Not sure how much food to give my dog.

  
Perry

1184087
 
 
Barked: Wed Feb 22, '12 4:36pm PST 
How much food should I be feeding my shepard beagle mix? Hes about 50lbs+ I give him 2 1/2 cups a day of purina one but hes really packed it on this winter. Am I giving him to much?