After her big “American Idol” win in 2005, Carrie Underwood was eating as few as 800 calories a day to lose weight.

During her time on the hit singing competition, the 36-year-old would read message boards critiquing her weight gain.

“I shouldn’t care what other people think about me,” she said in the new issue of Women’s Health, adding that she’d been living off quesadillas and pasta at the time. “I was tired, and I kept buying bigger clothes. I knew I could be better for myself, and I let my haters be my motivators.”

When the show ended in May 2005, Underwood decided to start counting calories while the group went on the “American Idols Live! Tour.”

“I was sleeping better, and I had more energy for our grueling schedule,” she said and continued to push the diet further and further until she was eating as few as 800 calories a day.

She said the diet wasn’t manageable at all.

“I would ‘fall off the wagon,’ then feel terrible and repeat the cycle,” she said. “Your body is screaming out, ‘I need more calories, I need more carbs!'”

These days, Underwood — who just released her own fitness manual, “Find Your Path: Honor Your Body, Fuel Your Soul, and Get Strong with the Fit52 Life” — has a more structured, healthy way of keeping in shape while still tracking her calories.

“I love rules,” the mother of two said. “This is how I feel good about myself, and this is how I operate.”

Using the MyFitnessPal app, she now tracks calories, as well as protein, carbs, and fat. According to the mag, she eats 45 percent carbs, 30 percent fat and 25 percent protein.

“I do have my vice, and it’s red wine,” she admitted. “It’s good for my heart, right?!”

On top of eating well, she tries to work out seven days a week.

“My self-care is my gym time, and that’s a stress reliever for me,” Underwood, who wed hockey player Mike Fisher in 2010, said.