Millions of people rely on wearable fitness trackers -- FitBits, Apple Watches, Microsoft Bands, PulseOn, etc. -- to measure their heart rate and track the calories they burn.

In one way that's great, because all of them will do a fine job of measuring heart rate. But if you're using a fitness tracker to track the calories you burn... the data generated isn't even close.

According to a study performed by researchers at the Stanford University School of Medicine, six out of seven devices evaluated -- Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2 -- measured heart rate with an error rate of less than 5 percent. While not perfect, that level of accuracy is just fine; after all, the difference in, say, a reported 60 beats per minute and an actual 63 beats per minute is basically irrelevant.

Yet those same devices did a terrible job of measuring energy expended. The least accurate device, the Samsung Gear S2, was off by 93 percent, and the best-performing device, the FitBit Surge, was still off by 27 percent.

Why do fitness trackers do such a poor job of tracking the calories you burn? Unlike your heart rate, which is a directly measurable number, energy expenditure requires the use of an algorithm based on a set of broad assumptions.

As one of the researchers said, "My take on this is that it's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone's fitness level, height and weight, etc." (Or in simpler terms, your fitness watch will never truly know you.)

That sucks if you're trying to lose weight, since losing weight always comes down to consuming less calories than you burn. Determining how many calories you've consumed is easy; you simply have to track everything you eat -- and be honest about the portions you consume.

Knowing how many calories you burn is tougher. You can use an exercise calculator (here's one I like) to estimate the number of calories you burn from a wide variety of exercises. That will get you close -- again, as long as you accurately estimate the effort you've expended. Just as we tend to underestimate how much we eat, we also tend to overestimate how hard we work out.

So how can you know if you're burning more calories than you consume?

Easy. Weigh yourself, on a fairly regular basis and under the same basic conditions. (I typically weigh myself just after I get out of bed.) Granted, your weight will sometimes fluctuate from day to day for what seems like no reason... but over the course of a few weeks, you will definitely know how you need to adjust your diet or exercise in order to reach your weight loss goal.

And that's why the accuracy of fitness trackers for determining calories burned isn't really a problem after all. For one thing, studies show that people who use fitness trackers tend to lose less weight than those who don't.

And even if you aren't one of those people, it's still really hard to lose weight simply by burning more calories. Most people can't or don't want to work out that hard, for that long.

It's a lot easier to cut back by say, 300 calories per day. Do that for ten days or so and you will have lost a pound. Cut back by 600 per day and it will take you less than a week to lose a pound.

And that rate of weight loss is fast enough; for most of us, the quicker we lose weight, the quicker we gain it back. (But if you're looking for a proven way to lose weight fairly fast, here's how you can lose 10 pounds in 30 days. I know it works, because I've done it.)

So forget using a fitness tracker to measure the calories you burn. They aren't accurate, and even if they were, you don't need them.

Instead, start changing your lifestyle.

After all, the proof will never be found on your wrist. The proof will be found in the way you feel... and the way you feel about yourself.