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Fitness apps 6 iPhone Apps You'll Want to Use With HealthKit 6 iPhone Apps You'll Want to Use With HealthKit With the myriad of fitness apps and gadgets available, it can be a pain to keep track of health information that's scattered all over the place, unless you use HealthKit. Read More and devices can do a lot of different things. One of the most important functions that people use them for is to make sure they’re getting the right number of calories in their diet. By subtracting the number of calories you eat from the number of calories you burn in a day, you come up with your calorie surplus or deficit, which, over time, can lead to weight loss or gain.

But how do those apps figure out how many calories you’re consuming and burning? How accurate are they? How do they know how many calories you should be eating each day?

The answers to these questions can be very helpful in understanding your fitness.

Calculating Calories In

This part of the equation is fairly simple. All packaged foods come with a nutrition label 7 Websites That Give You Nutritional Information To Eat Smarter 7 Websites That Give You Nutritional Information To Eat Smarter Nutritional information is crucial for understanding diets especially if you afflicted with lifestyle diseases like diabetes, hypertension, obesity etc. Nutritional guides help us to choose healthy foods. So, let these seven online nutritional guides help... Read More that contains relevant information; the number of total calories; the amount of fat, protein, and carbohydrate; fiber content; and so on. Much of this data is stored online, so when you run a search or scan a barcode, the app hits a database to find that information. It then pulls it into your food log and adds the number of calories to your intake.


While many fresh foods don’t come with nutrition labels, information on caloric and nutrient content has been measured and is also available online, so that can be pulled into the app, too.


Have you ever wondered how companies figure out how many calories their food products have?

Because calories are a measurement of energy (the amount of energy available to your body after digesting them), the old-fashioned way was to burn the food in a device called a bomb calorimeter and see how much energy was released.

Starting in 1990, however, a different system was put into place. The Atwater system, as it’s known, is used to estimate the number of calories in a particular sample of food by multiplying the amount of macronutrients (carbohydrates, proteins, and fats) by known constants.

Every gram of carbohydrate in food, for example, is worth four calories. Protein is also worth four per gram. Fats have nine calories per gram, and alcohol has seven. Because fiber isn’t very digestible, it’s counted as zero calories per gram. If you know how much of each macronutrient is present in a particular food, you can estimate pretty well how many calories that food had. Although the Atwater system is only capable of estimating the number of calories in a particular food, it’s generally considered to be accurate enough.

How do you figure out how much of a macronutrient is in a food?

There are chemical processes that allow scientists to measure each of them. They’re a bit complicated, but you can read more about them in this fantastic explanation on Reddit. In short, food is weighed, chemicals are added that eliminate a particular macronutrient, and the food is weighed again. The difference in weight indicates how much of that nutrient was present.

Calculating Calories Out

Okay, so food is analyzed, nutrients are multiplied, and labels are imported into your app to figure out how many calories you’re eating in a day. But what about how many calories you’re burning?

To understand this, you need to know about metabolic equivalents, or METs, which measure the number of calories burned compared to your resting metabolic rate (RMR), or the number of calories you burn while you’re just sitting down, doing nothing.

If you’re doing an activity that requires three METs, you’re burning three times as many calories per minute as you would by just sitting. There are published tables that assign MET values to different activities — for example, walking at 2.5 MPH requires about 2.9 METs. Riding a bike at under 10 MPH is close to 4 METs. And so on. The scale goes up to 23, which you’ll hit if you’re running a 4:17 mile.exercise-mets-tableVarious MET tables include a wide range of things, from housework to taking classes to real estate services to bowling. If you want to find the metabolic equivalent for a specific activity, you just need to run a search online.

So, we now know that every activity has a MET value, which serves as a multiplier for your resting metabolic rate, which is the number of calories you burn at rest. When you enter an activity into your fitness app, it multiplies the MET value by your RMR. Pretty simple. But how does it know your RMR?

As you might have guessed, it uses another estimation equation. Finding your exact resting metabolic rate is possible, but it requires a trained professional and some time. By estimating using a well-researched equation, your fitness app will get decently close. The equation takes into account your height, weight, and age, and gives you a rough estimate of the number of calories you burn just to stay alive.


Now, whenever you tell the app or device that you completed a certain activity, it multiplies that number by the MET value, and you have an estimation of your calorie burn. Because these calculations involve a number of estimates, there’s necessarily going to be some level of error. Your resting metabolic rate could be higher or lower than the estimate. You could be more or less efficient in completing an activity. And so on.

These equations are generally considered to be as accurate as possible without providing further information, like heart rate, power output, and other more complicated measurements, but there’s a definite possibility that the estimates could be off. Averaging out over time, however, they’ll get you the numbers you need.

Calculating Calories You Need Each Day

The final calculation your fitness device 17 Best Health and Fitness Gadgets to Improve Your Body 17 Best Health and Fitness Gadgets to Improve Your Body Over the past few years, innovation around health and fitness gadgets has exploded. Here are just a few of the amazing pieces of kit you'll be able to use to keep you feeling great. Read More will make is the number of calories you need to consume each day to meet your weight goal, whether that’s maintaining, losing, or gaining. All of these calculations start with an estimation of the number of calories you burn each day, which — like exercise — is calculated as a function of your resting metabolic rate. There are a number of different ways to estimate this, and it’s likely that different apps use different methods.

If your fitness app asked you what your general rate of activity is (the options are usually something like “sedentary,” “lightly active,” “moderately active,” and “highly active”), your daily calorie expenditure is probably what it’s trying to figure out.

As I mentioned, there are a number of different ways to estimate how many calories you need, but your general activity combined with your weight will usually give you that number.


To maintain your weight, all you need to do is eat that many calories per day, adjusting for exercise. You’ll burn the same number as you consume, and you’ll stay (approximately) the same weight.

To lose weight, you’ll need to consume fewer calories than you burn so you establish a deficit. How many fewer depends on how fast you want to lose weight — this is another question your app or device has probably asked you.

If your goal is to lose a pound a week, you’ll need to establish about a 500-calorie deficit each day (there are approximately 3500 calories in a pound of body fat). If you want to lose half a pound, you’ll aim for a more modest 250-calorie deficit.

How You Can Use It All to Control Your Weight

By combining these three calculations, your fitness app or device will give you a really good idea of the number of calories in the food you eat, the number of calories you burn through exercise, and the number of calories you’ll need to consume to meet your weight-loss goal.

In the days before fitness apps and FitBits, these calculations would be done by hand — fortunately, things are a lot easier now (though using IIFYM and Excel is still a great way to come up with healthy meal plans How to Build Perfect Meals with The IIFYM Calculator and Excel How to Build Perfect Meals with The IIFYM Calculator and Excel You can easily design meals that meet your nutrition intake targets. Achieve your fitness goals by using the free IIFYM Calculator and an Excel spreadsheet for healthy meals every day. Read More ). You don’t have to do these calculations yourself, but it’s still good to understand what information they’re taking into account and how it’s used.

Even if you don’t totally understand it, though, you can take advantage of all the science that’s gone into these calculators. Use your FitBit and IFTTT Take Your Exercise Routine to the Next Level With Fitbit and IFTTT Take Your Exercise Routine to the Next Level With Fitbit and IFTTT Here, we'll dig deeper into the many ways that Fitbit and IFTTT can work together to make your life better. Read More together to take your fitness tracking to the next level, get the right number of calories in your meals with these online meal planners Plan Weekly Meals in Under 1 Hour with 5 Web Tools Plan Weekly Meals in Under 1 Hour with 5 Web Tools There is a way to eat healthy food that doesn't cost much and do so in a way that doesn't require too much effort on your part. That secret is weekly meal plans. Read More , and put it all together with a fitness tracking app (or not Why Mobile Weight Loss Apps Suck More Than a Vacuum Why Mobile Weight Loss Apps Suck More Than a Vacuum Apps for working out are all over the place, but do they actually work? Read More ).

Do you count calories or keep track of your exercise? Did you know how your app or fitness device calculated calories? Or do you just make an effort to be disciplined, eat less, and exercise more? Share your thoughts in the comments below!

Image Credits:using calorie counter by Dragon Images via Shutterstock

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  1. Mansa
    June 22, 2017 at 4:08 am

    Hello, i am trying to build a fitness app as described above for my final year project, so was reading about for formula used to calculate calories loss. One factor that seem to be lost in time spend on the task. while some MET values have time factors some dont, and those with it have timeline with no know time factor. Can you help me with this problem please or can you point me a direction where i can get help. Thanks.

  2. Amanda Houck
    March 6, 2017 at 6:38 pm

    I am using all three in my quest to become thinner. I use MyFitnessPal to calculate my calories in. Initially, it calculated my target calories at 1200, but that was only about 250 less than I was eating before I started tracking calories. So I set my calorie target to 1000 per day and I have been doing fine with it. I use my Fitbit to track my calories burned. I just started weight lifting and walking for two hours six days a week. I find that my Fitbit calculates my exercise calories high! That is compared to the treadmill and other online calculators. But that is okay. I just account for how many calories I'm actually burning so that I can calculate my deficit. Thank you for the article. It was very informative.

    • Dann Albright
      March 19, 2017 at 1:23 am

      Glad you enjoyed the article; sounds like you have a great system!

  3. Zerenia
    August 5, 2016 at 8:12 pm

    You wrote that calories out are computed using RMR, but you gave the BMR formulas. BMR will be a lower value than RMR. So, which one is used to compute calorie expenditure - BMR or RMR?

    • Dann Albright
      August 11, 2016 at 3:04 pm

      Oops! Thanks for pointing that out. I missed that in the editing process. While BMR and RMR do differ, they're not usually different by a huge margin, and it's possible that different apps use different formulas. My guess is that most apps use BMR as—in my (admittedly limited) experience—there are more formulas that are easily calculated with BMR.

  4. shweta Ahuja
    August 5, 2016 at 5:15 am

    this is very interesting to know, thanks for sharing.

    • Dann Albright
      August 11, 2016 at 3:02 pm

      Glad you found it interesting! I love writing about this kind of stuff.