You use your fitness tracker for everything from counting steps to measuring your heart rate to judging exactly how many calories you torched during that high-intensity spin class. But at the end of the day, just how much can you trust the numbers you’re seeing on your wrist?

According to a recent Stanford study, it depends on what metric you’re looking at. When compared to heart rates measured by electro-cardiogram (or EKG), fitness trackers did a surprisingly great job accurately tracking your heart rate, with fitness trackers being “off” by only about 5% on average. That’s a pretty small amount, or what amounts to plus/minus three beats per minute if your heart rate is 60 bpm.

Tracking calories burned, on the other hand, was a much more error-prone story. The study found that the degree of inaccuracy ranged from 20% all the way up to 93% – meaning that the worst fitness trackers were wrong about calories burned a whopping 93% of the time when compared to a complex calculation of metabolism.

When confronted about the correctness (or incorrectness, as it turns out) of my fitness tracker, my immediate reaction was, “Eh, who cares?” By no means had I assumed that my fitness tracker was 100% correct in every bit of data it feeds me (who hasn’t experienced the pleasant surprised of hitting your ten thousand steps while bending over to tie your shoes). I figured my calories burned were an estimation, giving me a rough look at how hard I worked in a given hour as well as something I could compare over time to see how I improved in a certain physical activity.

But here’s the important point: I’m not using my Fitbit to track calories in and out for weight loss purposes. And for people who do try to keep their diet under a certain caloric threshold, a 93% inaccuracy rate makes it near-impossible to plan your meals. In fact, it could even sabotage you into thinking you burned more calories than you actually did.

If you regularly use your fitness tracker to analyze your calories burned, there are two takeaways to remember:

1. Calorie burning (and for that matter, most of the data your Fitbit or Apple Watch tracks) is merely an estimation. Don’t treat it as the end-all, be-all way to measure your fitness; a holistic approach that factors in your physical activity, nutrition and personal goals (can you do a pull-up now that you couldn’t do two months ago) will be a much better measurement of your big-picture fitness.
2. No matter how many calories you’re burning, a healthy diet is key to any fitness goals. As the saying goes, “you can’t out-exercise a bad diet.” More fruits, veggies and whole grains; less sugars, saturated fats and white carbs.

Do you think your fitness tracker is accurate? How much do you rely on your fitness tracker as a measure of your overall fitness?