This is one of those things that I forget isn't necessarily common knowledge anymore.
So, there are several ways to go about this. Some are more complicated than others depending on what KIND of brassier you are looking at wearing. But! For most American, off-the-rack, department store type bras you can get by with two simple measurements. This is easiest with flexible measuring tape, but can also be done with any flat ribbon long enough to go around your body, and a yard stick.
1) DO NOT WEAR A BRA WHILE MEASURING FOR BRA SIZE. Modern bras are largely designed to change how the breasts sit on the chest. Wearing a bra while doing your measurements WILL skew the measurements and put you in a funky fitting. Be topless if you can or are comfortable with it; wear a loose fitting top of thin material if not.
2) With a cloth measuring tape (or your ribbon), measure around your ribs directly under the base of your breast. The easiest way to do so it's to hold the end of it at the center of your ribcage at the base of your breasts and wrap the tape around you until it meets itself. Keep the tape measure horizontal and as straight around your chest as possible. Meaning it should not cross your back two or three inches below the starting point on you ribs.
Pull to comfortably tight; meaning it is snug, might dig into your skin just a bit, but is not actively uncomfortable. This is called the 'underbust measurement'. This is the basis for your band size. If your measurement is an even number, congrats! That IS your band size. 38" measurement is a size 38 band. If your measurement is odd, then your band size is the even number on either side. So you measure 37", and your band size will be 36 or 38 depending on how close to whichever even number you are, and how stretchy the material of your bra is. Most of the time you will size up and use the tighter hooks, but the other direction is fine too. It's all down to personal preference and the style/material of the bras you wear.
3) Lean forward at the hips so your spine is parallel to the floor. This is where having your breasts free from restrictions is especially necessary.
Measure around the widest point of your breasts, again keeping the measuring tape snug but not pinching. This the what's called the 'bust measurement'. You will use this number for your cup size.
4) The absolute most basic formula is busy measurement minus underbust measurement, and every inch of difference is a cup size.
And so on until you get to extended sizes.
So, for example, a bust measurement of 41 and an underbust measurement of 38 would be 41-38=3. Which put the bra size at 38C. This is the standard size that will generally fit those measurements.
5) Standing upright and facing a mirror, look at your breasts.
No, really, that is not a joke.
Look at the shape of them, how they hang, is one a different size than the other, are they equidistant, do they 'face' the same direction. These answers are not the same for everyone, and may not be the same for you five years down the line. Take honest stock of the shape and form of your breasts and consider that when purchasing a bra style. Not meaning 'are they lace or cotton, colorful or beige'. Meaning, is it under wire to support a heavier breast, or demi-cups for shallower breasts kind of style.
For example: If you want the smaller half cup style because reasons, but you have muffin top boobs happening, adjust to a related size more suited to that style. So the 38C with saggy boobs would switch over to a 36D, fitting the cup more to the breast and using the adjustable hooks to give them more breathing room in the band.
There are online calculators and wiki-how pages that probably explain it better, so please do feel free to look into it further. 😁
