When start-up brands come to us at Blue Associates Sportswear, most of the focus is on design, fabrics and getting into production. Sizing is often an afterthought. That is a costly mistake.
Poor sportswear sizing leads to returns, negative reviews and customers who never come back. According to McKinsey research into apparel returns, incorrect sizing and fit are the single biggest drivers of returns in the fashion industry. Getting it right from the very beginning protects your brand reputation and saves you significant time and money further down the line.
In this guide, we walk you through everything you need to know about sportswear sizing from deciding your initial size range and finding the right fit model, through to sampling, size grading, and building a size guide for your website.
Why Sportswear Sizing Is Different to Regular Clothing
Sportswear sizing is not the same as sizing for casual or fashion clothing. Performance garments are worn close to the body, which means any fit error is immediately obvious to the wearer.
Stretch, compression and fabric recovery all affect how a size feels compared to how it measures on a spec sheet. A size M in one brand can feel completely different to a size M in another, depending on the fabric construction and the intended fit. This makes it even more important that you define your sizing clearly and test it properly before going into bulk production.
Athletes notice everything. A seam in the wrong place, a waistband that rolls, or a leg length that rides up during movement will result in returns and complaints. In sportswear, fit is performance.
How to Decide Your Size Range and Opening Order Quantity
One of the first decisions you need to make is how many sizes to offer in your first collection. The temptation is to launch with a full XS to XL range, but this has real cost implications.
Each additional size means more fabric, more sampling rounds, more grading work, and a higher total order quantity. For most start-up brands, launching with a focused range — such as S, M, L and XL — is the smarter approach. Use your target customer to guide this decision. If you are launching a cycling brand targeting competitive male riders, your size distribution will look very different to a gym wear brand targeting women aged 25 to 40.
On order quantities, we advise our start-up clients to begin with a minimum of 150 pieces per colourway. This is the sweet spot. It is enough stock to fulfil initial orders, generate real customer feedback and test your fit in the market — without over-committing cash to unproven styles. Ordering fewer than this creates fulfilment problems and makes reordering difficult. Ordering significantly more before your fit has been proven in the real world ties up capital in stock that may need to be adjusted.
Think of your first production run as a live fit test with real customers. 150 pieces per colourway gives you the data you need to refine and scale with confidence.
Fit Models and Fitting on an Industry Size M
Before you send a single comment back to your factory, you need the right person wearing your samples. This is where many start-up brands go wrong.
A professional fit model is not the same as a standard photographic model. Fit models maintain specific, consistent body measurements and are trained to give precise, technical feedback on how a garment feels, moves, and performs. Peer-reviewed research published in Fashion and Textiles confirms that fit models provide irreplaceable comfort, fit and movement feedback that no other stage of the development process can replicate. They will notice things that most people simply would not — subtle pressure points, areas of restriction, seams that dig in during movement.
Using a friend, colleague, or yourself to approve fit is one of the most common and costly mistakes we see from first-time founders. Without consistent measurements, your feedback is unreliable, and your factory cannot make accurate corrections.
The industry standard is to develop and approve all initial samples on a size M. This is the base size from which the rest of your range is graded. If your size M fit is not correct, every other size in your range will also be wrong. Getting the size M right before you move forward is not optional — it is the foundation of your entire size range.
The Importance of Sampling — Perfecting the Performance Fit
Sampling is not just about how a garment looks. In sportswear, sampling is about how a garment performs.
Once you have your first sample and a professional fit model in place, the garment needs to be tested in the actual sporting environment it was designed for. A running top needs to be worn for running. Cycling bib shorts need to be tested on a bike. Gym leggings need to be squatted, lunged, and stretched in. This is the only way to truly understand how the garment behaves under the conditions your customer will put it through.
The specific movements that reveal fit problems include squatting, lunging, stretching overhead, running, and jumping. A garment that looks perfect on a hanger or standing still can fail in motion. Necklines that gap, waistbands that roll, seams that twist, panels that shift — these issues only reveal themselves during movement.
Expect multiple sample rounds before your sportswear fit is fully approved. At Blue Associates Sportswear, we rarely see a garment approved on the first sample. Two to three rounds are common, and that is perfectly normal. Each round gets you closer to a garment your customers will love and keep coming back for. For a deeper look at how to structure a professional fitting session, the Fashion Business Coach guide to fit sessions is a practical resource used widely across the industry.
What Is Size Grading and Why Does It Matter
Once your size M sample has been approved, the factory will grade the pattern up and down to create the rest of your size range. This process is called size grading.
In simple terms, size grading is the systematic scaling of a pattern between sizes. It is not simply making the garment bigger or smaller — it involves carefully adjusting specific measurements at specific points to maintain the correct proportions and fit across the range. Techpacker's guide to pattern grading breaks down the grading process in detail if you want to understand exactly how grade rules work in practice.
When grading goes wrong, the results can be significant. A size XS that is too tight across the shoulders, or an XL with a waistband that sits in the wrong position, are both grading errors. These issues are difficult and expensive to fix once you are in bulk production, which is why the size set approval stage — covered in the next section — is so important.
Poor size grading is one of the most common and costly mistakes in sportswear development, and one that is almost entirely avoidable with the right process in place.
Fitting Size Sets on Jumping Sizes
Once the full range has been graded, you do not need to physically fit every single size. Instead, the industry practice is to fit on jumping sizes — typically XS, M and XL, or S, L and XXL depending on your range.
This approach is standard across the industry because it efficiently catches grading errors across the full range. If your XS and XL both fit correctly, you can be confident the sizes in between are accurate.
During size set approvals, you are looking for the same things as in your initial sampling — proportion, fit through key areas such as the chest, waist, hips, and seat, and how the garment moves and performs. Any issues flagged at this stage must be resolved before you approve bulk production. Do not rush this step. Approving an incorrect size set is one of the most expensive mistakes a brand can make.
Building a Size Guide for Your Website
Once your size range has been approved and you are heading into production, you need to translate all that technical sizing work into something your customer can understand on your website.
A clear, accurate size guide is one of the most effective tools for reducing returns. Customers who can measure themselves and find their correct size with confidence are far less likely to order the wrong size and return it.
Your size guide should include the key body measurements for each size — chest, waist, hip, and inseam at a minimum, with additional measurements relevant to specific garment types. For example, a cycling bib short size guide should also include inside leg length. A sports bra guide should include both band size and bust measurement.
Present your sizing in a way that speaks to your target customer. If you are selling internationally, include both centimetre and inch measurements. If your fit runs small or large, say so. Honest sizing guidance builds trust and reduces the frustration that leads to returns and negative reviews.
Final Thoughts
Sizing is not a detail to sort out at the end of your product development process. It is one of the most important decisions you will make as a sportswear brand.
Get your sportswear sizing right — with the correct fit model, thorough sampling, accurate size grading and a clear size guide — and you build a product your customers trust and come back for. Get it wrong, and no amount of great design or marketing will save you from returns, complaints, and damaged brand reputation.
At Blue Associates Sportswear, we have been guiding start-up brands through this process since 1997. If you want to get your sizing right from day one, get in touch with our team today.