Numbered Sizes vs. Letter Sizes (S/M/L): Which Is More Reliable
Neither numbered sizes nor letter sizes are inherently more reliable across brands — both are ultimately mapped by each individual brand to its own specific measurements, so the real reliability comes from checking a specific brand's own size chart rather than from choosing one labeling format over the other.
It's a common assumption that one format is somehow more precise than the other, but the underlying issue — brand-specific interpretation — affects both equally.
Why letter sizes feel less precise but aren't necessarily worse
A letter size (S, M, L) covers a broader range by design, which can feel less precise than a specific number — but a numbered size doesn't guarantee more consistency across brands either, since each brand still independently decides exactly what measurements correspond to "size 8" just as it decides what measurements correspond to "medium."
What actually determines reliability
The real driver of fit reliability is whether a specific brand publishes and adheres consistently to its own detailed size chart with actual body measurements — a brand with a clear, detailed chart (whether using letters or numbers) is more reliably predictable than a brand with a vague or inconsistent chart, regardless of which labeling format either one uses.
A practical takeaway
Checking a specific brand's actual measurement chart (bust, waist, hip in inches or centimeters) and comparing it against your own measurements is more reliable than trusting either a letter or number size label alone, especially for an unfamiliar brand.