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This blog is a little different to our normal content. We have partnered with peBe (said “PB”), a challenger sports bra brand, to share some useful tips on checking your sports bra fits correctly and why this is important for running. Over to Charlotte, peBe’s founder, to share some insights for the women in this community. Guys you might also learn a bit if you read on!

Today I’m going to answer four questions:

  • Why is it so important to wear a decent sports bra for running?
  • What bra size am I?
  • How do I check my sports bra fits correctly?
  • What type of sports bra should I use for running?

Why is it so important to wear a decent sports bra for running?

Did you know that when you run, your breasts move in a figure of eight shape? Most of the movement is up and down, but they also move side to side. This creates a swinging motion that can increase as your breasts get bigger and also as we age and our skin loses its elasticity.

Because we all love stats – for context – this can result in between 4cm to 14cm movement in your breasts when you run!! Remember this number as I give you some more in a moment. Our boobs are heavy and unsupported by muscle – only skin, glands and fatty tissue to their make-up. This means that they will move out of sync with our torso when we run, and can be the cause of pain for lots of women. Add to this that the average cup size in the UK is a DD, which is approx. 1.1kg of unsupported weight, now you can see why boobs can literally be a pain in the back!!

Let’s recap… 1.1kg swinging around with approx. 9cm movement. The argument for investing in a well fitted high impact sports bra designed for your comfort becomes very strong!! Especially when you realise that all this movement can actually shorten your stride length whilst running and therefore increase the amount you cover over a distance! Research has found that marathon finish times increase by as much as 4.6-8.6 minutes for each increase in cup size. This means a woman wearing a 32DD sports bra could finish a marathon over 18 minutes behind a woman wearing a 32A, even if their fitness levels and training experience was similar. This difference is more pronounced with larger underband sizes.

So if I have your attention based on the research and stats, let me help you reach that PB by making sure you a) have the right bra size and b) have the best support to minimise that bounce and gain an advantage, or just feel a bit more comfortable running!

What bra size am I?

A question you should ask yourself at least every six months. Over 80% of us are wearing the wrong bra size. Finding your right fit requires a little more of your time, then just slipping into a XS, S, M, L top but it is SOOO worth it!

What you have to understand is that boobs are wonderful 3D things and every pair is a unique shape. For this reason, a tape measure acts as a starting point to get you into the right bra size and then you should use our fit check list to see if you need to go up/down in the underband or cup size.

From experience, I tend to find lots of us are wearing an underband that is too big for us and/or a cup size that is too small for us. So be prepared to go up some cup sizes!

To measure from scratch:


  • Underband measurements are in even inches – 28, 30, 32, 34 etc.
  • If your measurement is an odd number, you write down the next even number i.e. 31 becomes 32.
  • Firmly hold your tape measure flat around your waist/ under your boobs to measure.
  • Write this measurement down in inches.


  • A deceiving measurement that can often require some subjectivity. If in doubt, go up a cup size.
  • Loosely hold your tape measure flat around your boobs (in line with/ over your nipples) to measure.
  • Write this down in inches.
  • Make a note of how many more inches this is compared to your underband measurement (not size). e.g. 29 inch underband / 35 inch cups = +6

Now follow our grid below to find your bra size:

If you don’t have a tape measure, you can check your bra size using your existing bra whilst on:

First make sure your straps are done up tightly, you are on the loosest back hook and you have scooped your boobs inside your cup (scoop underneath them from the side into the middle).

  1. UNDERBAND:if you notice your underband riding up your back, and can pull it away from your body by 2 inches, go down one back size. If you can pull it away by 3+ inches go down 2 back sizes.

Warning: if you’ve measured your back size and it’s smaller, you need to go up by the corresponding number of cup sizes. e.g. if you are wearing a 34D but you can pull it away by 2 inches, you’ll become a 32DD.

  • CUPS: if the wires are digging into the side of your boob or not sitting flat against your rib cage, you need to go up at least one cup size, if not two. This should be added on to any cup size increases if you’ve made in the underband step.

How do I check my sports bra fits correctly?

✔ Boobs comfortably inside ✖ Boobs spilling out the side / showing lots of cleavage (means cup size is too small) ✖ Gaping fabric (means cup size is too big)✔ Fits firmly on the loosest hook ✔ Level all the way around ✖ Riding up your back (go down a back size) ✖ Pinching into your back (go up a back size)✔ As a measure, the hook should be above your armpit. ✔ Fits firmly ✖ Digging in (then loosen) / too loose (then tighten)

Sister sizing

Now you’ve really lost me Charlotte… bear with me! Sister sizing is the phrase used for alternative bra sizes that will keep your cup size the same but the underband will feel tighter/looser. Think about underbands and cup sizes like a sliding scale. If you go up in one, you go down in the other. So –

1. Go up a back size = go down a cup size. e.g. a 30DD becomes a 32D.

2. Go down a back size = go up a cup size. e.g. a 36DD becomes a 34E.

If you go up a back size but stay the same cup size, you’ll notice extra room in the cup. If you go down a back size but stay the same cup size, the opposite will happen, and you’ll notice the cup is tighter. Choosing to move around sizes, is usually done on comfort/ fit preference.

If it sounds like I’m speaking a foreign language now, then just take a look at our simple +/- scale below, where you can find your two sister sizes of your original bra size.

What type of sports bra should I use for running?

Two no no’s for me:

  1. Never wear two sports bras on top of each other! Double layering for support is not necessary.
  2. Never run in a really old sports bra. Think of sports bras like trainers. A lot of impact goes through both as you train and race, so they naturally wear over time. It is important to replace both for optimum support, or have a spare of each, so you can rotate usage and prolong their lives.

So what is right? Well that can depend on your bra size and also what you like wearing!

A-C cup: a comfy compression, over the head job for you ladies.

D+ cup: go for a bra that combines compression and encapsulation (two separate bra like cups), this is where you’ll get the best support or that is just encapsulation support. There is more weight and movement to handle, so make sure you pick a sports bra that has low stretch fabrics and adjustability. You want a back clasp that has several adjustments, and straps that you can tighten as the elastic stretches.

Other things to consider when buying a sports bra:

  • Does it pass the high knees test? Check for wiggle & jiggle. If you have some, try a different size, or a different bra!
  • Has the brand invested in Technology? Look for sports bras brands that have invested in their design and based it on research to make sure the support aligns to the breast biomechanics. You’d do this for your feet, so make sure you do it for your boobs too!
  • Avoid under wires, they will damage the breast tissue when running, and apart from anything else are just uncomfortable!
  • Check the fabric. Is it sweat wicking, so you can avoid chafe and boob sweat? Is it smooth, so you can avoid chafing? Is it really stretchy? If so avoid it like the plague if you are busty as it’ll do no good on the support front.

If you’ve got any other questions on fit questions, please drop us a DM or email at You can find plenty more size info over on our website

Bonus question (a tiny plug!) – How is peBe’s Impact Sports Bra different and better to the current options?

Well I’m glad you asked. Our world-first WRAPA® technology and supportive women-led design features make us stand out.

WRAPA® is our primary tech. Engineered contoured loops that wraparound each breast, act as a sling like support, cushioning both horizontal & vertical breast movement. As I mentioned before, breast health research shows that breasts move in a figure of eight shape & out of sync with your body when you run, causing pain. So it made sense to us that your sports bra delivers comfortable & contoured support, to remove that pain.

Our supportive design compliments our WRAPA® tech. Our fabric is smooth but stiff which provides world-class support without wires or padding. Our seam-free cups make for a chafe-free run. Our front adjusting straps, mean that you can adjust the level of support without the help of someone else! And last but not least, our free-form cross-over straps mean you can have an uninterrupted run.  

Wherever you run, I hope you are supported by a decent sports bra!

Big love,

Charlotte x

Written by Charlotte Gill, peBe founder.