If you're like most women, you probably determined your bra size sometime around the time you learned to drive, and haven't given it much thought since. That would put you squarely among the bazillions of people wearing ill-fitting underthings who don't even know it.
But you can fix that! And there's good reason to: Nothing does more to instantly trim inches off your silhouette and subtract years from your figure than a bra in the correct size. Here's how to fit yourself for a bra at home, courtesy of Kim Caldwell of Linda's Bra Salon in New York City.
First, she says, get a soft measuring tape and figure out your bra's band size in inches by measuring directly underneath the breast tissue. "Lift them up if you have fuller breasts," Kim says. Then round up to the nearest even number—or a bit higher if you have smaller breasts. In that case, add about one to three inches to that figure to determine your band size. "That's just because your band doesn't have to be so tight that it's choking you if you have smaller breasts. If your breasts are heavier, the back band has to be snugger so it's working harder."
Then, with your bra on, wrap the measuring tape directly around the fullest part of your bust. This will be your cup size measurement. Take that measurement and subtract your band measurement. Every difference of one inch equals one cup size. So a one-inch difference means an A, two inches a B, and so on.
For instance, Kim says, "Let's say I measure 29 below my bust and I have a bust size of 35. Then I'm a 32F. If I were 40 around my under-bust and 50 around my bust, I'm a 40J."
Does that sound like a big bra? Maybe. But Kim says too many people just assume they're what they consider to be middle-of-the-road sizes like 32B or 34C, when in fact those assumptions aren't based in reality.
"It's pretty basic, but a lot of women take for granted that their bras fit. You have no idea how many women come in and say, 'I've been a 34B my entire life.' I say there's no way! That's like saying i've been a size 4 my entire life. It's impossible. We're women. Our bodies change," she says.
And the right fit might be more important than you think. "If you're in a good bra, you'll always look and always feel better. If you spend $500 on a dress and wear an ill-fitting bra underneath, it's going to look terrible. But a $40 dress will look great with a bra that fits. If you have the right bra on and your breasts are lifted, suddenly you look taller, thinner, more youthful, and you feel more confident. You feel ready to take on anything that day."
So what are you waiting for? Get out that measuring tape!
A version of this story originally appeared on iVillage.