Three Ways to Determine Your Cup Size

What's your cup size? 

I'm sure this isn't a question you get often but the answer is really important if you want to achieve a good fit on the garments you create. Keep watching to discover three ways to determine your cup size and why one method might work better for you than the others.

Cup Size Theory

Before we dive into the three ways to determine your cup size, let's talk about the theory behind cup sizing. The theory goes like this: Each cup size represents a change in breast size or bust projection of 1.25 cm or ½".

In our little sewing world this means that the front bodice width and length must increase or decrease to accommodate the various cup sizes. If your cup size doesn't match that of the sewing pattern you will need to make a bust adjustment. In order to make that adjustment,  you'll need to know your personal cup size. I'd like to share three methods to determine your cup size and provide some insight as to why one method may work better than another for you.

Body Measurements

In order to follow along with each of these methods, you'll need to take four body measurements. The high bust measurement, the full bust measurement, the under bust measurement and the bust cup depth.

High Bust Girth
Full Bust Girth
Under Bust Girth
Bust Cup Depth

Watch the video to see how to take these measurements.

Method 1

The first method is one that I'm sure most of you are familiar with. This is the method most sewists have become accustomed to seeing. For this method you will take your Full Bust measurement and subtract your High Bust Measurement from it. Your cup size will be chosen according to the result. 

A 1" (2.5 cm) difference will give you an A cup, a 2" (5 cm) difference will give you a B Cup, a 3" (7.5 cm) difference will result in a C cup and so on with each cup size increasing by 2.5 cm or 1" per cup size. If your result lands somewhere in between the measurements given, simply round to the nearest number and choose that cup size.

This method will work well for many which is why it's become so popular, but if your back body is more rounded or more muscular than average you may find that this method doesn't work very well. In that case, I'd like you to try method #2.

Method 2

In this method you will use the full bust measurement and the underbust measurement. First you will need to consider your under bust measurement. If that measurement is an even number, you will add 4" (10cm) to it. If it is an odd number you will add 5" (12.5 cm) to it.  if your under bust measurement lands in between and even and an odd number use the whole number (that's the one before the decimal) as your guide. Once you've completed that calculation you will subtract that result from your full bust measurement. The final result of this calculation will reveal your cup size.

A 1" (2.5 cm) difference will give you an A cup, a 2" (5 cm) difference will give you a B Cup, a 3" (7.5 cm) difference will result in a C cup and so on with each cup size increasing by 2.5 cm or 1". If your result lands somewhere in between the measurements given, simply round up to the nearest number and choose that cup size.

This method is a little less popular as of late, but may work better for those with a more rounded upper back but if your back is broader or more muscular than average you may find this method a little suspect too. If you're feeling a little unsure of the results of methods 1 and 2, I'd like you to try method #3.

Method 3

Method #3 uses just one measurement, the bust cup depth measurement. You'll remember that this measurement is the distance between your bust point and the base of the breast where it meets the rib cage.

A measurement of 2 1/2" (6.5 cm) indicates an A Cup, 3" (7.5 cm) is a B cup, 3 1/2" (9 cm) indicates a C cup and so on with each cup size increasing by an average of  1.25 cm or ½". Once again if your measurement  lands somewhere in between the measurements given, simply round up to the nearest number and choose that cup size.

This method, in my opinion, will give you the most accurate read of actual breast size because it measures the actual breast and is independent of any other body parts. However, for this method to work you need to be wearing a supportive, well fitting bra while taking the measurement.

Comparing Results

Now it's time to compare the results. Did you get the same result for all three methods or did your results vary with each of the methods? If you had varying results don't be concerned, we are all incredibly unique and bust cup sizing is not a perfect science. Since linear measurements do not  indicate shape, the calculations can be affected by the amount of support your bra provides, the distribution of measurement around the body as well as our posture. As a result some of us will need to experiment to discover our perfect bust cup size.

If you're still not sure what cup size to use, here's my advice. Gauge the results of these calculations against the cup size of your best fitting, un-padded bra. If there is a large discrepancy between the calculations you've made and the bra cup size that you wear, choose the average of the two and test a sample. A quick sample will tell you instantly if you're on the right track.

While your bra cup size does not always correspond to your sewing cup size, you can use it as a guide to help you choose a starting point if none of these methods result in a cup size that you believe is workable for you.

I know I've shared a ton of information with you in this video so in order to make it easier for you to put what you've learned into action,  I've created a free downloadable worksheet for you. Just click the link below to get your copy sent to your inbox.

Once you've determined your bust cup size, and made the appropriate bust adjustment to your patterns you'll be one step closer to your perfect fit.

I hope you found the information I shared with you helpful. I'll chat with you soon!

All My Best,
Alexandra