Resistance Calories?

Having recently started at my local Gym I am rotating between one session of Cardio, next session Resistance etc etc. The Cardio machines tell me how many calories I have burned so I can eat the correct amount for that day to lose weight. I am aiming for a 14-21lb loss to reach a goal weight of 8 and half stone or there about. However, how do I ACCURATELY work out calories burned from resistance when everything I have been told and read indicates that I will continue to burn for a few hours after the resistance work and checking on the Strength Training part on Cardio Exercise setting on this app says I burned no more than 130 calories for 45 minutes of serious resistance work. Surely I have burned more than that???