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Why Sizes Differ Between Activewear Brands
October 19, 2026 · Alo Editorial · 9 MIN READ
The practical consequence is that the size chart is not a reliable tool for comparing brands, and the only measurement that transfers is the one taken on your own body in a garment you already...
Every activewear brand has its own size chart, and the charts disagree with each other in a way that is not random. A small in one brand is not the same garment as a small in another brand, and the difference is large enough that someone who has fitted well in one brand for years can be wearing the wrong size in the next one without any obvious sign.
The reason is that a size label encodes a grade, and brands use different grades and different reference bodies. It is also that brands describe fit differently without ever saying so. One brand's medium is designed to be close to the body, another's is designed to sit over a base layer with room to spare. Both are labelled medium. Nothing on the label tells you which one you are buying.
The practical consequence is that the size chart is not a reliable tool for comparing brands, and the only measurement that transfers is the one taken on your own body in a garment you already like.
Three variables under one label
A size is a position on a grade, and a grade is an increment system built on a reference body. If brand A builds its grade from a reference measurement set and brand B uses a different one, then their labels refer to different people. This is not sloppiness, it is how the industry works, and the reference measurements are often proprietary.
The second variable is the ease built into the size, which is the gap between your body and the garment. A performance legging graded with negative ease is meant to compress you. A cotton-blend pant graded with positive ease is meant to hang off you. Both can be a medium. The compression garment in a medium and the loose garment in a medium will not look remotely alike on the same body, and this is the variable that catches people out most often because it is invisible in a chart.
The third is the intended layering. Brands that expect you to wear a base layer under a mid-layer build more room into the upper body and often into the waist. Brands that expect a single layer build close. A medium designed to sit over a long-sleeve base will feel loose if you wear it on its own, and a medium designed as a single layer will feel restrictive the moment you add something underneath.
Because all three variables exist simultaneously and none of them are printed on the label, the only way to compare two brands is to compare what the garment does rather than what it is called. That means the next section.
The measurements that actually transfer
The most reliable number you can own is not a body measurement. It is a worn measurement taken on a garment that already fits the way you want. Lay the garment flat, relax it, and measure the waist at the top edge, the hip at the widest point, and the rise from the crotch seam to the top edge at the front. Write the numbers down. That is your reference set, and it is specific to the fit you have already confirmed is right.
Body measurements are still worth having, but they serve a different purpose. A waist measured at the narrowest point of your torso tells you how a garment with a specific rise and a specific waistband construction might sit on you, and it is genuinely useful when comparing a garment with a 25-centimetre rise against one with a 30-centimetre rise, because rise changes where the waistband lands on the body.
Girth is the third measurement and it is the one most people never take. It is the circumference of your body at the fullest point of whatever area you are measuring, and it is the only way to know whether a sleeve will reach across your shoulders or a waistband will have to stretch. Sleeve length is the usual failure: a garment that fits everywhere and whose sleeves stop four centimetres short is a common and fixable outcome, and it is visible to everyone.
Two garment measurements worth having: the inseam of a pair you like, so you can match it when a chart gives a range, and the shoulder-to-hem length of a top, because tops vary enormously and the chart rarely tells you the drop shoulder situation.
If you do all of this for one brand and not others, you will find the shop is faster for the brand you have measured and slower for the new one, and you will know exactly which number to check when it is.
Returns, exchanges, and the one thing to ask staff
The process that works is unglamorous. Find one garment from each brand in a fit you like, take the worn measurements, and use those as your reference for everything else from that brand. After that, a cross-brand order needs two checks rather than a full fitting: does the waistband sit at the same height, and does the sleeve or hem reach where you need it to.
Brand-internal consistency is much higher than cross-brand consistency, which is the whole reason this works. Most brands run a given style across their size range on a single block, so if you know how a medium in one style fits, you have a strong basis for a medium in a different style from the same brand, with exceptions for anything cut on a different block.
Those exceptions are worth knowing. Joggers and sweatpants are often cut generously at the hip and thigh. Underwear-adjacent and seamless pieces run small because they rely on stretch. Outerwear is usually cut large to layer over what is underneath. When a brand offers multiple fits in one category, the fit names are more informative than the size and are worth asking about directly.
The one question worth asking staff is what the garment is designed to go over, or what the intended layering is. Staff know this because they see it constantly, and it is rarely on the label. A single clear answer will save you a return, and it is the piece of information that converts a size label into a fit description.
Returns are the safety net, but treating them as the primary strategy costs time and often money. A process that gets it right the first time is more valuable, and the difference is entirely in the two measurements you took before you ordered.
Measurements worth taking before you cross brands
- A worn waist measurement on a garment that already fits the way you want, as your reference number.
- A worn hip measurement at the widest point, taken flat and relaxed rather than stretched.
- A rise measurement from the crotch seam to the front top edge, since rise decides where a waistband lands.
- Girth measurements where a stretch fabric has to reach, particularly the chest, because a sleeve that runs short is immediately visible.
- An inseam from a pair you like, to resolve the ranges that charts leave open.
- A shoulder-to-hem length on a top, since drop shoulder cuts vary more than any other measurement.
- The intended layering for each category, asked directly, because it explains most of the confusion between brands.
The Short Version
A size label encodes a grade, a reference body, and an intended fit, and only the first of those is printed on the tag. The measurement that transfers between brands is not on the label at all, it is the one you take off a garment you already like.
Questions people actually ask
Is a size M the same at every activewear brand?
No. A size encodes a position on a grade, and brands build grades from different reference bodies. On top of that, some brands cut a medium to compress and others cut it to sit loose over a base layer. Both are labelled medium, and nothing on the tag distinguishes them, which is why the same number can fit differently across brands.
What is the most useful measurement to know?
A worn measurement taken on a garment that already fits the way you want, usually the waist measured flat at the top edge. That number is specific to a fit you have confirmed is right, so it transfers to other garments from the same brand far more reliably than a body measurement or a chart number does.
Why do sleeves and hems vary so much between brands?
Because they are usually the least engineered measurements in garment making. Shoulder seams move, drop shoulders are used in different degrees, and hem lengths get adjusted for styling reasons rather than for fit. These are the two things worth checking before you commit, because a garment that fits perfectly everywhere and has short sleeves is a common and frustrating outcome.
Should I size up for activewear?
It depends on the fabric and the cut rather than on the category. Compression styles are intended to be worn close and will feel tight in your usual size, which is correct. Seamless styles run small because they rely on stretch. Outerwear is normally cut to go over other layers. If you are unsure, ask what the garment is designed to be worn over rather than guessing from the size.
FREQUENTLY ASKED QUESTIONS
- Is a size M the same at every activewear brand?
- No. A size encodes a position on a grade, and brands build grades from different reference bodies. On top of that, some brands cut a medium to compress and others cut it to sit loose over a base layer. Both are labelled medium, and nothing on the tag distinguishes them, which is why the same number can fit differently across brands.
- What is the most useful measurement to know?
- A worn measurement taken on a garment that already fits the way you want, usually the waist measured flat at the top edge. That number is specific to a fit you have confirmed is right, so it transfers to other garments from the same brand far more reliably than a body measurement or a chart number does.
- Why do sleeves and hems vary so much between brands?
- Because they are usually the least engineered measurements in garment making. Shoulder seams move, drop shoulders are used in different degrees, and hem lengths get adjusted for styling reasons rather than for fit. These are the two things worth checking before you commit, because a garment that fits perfectly everywhere and has short sleeves is a common and frustrating outcome.
- Should I size up for activewear?
- It depends on the fabric and the cut rather than on the category. Compression styles are intended to be worn close and will feel tight in your usual size, which is correct. Seamless styles run small because they rely on stretch. Outerwear is normally cut to go over other layers. If you are unsure, ask what the garment is designed to be worn over rather than guessing from the size.
WORTH A LOOK
ABOUT THIS GUIDE
Written by Alo Editorial. Every fit and fabric claim in this guide is checked against the pieces we actually sell — if something does not apply to your size, body or climate, our customer care team will say so. Corrections get published, not quietly deleted.
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