Grab a Tape Measure

Make sure it’s soft and pliable, preferably made of fabric (like tailors use). If you don’t have one, head to a local fabric store, drug store, or even a mass retailer.

Make Sure You’re Wearing Your Best Bra

You wouldn’t go on a first date without first putting on your best outfit, so don’t measure your bra size without wearing your most well-fitting bra. It should fit snugly, preferably have an underwire and with no padding. 

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Pro tip: Be sure your breasts aren’t sagging; if your nipple isn’t located approximately halfway down the top half of your back, adjust your bra straps to make them tighter.

Measure Your Band Size First

As you know, most bras are sized via band then cup., i.e., a 34-D bra is a bra with a band measurement of 34 and a cup measurement of D. But don’t be fooled by appearances: The band size often isn’t precisely the measurement of the circumference of your torso. While wearing your bra, wrap the tape measure snugly around your rib cage, directly under the bottom bra edge. The tape should be snug, but not tight.

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On to Measuring Your Bust Size

Compared to measuring bust size, the band size is a relative walk in the park. Cups are trickier because the size of your breasts can actually change depending on myriad factors, like what day of the month it is and if you are experiencing bloating or not. Try to pick a day to measure your cup size when you feel (relatively) true-to-size.

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Your posture also plays an important part in measuring an accurate cup size. To counteract a mis-measurement on account of slouching or standing up abnormally erect, try this: Bend forward at your hips before measuring. So your body, from your feet to the top of your head, looks like an upside-down letter “L.”

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This helps ensure that you are getting all the breast tissue in the tape. Then carefully place the tape so it’s straight across your back (again, you can use a mirror to help you do this) and measure your breasts at the fullest point, which typically is where your nipples are. Take a mental (or literal) note of that number.

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Calculate the Cup Size

Because in the land of bras, everything is more confusing than it really needs to be: Your bust size and your cup size aren’t the same. The bust size is the literal number you just read on the tape measure while contorting yourself into the “L” shape. The cup size, however, is the difference between your band size and your bust size.

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Here’s a chart of standard size differences and how they equate to cup sizes:

• If your bust size is less than 1 inch bigger than your band size, you’re a cup size AA.

• 1-inch difference = A

• 2-inch difference = B

• 3-inch difference = C

• 4-inch difference = D

• 5-inch difference = DD

• 6-inch difference = DDD or F

• 7-inch difference = DDDD or G

• 8-inch difference= H

• 9-inch difference = I

• 10-inch difference = J

• 11-inch difference = K

• 12-inch difference = L

• 13-inch difference = M

• 14-inch difference = N

• 15-inch difference = O

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Here’s an example from start to finish:

Let’s say you measure around your rib cage and get the measurement of 33 inches. Because it’s an odd number, you should round up to 34.

 

When you measure around your full bust, you get 36 inches. That means there’s a 2-inch difference between your full bust size and your band size. The chart above says a 2-inch difference is equivalent to a “B” cup size, so your final bra size is: 34B.

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(by MEGHAN BLALOCK JUN 29, 2018 AT 8:00 AM EDT, https://stylecaster.com/how-to-measure-bra-size/)