, :delimiter => ',') %>

Alternatively, you can use the number_with_precision method to ensure that the number is always displayed with two decimal places of precision:

<%= number_with_precision(@number, :precision => 2, :delimiter => ',') %>

For anyone not using rails:

number.to_s.reverse.gsub(/(\d{3})(?=\d)/, '\\1,').reverse