American Idol mentor Mariah Carey says she can now fit into the same size jeans she wore when she was 16.

The diva told US chat show host Oprah Winfrey that she is “down to my tenth grade size” and can now fit into clothes she has not worn since high school, FemaleFirst reported.

Earlier this week, the Daily Mail revealed images of Mariah’s appearance on Oprah, where she shared her slimming secrets.

The Touch My Body singer revealed she flies personal trainer Patricia Gay from St Barts to her apartment in New York City, where they have developed a one-hour daily exercise routine.

“Working out and being on a diet has made me feel healthier, so that’s a good thing,” Mariah said, showing off a svelte figure in a red bikini.

Mariah’s music was the focus of the most recent week of competition on American Idol - with songs such as Hero and Vanishing performed by the contestants. According to MTV.com, the diva was an “awesome coach”.

The first of two episodes of American Idol - which has already aired in the US - is set to be shown this evening on ITV2.