Los Angeles: Reality TV personality Kim Kardashian has admitted falling off her diet and gaining 8 pounds (8.1 kgs) within a year.

Realising that she should not keep her new habit, especially after learning how much weight she has gained over the past year, the 39-year-old make-up mogul is determined to lose weight and hit the gym more frequently than before, reports a popular website.

Taking to Instagram Stories to give her fans an update on her new workout routine, Kim admitted to gaining 18 pounds within a year.

"OK, so we're doing a night workout before my dinner, which I never do with this one, but I figured we have to get into it at some point," she said in the video, referring to her trainer Melissa Alcantara.

She added: "And yeah, I mean, sometimes we fall off and sometimes you really gotta get it together, and this is one of my fallen-off times where I am, I think, 18 lbs (8 kgs) up from what I was about a year ago, a year and a half ago."

Kim then revealed that she's working hard toward her "goal weight" by her 40 birthday, which falls on October 21 next year.

"To feel good, I just want to get it like, really good goal weight and so that means working out at all cost. It's my eating. I work out, but it's like my eating. But Melissa and I have a major goal for by the time I'm 40."

Kim is determined to lose her weight because later on Sunday morning, she was already back in the gym.

"A morning workout after a night workout is the best!!" she wrote along with a video of her working out.