Your Fitness Tracker Has No Idea How Many Calories You’re Burning

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                It would be fantastic if Fitbits and Apple Watches were precise, however sorry-- they're not.






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Beth Skwarecki

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        <span>Beth Skwarecki</span>
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                                        Senior Citizen Health Editor
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    September 22, 2026




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Here are all the methods to discover the ideal physical fitness wearable for you, and how to maximize the information it can provide.

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    When I evaluate smartwatches and physical fitness trackers, I position a specific focus on precision. Is my running speed remedy? Does the gadget capture the ups and downs of my heart rate? I even got a VO2max laboratory test to examine a lot of watches'physical fitness ratings. One thing I <em>never ever</em> test for precision is calorie burn. None of my gadgets come close to settling on the variety of calories they believe I'm burning, and I do not anticipate them to. Even researchers who study the precision of wearables can't respond to the concern in a manner that's beneficial when you're looking for this year's gadgets-- however we'll enter into why that is below. 

There was a time, before Fitbits, when no one understood rather the number of calories they were burning daily. Sure, you might determine a rough quote based upon your body size, sex, and age. You might pick whether to think the calorie readout on the cardio devices at the health club. (You should not.) The concept that a gizmo on your wrist might inform you how numerous calories you personally burned throughout one specific day was innovative. It was likewise incorrect.

How physical fitness trackers compute calorie burn

Before I go over how precise physical fitness trackers are, let’s take a look at where they get their numbers. For calorie computations, the primary sources are movement and heart rate information. For movement, trackers utilize accelerometers to find out when your body is moving, and by just how much. If you have a watch on your wrist, and the watch swings backward and forward rhythmically while sort of bouncing up and down, your device guesses that you should be strolling. If there is quicker bouncing and your wrist makes a smaller sized motion, you’re most likely running.

This is the standard concept behind how trackers discover the number of actions you’re taking. If you’ve focused on your action count, you currently understand a few of the methods this can be incorrect. If you’re going shopping, for instance, keeping your hand on the shopping cart deal with might lead to you not getting credit for the actions you’re taking, though this depends upon the gadget. (For a best illustration of this problem, see these tests I did comparing a Garmin to an Apple Watch on a treadmill. When I rested my hands on the treadmill deal with, the Apple Watch taped 318 actions while the Garmin taped none.)

There’s the heart rate sensing unit: Since your hands do not constantly move naturally throughout workout, it can be simpler to simply inform your watch that you’ll be biking or doing yoga or whatever. The device then utilizes your heart rate to make an informed guess about just how much work your body is doing.

Whatever the source of the information– heart rate, motions, or a mix– the device processes it through a formula to compute the number of calories it believes you’re burning. Your age, weight, and sex might figure into this formula. Notably, however, the physical fitness tracker does not really understand the number of calories you’re burning; rather, it’s determining a possible number based upon insufficient details.

Why there’s no easy test for calorie burn precision

If people were robotics, all constructed the very same, all relocating foreseeable patterns, this formulaic method may work. Human beings are made complex, and innovation frequently gets puzzled. You might get various action counts if you put a gadget on your ideal versus left wristAnd the optical heart rate sensing units that a great deal of trackers utilize might be less precise on dark skin compared to lighter skinThese issues associate with the information that the trackers collect, however calorie burn isn’t a direct measurement. It’s a quote originated from doing mathematics with the information the watch procedures, and various algorithms can create various calorie burn numbers depending upon how the algorithm is created.

The business that make physical fitness trackers aren’t needed to release their algorithms or confirm that their calorie counts are precise. They can simply put a gadget on the marketplace, and there you are, comparing wearables on shopping websites with no details about how precise they are, beyond the business’ claims.

Scientists have an interest in physical fitness trackers’ precision, which would appear like an advantage. They wish to have the ability to utilize wearables in research study or suggest them for people and doctor, therefore they’ll run research studies comparing customer gadgets to laboratory devices. This seems like an excellent method to address our concerns! There’s a substantial hold-up in really getting that info, and it’s typically released too late to be helpful. By the time a scientist purchases a batch of the current design, runs a research study, composes it up, sends it to a journal, and lastly gets it released, numerous years might have passed, and the business has actually proceeded to the next design.

That hold-up is why I (normally) can’t utilize clinical research studies to weigh in on the gadgets I discuss. Here’s a fantastic example of how aggravating it can be: This evaluation was released in 2025and discovered the Series 1 was the Apple Watch that showed up the most typically in the research studies the authors had the ability to collect. The Series 9 and 10 watches were totally missing out on from the readily available information– and as a tip, we’re now as much as the Series 12. With that caution about hold-ups, I still believe it’s helpful to take a look at the research study on physical fitness trackers to see what styles emerge. Are any of them proficient at approximating your calorie burn?

What research studies state about physical fitness trackers’ precision

Time for the problem. A research study from 2020 that took a look at a range of gizmos from Apple, Garmin, Polar, and Fitbit discovered that all the gadgets are unreliable regularly than they are precise. The authors thought about a gadget to be precise if its reading was plus or minus 3% when compared to a more trustworthy procedure of energy expense (that is, calorie burn) in a laboratory setting. Here’s how a few of the leading brand names fared:

• Garmin gadgets undervalued calorie burn 69% of the time.

• Apple views overstated calorie burn 58% of the time.

• Polar gadgets overstated calorie burn 69% of the time.

• Fitbits ignored calorie burn 48% of the time and overstated 39% of the time.

The reality that Fitbits were approximately proper usually does not suggest they worked. If often your gadget overestimates and often it undervalues, it’s not extremely handy unless you understand which is which.

A 2018 evaluation particularly of Fitbits discovered that precision differed considerably depending upon elements like where they were used (upper body was more precise than wrist), whether you were strolling uphill, and whether you strolled at a consistent speed or stopped and began. The precision likewise differed by gadget, with the Fitbit Classic undervaluing calorie burn and the Fitbit Charge normally overstating. The gadgets simply aren’t precise adequate to understand the number of calories you’re actually burning

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            What do you believe up until now?

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A 2022 research study compared the Apple Watch 6, the Fitbit Sense, and the Polar Vantage V. The scientists had volunteers use all 3 devices while sitting silently, strolling, running, biking, and strength training. Every device, for every single activity, was granted a judgment of “bad precision,” with coefficients of variation varying from 15% to 30%.

This 2025 evaluation of Apple Watch research studies– the very same one I point out above– discovered that calorie burn was, typically, off by about 18%. The authors discuss a suggested requirement from the International Electrotechnical Commission that advises physical fitness trackers not be off by more than 10%. That’s plainly not being fulfilled, although the scientists kept in mind that more recent designs might be more precise than earlier ones.

And this 2026 research study looked particularly at how precision associates with complexion and to the quantity of fat an individual has. To the gadgets’ credit, complexion didn’t appear to impact calorie counts quite, however adiposity (fat material) did. The greater a volunteer’s body fat portion was, the more unreliable their calorie burn outcome. The Garmin Forerunner 955 and Samsung Galaxy Watch 5 seriously overstated calories; the Apple Watch Series 8 likewise overstated however not as much; and the Fitbit Sense 2 undervalued when it wasn’t producing wild outliers. The authors concluded that all gadgets “substantially misestimate” calorie burn throughout workout.

To get a sense of these portions, let’s state your real calorie burn is 2,000 calories daily. A gadget that’s off by 15%– on the low end of what these research studies discovered– may report that you burned 1,700 calories, or that you burned 2,300. If you’re utilizing your gadget to find out just how much to consume, you might be method off in fulfilling your calorie objectives.

If these gadgets are all incorrect, how can you understand the number of calories you’re burning?

It’s most likely most beneficial if you consider your calorie burn as a number you can not determine straight. Treat it as a black box: I burn some unknowable number of calories; now what?

The only typical factor you would require a precise price quote of calorie burn is if you are attempting to find out just how much food you require to consume. If you wish to reduce weight, you wish to consume less than you burn; if you wish to put on weight, you desire the reverse; and if you’re attempting to keep your weight, you wish to consume approximately the like what you burn. Believe about it this method: you do not really require to understand your calorie burn if you have the other 2 terms in the formula– your calorie consumption and your weight.

It’s thought about more precise to change just how much to consume based straight on your weight, instead of utilizing calorie burn quotes as an intermediary. Let’s state you’re training for a marathon and you wish to ensure you sustain yourself properly. Well, if you’re under-eating, you’ll begin to reduce weight. When you begin to see the scale trending downward, that’s your signal to include a couple of hundred calories to your diet plan. If, after that modification, your weight remains stable, then you understand you’re consuming the correct amount. As you increase your training (or if you require time off to rest a sprained ankle), you can make more changes as you go.

I have a post here detailing how to make these modifications with the assistance of either a paid app, a group of complimentary apps, or a DIY spreadsheet. If you’ve been utilizing a physical fitness tracker rather, and it’s working for you, do not hesitate to keep utilizing it. If the tracker ever stops offering you the outcomes you desire, you can securely leave it out of the formula.

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            Beth Skwarecki
                                Elder Health Editor
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                                        Covering health, physical fitness tech, home fitness center devices, and more.
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