My hp48 calculator had unit conversions built-in. You could add units to any number and then conversions are done automatically when you do math:
(1_ft + 1_in) / 20_s + 5_mi/h
if the units don't match then the operation was an error: 1_ft + 1_gal => error
As other people have mentioned I really wish spreadsheets had this built-in where units are like formats for a cell. Ii controls about how you want things displayed and will prevent you from making conversion mistakes or doing illegal operations.