The hottest-selling gadget for fitness-minded people is the activity tracker, a wearable device that tells you how much you're moving.  Consumer Reports took six popular trackers for a whirl and found out some are better than others.

Joseph Jose never leaves home without  his activity tracker.

"What I found out is that I was actually  moving a lot less than I thought I was," Jose said.

An activity tracker uses sensors to gather information about the steps you take and the calories you burn.  Jose's tracker syncs to his phone or computer when he's ready to crunch the numbers.

But are activity trackers accurate?

Consumer Reports' volunteers wore them and hit the treadmill, elliptical trainer, stairs  and even picked up toys.  The counted every step. 

Turned out all of the trackers counted steps correctly.  However, some were better than others at counting calories.

"Three of the trackers show how you're doing while you're exercising, and most of the panelists said they really like that feature," said Consumer Reports' Nicole Sarrubo.

One that gives instant encouragement is the Fitbit One.  It got top ratings and costs $100.