Size Grading Rules Explained (Why Your XL Fits Badly)
Grading is how a garment's measurements change from one size to the next. Get it wrong and your sample size fits beautifully while your largest and smallest sizes fit nobody — a problem most brands never diagnose.
Quick answers
- What is grading?
- How measurements change between sizes
- Why does my XL fit badly?
- Likely linear grading applied too far from the base size
- What's a base size?
- The size the garment was actually designed and fitted on
- The fix?
- Non-linear grade rules and fitting at the extremes
How Grading Works
| Concept | Meaning |
|---|---|
| Base / sample size | The size the garment was designed on |
| Grade rule | How much each point changes per size |
| Linear grading | Same increment every size |
| Non-linear grading | Increment changes at the extremes |
Notes
Everything is designed on one size. A garment is fitted on a single base size — often an M or a US 8 — and every other size is derived from it by grade rules. The further a size sits from that base, the more the derivation can drift from how a real body of that size is shaped.
Bodies don't scale linearly, so grading shouldn't either. Adding 2 inches to the chest and 2 inches to the shoulder for every size up is simple and wrong. Bodies broaden differently than they lengthen. Mechanical linear grading is precisely why so many brands' XL is wide but too short, or whose XS is narrow but too long.
Fit-test the extremes, not just the middle. If you only ever fit the sample size on a fit model, you're guessing about your largest and smallest sizes. Fit a garment at both ends of the range. This is the single most effective thing you can do to fix returns concentrated at the extremes.
Your return data will tell you. If returns cluster in your largest or smallest sizes with 'didn't fit' as the reason, that's a grading problem, not a chart problem. No amount of size-chart accuracy fixes a garment that was never fitted at that size.
FAQs
What is grading in clothing?
Grading is the set of rules that determine how a garment's measurements change from one size to the next. Every size is derived from a single base or sample size that the garment was actually designed and fitted on.
Why do my largest sizes fit badly?
Usually mechanical linear grading — adding the same increment to every measurement for each size up. Bodies don't scale that way, so the further a size is from the base size, the worse the fit drifts. Fit-test at both extremes of your range.
How do I know if I have a grading problem?
Look at where returns cluster. If 'didn't fit' returns concentrate in your largest or smallest sizes, that's grading — the garment was never properly fitted at that size. A more accurate size chart won't fix it; the pattern has to change.
Need this on your store?
Tailor Size Guide ships pre-built size charts for Shopify.