Stop Sizing Up Your Running Shoes! Here’s Why…

The myth to size up for running shoes has been perpetuated for too long. We've reviewed hundreds of shoes and I'm setting the record straight.


All opinions are our own and never influenced by brands. If you buy through links, we may earn a commission.

When you first look at buying a pair of running shoes, what’s one of the most common things you get told in the store, or by googling ‘what size running shoe to buy’, or similar?

How about, ‘make sure you wear a half size larger than your regular shoe size’? Or ‘be sure to size up when choosing running shoes’?

At some point, someone had the bright idea to share this nugget of advice with people looking to buy running shoes, and now it’s such a common piece of advice that it’s probably something you’ve been told if you’ve visited a running store or searched online looking for advice on buying your first pair of trainers for running.

Now maybe this was true 10 years ago when running shoe ‘uppers’ were less technical, using less stretchy materials with lacing systems that had no ‘give’ in them. A lot has changed over the last few years with some excellent innovations in uppers, such as the case of MATRYX fabrics that are increasingly used on high-end trail shoes. In case I use a shoe term in this article or on our site in general, have a read of our anatomy of a running shoe post to learn all the different parts of a shoe.

Over time, it’s been picked up and regurgitated by other, less reliable sources, including the worst of all… spammy websites that purport to know about running shoes but actually are simply repeating each other’s mistruths using AI content writing tools so they basically all say the same thing, just in a slightly different way.

It’s a mad world out there, people, and you need to fact and source-check everything these days!

With all that said, that type of advice is not something you’ll find thrown around on this site or in our handy best road running shoes and best trail running shoes roundups. Let me explain why.

When you may need to size up for your running shoes

There are a couple of reasons you may need to size up for your running shoes. However, in almost every other scenario, you probably don’t.

The first reason, and probably why people are often told to go up a half size when buying trainers, is because you’re used to wearing shoes that are not trainers – other styles of shoe can run large.

For example, I would always wear a UK size 5 in high heels and work shoes. When I started running, the running shoe size was a size 5.5. Why? Because the heels and work shoes were sized incorrectly. Actually, when I measured my feet, they were a size 5.5, but to me, I was indeed, ‘sizing up’ because I’d thought for years that I was a 5.

So the moral of the story is, don’t assume you know what size feet you have, if you’ve never had them measured. Get your feet measured and then you’ll know what your correct shoe size is.

Once you know your correct shoe size… DON’T SIZE UP when shopping for running shoes. Buy your actual shoe size.

The only exception is if the brand themselves tells you a shoe runs small, or, even better, a running shoe expert review (like our team here at Trail & Kale) has tested and reviewed a specific model of shoe and tells you most people will be better off sizing up.

Another reason you may get told you need to size up for running shoes is because they tell you that when you run, your feet may get hot and swell up.

While this is true to some degree – most peoples feet will get hot and swell up a little for long runs or runs in particularly hot weather – most breathable running shoes are designed to accommodate that in the design of the upper, particularly in terms of its breathability (to allow heat and sweat to escape), the mesh fabric used to create it, which normally has some stretch to accommodate a little swelling and toe wiggle, and a lacing system (including the laces themselves) which also has some ‘give’ in them.

Now, everyone is different, and this may be a specific consideration for you, especially if you live and run somewhere hot and humid (hello, Florida), or you have feet that tend to do that to a noticeable degree, then sizing up could make a lot of sense for your particular case.

Sizing up your running shoes can cause blisters and other issues

So when I drop in and say ‘hold up, STOP SIZING UP!!!’, you may be wondering what the downside is of just doing it anyway.

Well, the main downside is that if you run in shoes that are too big for your feet, that leaves you more prone to running injuries, such as discomfort that can lead to blisters from your feet moving around inside the shoe, and other muscular or joint-related injuries because the shoe is not supporting you in exactly the right places like it was designed to do – you may find yourself with an issue like plantar fasciitis or Achilles’ Tendinitis, for example.

You may also be tempted to wear thicker socks for running, which is generally something I would avoid. Thicker socks may make your feet unnecessarily warm, and can make it harder to get a good fit when cinching down the laces, due to the sock thickness needing to compress sufficiently for a secure shoe.

By the way, your choice of running socks can make a huge difference to your overall comfort when running. If you need some good running socks that will help keep your feet comfortable and happy, check out our best running socks roundup for some recommendations.

Another, less noticeable, downside of wearing shoes that are slightly too big for your feet is that the larger the shoe, the more it weighs, so although the difference in weight between half-sizes may be very small, if you’re running a long way, such as a marathon or ultramarathon, then you may prefer to not be carrying any extra weight for all those hours on your feet.

Why running shoe sizes may not be ‘true to size’

One of the top reasons for running shoes not fitting ‘true to size’ is that the translation between US, EU and UK sizes can be inconsistent across brands. There isn’t necessarily a logic or rule to this, it simply depends on factors such as what country the brand is from, and how they’ve sized a particular model of running shoe, as well as how accurate they’ve been when stating the shoe’s size for other countries, on the label.

How do I know this? At Trail & Kale Running Co. we’ve test, run in and reviewed hundreds of running shoes, and each shoe needs to fit us correctly to enable us to do that. So you could say we’re experts in understanding how running shoe sizes vary between running shoe brands and models.

What’s interesting is that many running shoe brands appear to be better at being consistent with the sizes in US sizing, than they are in European or UK sizing – even if they’re not a US company. So, if you’re shopping for running shoes in Europe or the UK, then it’s possible you may need to size up or down because of how the officially-stated shoe size has been derived from the US size – even if you’re shopping a European brand of running shoe.

To give you an example, here is a selection of shoes that I currently own and have either reviewed or have in the pipeline for review. They all fit me well, so I can confidently tell you that these shoes in a US women’s 7.5 are the right size for me. But, look how the US size translates to EU and UK sizes:

Shoe brand and modelUS Women’sEuropeanUK
HOKA Clifton 97.539 1/36
Lululemon Chargefeel7.539 1/36
La Sportiva Levante7.5396
Altra Torin 77.538.55.5
On Cloudspark7.538.55.5
New Balance FuelCellRebel v47.5385.5
Merrell Moab 2 Low7.5385
Sample running shoe size conversion comparison | Trail & Kale Running Co.

Notice anything strange?

Yep, while they’re all size 7.5 in the US, the UK and European sizes vary significantly – enough to make a difference to your shoe size choice, if you’re looking at buying them. In some of these shoes, I’d be a UK size 5… and others I’d need a UK size 6 to get the right fit, even though, as I mentioned earlier in this article, I’ve had my feet measured, run in hundreds of shoes, and when in the UK would almost always wear a size 5.5!

Even the translation between EUR sizes and UK sizes is off. For example, the New Balance Rebel v4 is a UK 5.5 and an EU 38, but the Altra Torin, which is also a UK 5.5, is an EU 38.5.

How should you ensure you’re getting the right size running shoe?

At Trail & Kale Running Co, we test and review running shoes referencing our regular sizes, which for our co-founders and Managing Editors are US women’s 7.5 for me, and US men’s 9 for Alastair.

Because of the apparent consistency between US shoe sizes, for the most part, among brands and models of shoes, you could try shopping for running shoes based on your US shoe size – that may yield a more consistent result for you.

As part of our review process, we will always call out whether or not a shoe fits true to size, in length as well as whether it runs particularly wide or narrow.

Insights from running shoe reviews like ours, that have been put together by someone who reviews a lot of shoes, and actually runs in them (i.e. who doesn’t just take pictures of them and cut them in half to analyze the midsole to the nth degree!) will also help you find the right size for a particular brand and model.

One of Trail & Kale's co-founders, a mom, and guardian of our resident trail dog, Kepler, Helen can be found trail running with Kepler and enjoying road runs with her mini in a jogging stroller, all while testing out the latest running gear for our readers.


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