Exercise calories

My afternoon walk : according to garmin forerunner (model without heart rate monitor): 7.85 km, 1:40:55 min,154 metres ascent and 926 calories
My afternoon walk , according to garmin vivofit with a heartrate monitor .7.68 km, 1.41 minutes (doesn't measure elevation), 247 calories.
(the 247 is the figure transferred to this site)

Gut feeling is one is too low, the other too high but how then do I decide just how much I have expended? (my BMR is only about 1250 so 926 cals is a significant amount)