I haven't been using the HR strap on my Garmin and it still gives me calories, so I don't think it requires HR data. I think it factors elevation gain, weight, speed, etc. in to the equation. I think the Garmin data may run a bit high. I know there've been discussions about the Polars running high on calories. I will say the Garmin data I get is lower than plugging the info into some calorie counting sites.