Formula Used:
For Male,
    Calorie Burned = ((-95.7735 + (0.634 × HR) + (0.404 × VO₂ max) + (0.394 × W) + (0.271 × A)) / 4.184) × 60 × T
For Female,
    Calorie Burned = ((-59.3954 + (0.45 × HR) + (0.380 × VO₂ max) + (0.103 × W) + (0.274 × A)) / 4.184) × 60 × T

Where,
   HR = Heart Rate
   W = Weight in kilograms
   A = Age
   VO₂ max = Maximal oxygen consumption (in mL kg^-1 min^-1)
   T = Exercise duration time in hours

Heart rate based calories burnt calculation is made easier with this online calories calculator.