The 28-year-old singer revealed she often wears Spanx to help fit into designer dresses because she often gets bored of exercising.
She told Us Weekly: "I almost always wear Spanx.
"I like to say I run on the elliptical for 30 minutes watching CNN, but I don`t do it often enough. So I`m just like, `Oh, I guess I`m not fitting into this Lanvin dress for these couple of months.`"
It was recently revealed Katy - whose 14-month marriage to Russell Brand ended last December - is planning to take the next step in her relationship with John Mayer and they have been house-hunting in Los Angeles.
A source said: "John and Katy have decided to give it a proper go and want to find a place in LA to share.
"But lots of people are willing them to fail. They row a bit and have quite a turbulent up-and-down relationship, despite it only being a few months old.
"Many of her friends are certain it will all end in tears.
"But that`s brought them closer in a way. It`s given them an `us against the world` mentality."
Sources say the couple - who had a brief split in October before getting back together - are smitten with each other and like to spend every moment they can together.
One said: "They are together virtually every day.
"They hang out at John`s place, cooking, watching movies and playing music.
"They take the odd trip away but generally they`re trying to keep it behind closed doors."