Most sizing problems start the same way. You order the size you always order, the top is fine, the shorts sit weird, and now you are packing a parcel back up on a Tuesday night. Getting ASICS sizing UK buyers can trust takes about five minutes of prep, and it saves the whole round trip.
Here is the honest bit up front. We are not going to print a chest chart from memory or tell you the Core tee "runs half a size small" like that is a universal law, because fit depends on your build and how you like your kit to sit. What we can do is show you how to decide. For the wider context on the range, start with our complete ASICS sets buying guide for the UK.
For the actual numbers, everything below points at one place: the official Tempo size guide. That page is the source of truth. This post is the method.
The one thing to do before you order
Measure a t-shirt you already own and like. Not your body. A tee.
Lay it flat, do up nothing, smooth out the creases and take two numbers with a tape measure:
- Pit to pit. Straight across from one armpit seam to the other. Double it and you have the chest measurement of the garment.
- Length. From the highest point of the shoulder seam straight down to the hem.
Write those numbers on your phone, open the sizing guide page and find the size that sits closest. That beats "I'm usually a medium", because a medium in a slim gym brand and a medium in a boxy streetwear brand are not the same garment.
Do the same with a pair of shorts or joggers you rate: waist flat across, then the inside leg from crotch seam to hem. Two minutes, both halves covered.
How set sizing works when the top and bottom come together
This is where people get caught. A set is sold as one coordinated purchase, top and bottom together, designed to be worn straight out the bag. So you are picking one size for two garments.
Most people are the same size top and bottom, so it is a non-issue. If you are not, the usual advice runs like this:
- Broader up top, slimmer legs? Size for the top. A short with a drawcord waist has more give than a t-shirt across the shoulders.
- Slim up top, bigger through the thigh? Size for the bottom, and accept the tee will wear a bit roomier. Roomier reads fine on a Core tee. Tight across the back does not.
- Genuinely split by two sizes? Buy the pieces separately instead. A single ASICS Black Core T-Shirt is £34.99. Pair it with shorts and you land in the same place as the ASICS Black Core T-Shirt Set at £74.99, just with independent sizing.
If you are still working out what a set includes and how the pieces are cut to match, our breakdown of the Core T-Shirt set covers it properly.
Tops: the tee and the vest are not cut the same
Do not assume your tee size carries over. A vest and a t-shirt are different shapes doing different jobs.
The tee has a sleeve, so shoulder and bicep matter. If your tee size is already a compromise because of your arms, you are sizing on the sleeve, not the chest. The vest has no sleeve, so the questions move to armhole and strap. Armhole depth is the thing people notice.
So check the listed measurements for the specific piece. If you are choosing between the two styles for summer, compare the ASICS Black Core Vest against the tee, or look at the Black Core Vest Set as a pair.
If you are between sizes on a top, the safer call is usually the larger one. A slightly relaxed tee looks intentional. A tee that pulls at the chest print does not, and it gets worse after a wash.
Bottoms: 5" vs 7" shorts, and woven pants
The number in the name is about leg length, not waist. A 5" short sits higher on the thigh, a 7" sits lower. Same waist sizing, different amount of leg on show.
- 5" shorts suit you if you are shorter in the leg, train in them, or just prefer a higher hem. See the Black Core Woven 5" Shorts.
- 7" shorts land nearer the knee and read a bit more everyday-wear than gym. The Black Core Woven 7" Shorts are the reference point.
Tall lads often prefer the 7", because a 5" rides higher on a long thigh than the product photos suggest. If you are 6ft plus and unsure, lean that way.
Woven pants are a waist-and-inside-leg decision. Check both numbers on the published size charts before ordering the ASICS Black Core Woven Pant. Length especially, because a pant that breaks badly over the shoe is the one thing a drawcord cannot fix.
Windrunners and outer layers
Outerwear needs a different question: what are you wearing under it?
Over a tee only, your normal size is usually right. Over a hoodie or a half-zip in October, think about whether you want that extra room. Some people size up on a jacket by habit, some hate the volume it adds. Neither is wrong, but decide deliberately.
The ASICS Black Core Windrunner at £89.99 is the standalone piece. Buying it in a set or tri-set means sizing three garments at once, and the full windrunner guide explains how those bundles work. Same logic on the ASICS Black Core Tracksuit at £149.99 (price correct at time of writing). Torn between a tracksuit and a two-piece? The tracksuit versus set comparison covers it.
Trainers are a separate conversation
Do not carry your clothing size logic across to footwear. Different system, different chart.
Two things worth doing. Measure your foot late in the day, standing, heel against a wall, in the socks you actually wear, because feet swell and a morning fitting flatters you. Then check the specific model on the footwear charts, since a chunky retro runner and a stability trainer are built on different lasts.
Between sizes in trainers, most people find the larger the safer call, particularly for anything you walk distance in. A pair of 'Grey Graphite' Gel Kayano 14 that runs a touch generous beats a pair that runs a touch short. Browse the rest in the ASICS trainers collection.
ASICS vs Nike vs Under Armour: how the fits compare
The honest version: the three brands share no single standard, and each has pieces that sit differently within its own range. A performance-cut Nike top and a lifestyle Nike top are not the same fit as each other, never mind against ASICS.
Treat brand as a starting hypothesis, not an answer. What works:
- Compare the garment measurements, brand to brand, on the size guide. Numbers travel. Letters do not.
- Remember that anything sold as a training or compression piece is designed to sit close by intent, so it will feel smaller than a relaxed everyday tee at the same letter.
- Judge by what you already own from that brand and how it fits you, not by the label on the collar.
For the wider comparison on price, cut and what each brand is best at, our three-way brand comparison goes deeper than fit alone.
If it doesn't fit
It happens, and it is not a drama. Keep the packaging, keep the tags on, and read the current terms on the returns and refunds policy before you send anything back.
Better still, ask before you order. Message the team through the contact page with your measurements and the piece you are looking at, and you will get a straight answer.
Quick answers
Should I size up in an ASICS set?
There is no blanket rule, and anyone giving you one is guessing. Take the pit to pit and length off a tee you like, match it against the published figures, and let the numbers decide.
What if the top and bottom need different sizes?
Buy the pieces separately. A single tee plus a single short costs about the same as the set and lets you size each half independently.
Does a 5" or 7" short change the waist size?
No. The inches refer to leg length only. Pick the waist as normal, then choose the leg length you prefer.
Is my trainer size the same as my clothing size logic?
No, they are unrelated. Measure your foot properly and check the footwear chart for the model you want.
Ready to order
Sizing done properly is boring, and boring is the point. Five minutes with a tape measure and the numbers on the page beats a week of return labels.
Have a look through the full ASICS range at Tempo, keep the measurements page open in a second tab while you shop, and remember free UK delivery kicks in on orders over £100 with same day dispatch on your order.