need some help please.

recently bought a F6 polar heart rate monitor. After running i have checked the amount of calories burned but when i compare this to mapmyrun.com the reulsts are very different. For example a half hour run about 2 miles showed 400 calories burned on my HRM but acording to maymyrun.com it was 220 calories.

Can some one let me know why this is?

many thanks

You may need to calibrate your monitor.