It's the latest fitness craze.
People who use them say fitness apps are like having a personal trainer and dietician in the palm of your hand.
"I never thought that this would be me," said Stacey Clover.
These days, Clover is a gym regular.
"I love it! I love being in the gym and seeing results," she said.
It wasn't always her second home.
Not so long ago, Clover topped out at 265 pounds. But with the help of trainer Terrance Sterling. she's lost -- and kept off -- more than 100 pounds. When he isn't around, she stays on track with the help of her iPhone and the 'My Fitness Pal' app.
"My favorite thing is that it keeps me accountable," said Clover. "It makes me look back and see what I've eaten through the day and maybe make a healthier choice."
You put in your start weight and goal weight, log your foods from a list of hundreds of choices and it does the math for you.
"If it's not in here, it has a bar code that you can use and it will show you the nutritional value," said Clover.
"It tracks your carbs, fat and sodium and sugar," added Sterling.
Sterling recommends the app to hold his clients accountable 24/7.
"You'd be amazed what you're doing and what you thought you was doing," he said. "People are always surprised when they see the numbers on paper."
Meanwhile, the apps are also popular among groups of women who don't have access to a gym.
"I think they completely underestimate; they don't realize what's in there at all," said Lindsey McClellan. She said it's shocking to see how many calories are in your favorite foods, and how quickly they add up.
They not only like My Fitness Pal, but also 'Noom' for Android.
"It logs all your activities, all your food and then it sets goals for you to do each day," said Kia Sadler.
Sadler says Noom makes it easy, with smiley faces and colors to signal good and bad choices.
And then there are apps to get you moving.
Mendy Henson said she started running with the help of 'Couch to 5k' apps, specifically one called 'Get Running.'
"It is a nine-week program. The first week you run a mile, walk a mile. It increases by week six and you're running, not walking," said Mendy Henson.
Henson said she's really built some stamina.
With apps like Noom and My Fitness Pal, you can put in almost any food you can think of -- even grocery and restaurant brands -- and it will show you all the nutritional info.
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