Atrial flutter on an EKG does not have a p wave, so how does one calculate the atrial rate?

Actually, it does. The P wave reflects activation of the atria (upper chambers), but in atrial flutter, the atria activity is reflected in a "sawtooth" pattern ripping along at about 300 beats per minute, which also represents the atrial activation but from another source and conduction pattern. The important clinical item is the ventricular rate, dependent on the av node which is the controlling "relay station".
F waves. While you are correct that atrial flutter does not have a 'p' wave on ecg, there are 'f' (flutter) waves on the ecg. Counting the milliseconds between two consecutive 'f' waves gives you the atrial rate.