Lots of people use diet and activity trackers to log their food intake and exercise. After all, there’s an old saying that “you can’t manage what you don’t measure.” And yet it seems to be backfiring.

I get email after email from people using these trackers who can’t understand why they’re not losing weight. They’re entering in every morsel of food and logging every activity. According to their trackers, they should be shedding two or three pounds a week. And yet the scale hasn’t budged—or they’ve actually gained weight!

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The problem with net calories

Here’s how many of these trackers work: You start every day with a certain number of calories to spend. That number is based on your height, weight, age, sex, activity level, and your goals—that is, whether you’re trying to lose, gain, or maintain your current weight.

Calories are subtracted from your balance as you log your meals into the diet tracker over the course of the day. Ideally, you don’t get to zero too early in the day. But if you do, there’s a solution. Let’s say it’s 5 pm and I’m down to my last 400 calories. But wait! I can take an evening run, log it into the app and now I’ve got 840 calories to spend on dinner! How awesome is that?

The general principle here is sound: The more you move, the more you can eat. In practice, however, these “net calorie” calculations are inaccurate and misleading—and they are suckering people into eating too many calories. Let me explain.

How logging exercise leads you astray

Although diet tracking apps can help you get an accurate picture of your calorie intake, they are much less reliable in determining how many calories you burn. Here are at three ways they tend to get it wrong.

Mistake #1: Your baseline may be too high.  In order to calculate your baseline calorie requirements, you indicate your activity level: sedentary, lightly active, moderately active, or very active. This does not refer to how much you exercise (we’ll get to that in a moment). This is just about your daily activity level. And guess what? Most people select an activity level that’s one or two categories higher than their lifestyle actually warrants. Unless you rope cattle eight hours a day, your lifestyle probably does not qualify as “very active.”

If you use a wearable fitness tracker like a Fitbit or Apple Watch or even a low-tech pedometer or step counter, you can use that to help you select the proper category for your lifestyle. Here’s an easy cheat sheet:

• Fewer than 1,000 steps a day is Sedentary.
• Fewer than 10,000 steps or about four miles a day is Lightly Active.
• Ten to 23,000 steps or four to 10 miles a day is considered Active.
• More than 23,000 steps or 10 miles a day is Highly Active.

If you walk or run for exercise, you can count those steps and/or miles toward your baseline activity level if you want, but then you can’t enter them again as exercise. They’ve already been counted.

Mistake #2: The calories burned from additional activity are often overestimated.  Most diet tracking apps give you a place to manually log physical activities and exercise, such as a spinning class or yard work or ballroom dancing. Alternatively, there are wearable devices that sense your movement and changes in your heart rate.  Either way, you may not be burning anywhere near as many calories as your app thinks. As with the readouts on the aerobic equipment at your gym, diet and fitness trackers may overestimate calories burned by anywhere from 10% to 25%.

For one thing, the more you do a given exercise, the more efficient your body becomes at performing that motion. As a result, you burn fewer calories. The first time I run an eight-minute mile, I’m probably going to burn more calories than the 100th time I run an eight-minute mile.

Not only that, but when we burn a bunch of calories exercising, our body actually adjusts by burning fewer calories the rest of the day.  A new study finds that the body of a reasonably fit person may “recover” up to 28% of the calories burned through exercise by burning fewer calories at rest.  Ironically, the more fat tissue you have, the greater this effect.  As researcher John Speakman explains,

“When your smart watch tells you that you burned 300 calories on your run it may be correct (probably isn’t). But even if it is correct, you should not be deluded into thinking you can now eat 300 calories more food.”

Mistake #3: You may be counting some of those calories  twice.  If I spend the next hour sitting at my computer writing this podcast episode, I’ll burn about 100 calories. Those non-active calories are already accounted for in my daily calorie allowance. If I spent the next hour on the stationary bike instead, I’d burn 500 calories. That’s 400 more calories than I would have burned writing this episode. But if I log my bike ride into my diet tracker, it doesn’t add 400 calories to my total allowance … it adds 500. Essentially, it counts those 100 baseline calories twice.

The more activities you enter in to your exercise diary, the more this double-dipping error compounds—especially if you’re logging a lot of low-intensity activities like housecleaning or yoga.

I recently heard from a Nutrition Diva listener who said she burned 3,000 calories a day. She was only eating 2,500. She couldn’t figure out why she was gaining weight.  Sure enough, she was using an app to track her food intake and exercise.

According to her app, her baseline calorie needs were about 1,800 calories a day. She then logged activities for almost every hour of her day: making beds, folding laundry, unloading the dishwasher, yoga class, walking the dog, grocery shopping, car-pool, weeding the garden, playing the piano, cooking dinner, folding laundry, and so on. According to her tracker, all those activities were burning an extra 1,200 calories a day—which gave her a total “net calorie” allowance of 3,000 calories a day. She figured she could eat 2,500 calories a day and still lose weight.

In reality, all of her routine activities probably only burned a couple hundred calories above and beyond her baseline. Instead of eating 500 calories less than she burned each day, she was really eating 500 calories more than she burned each day. No wonder she wasn’t losing weight!

How to avoid the net calorie trap

Activity trackers are a great way to keep track of how active you are, but they aren’t very accurate at estimating your calorie expenditure. Adding “calories burned” to your daily calorie allowance can result in unintended weight gain.  So, I’d suggest that you don’t log your exercise and other activities into your diet tracker or sync your wearable fitness tracker to your diet log. Even better, I’d encourage you to stop thinking of exercise as simply way to burn calories or earn food and focus on the many other benefits it provides instead.

See also: What if exercise burned zero calories?