Sydney designer Gill Milligan is the  mastermind behind the design, which mimics bra sizing.

"It's the bra for your bum," Mrs Milligan said.

The Sweet Cheeks Fit System by Milk and Honey is currently available in A-fit for boyish figures with a smaller backside and B-fit for curvier figures.

"Like you have a 10A or 10B in a bra you have a 10A or 10B in these jeans," she said.

The Bondi mother-of-two said she had developed the jeans over 18 months after desperate calls for "denim that fits".

Traditional jeans sizing is based on women's waist measurements and leg length but Mrs Milligan said taking into account bum size and shape was more relevant.

"You can have a woman who is a size eight but still have a curvy bum and would need a B-fit," she said.

She said pocket pop star Christina Aguilera - who is a size 6 but also has a prominent backside -- would be a B-fit.

And the woman who has made her curvy backside her trademark, Jennifer Lopez, would also slip into a B-fit.

"Cameron Diaz is my A-fit muse and Beyonce would be a C-fit," she said.

While the range, that sells for between $165 to $185, is limited to A-fit and B-fit in sizes 6 to 14 - there are plans to expand into larger sizes.

"The range of denim jeans has become expansive with different washes and styles, but people have forgotten that what women really want is denim that fits well," she said.

"I'm extremely straight through the body and if I found jeans that fitted in the waist they were too tight in the seat," she said.