You need a calibrated microphone, along with appropriate software.
Room EQ Wizard will work with any calibrated microphone like this one. Youc an also buy OmniMic from Parts Express which is a mic/software bundle.
Most use REW and a Umik-1. Be aware that Umik-1 is $50 cheaper if you buy from miniDSP instead of Amazon! Apparently Amazon has it on Prime delivery:
https://www.minidsp.com/products/acoustic-measurement/umik-1