Kelly Clarkson credits her fiance with inspiring her to slim down.

In the February issue of Cosmopolitan (excerpted by Radar Online), the 30-year-old "Catch My Breath" singer opens up about her 30-pound weight loss in 2012. "I think that happens when you fall in love or date someone," Clarkson explains. "If I'm going to be messing around, I gotta look good."

Clarkson promises she didn't try a "fad diet" to get in better shape.

"I was working with a trainer, and she said, 'You'd be amazed the amount of weight you can drop by halving your portions. I know it sounds crazy, but do it for two weeks and watch what happens.' I just cut back my portions, I stopped eating late at night, and I work out a little more than I used to, and I literally dropped 18 pounds in a month," the American Idol winner reveals. "I think it happens when you get really busy. But when I'm on my off-time -- girl, those 18 might come back!"

Clarkson, who got engaged in mid-December 2012, says she's "not willing" to go to extremes to be a smaller size. "I'm willing to work a little bit because I want to be a healthy individual -- which I always have been," she says. "I've put on weight, but that's just what I'm happy with at that moment."

Talent manager Brandon Blackstock, 36, helps his fiancee stay fit by participating in the same workouts. "Brandon and I both have bikes on the back of the bus, and we both love the outdoors," Clarkson tells Cosmopolitan. "Once your lifestyle becomes more active, obviously you drop weight."

This article originally appeared on Usmagazine.com: Kelly Clarkson: "I Literally Dropped 18 Pounds in a Month"