The shop number is not standardised
No law requires a size 10 in one shop to match a size 10 in another. Voluntary industry sizing standards exist and brands are free to depart from them, which they do — every retailer sets its own chart to suit its own customer and its own house fit.
On top of that sits the well-documented drift usually called vanity sizing: over decades, the body measurements attached to a given number have tended to get larger, so the same physical body has been assigned progressively lower numbers. Nobody announced it and no single date applies, because it happened brand by brand.
The result is that a ready-to-wear size is a label attached to a garment by the company that made it. It is genuinely informative about that company's clothes, and it carries no information anywhere else.
A pattern size is a coordinate, not a label
A pattern size number points at a row in a published table of body measurements. The Big 4 chart says a size 16 corresponds to a 38-inch bust, a 30-inch waist and a 40-inch hip, and it says so in print, for anyone to check. The number is an index into that table.
That makes it a much more honest number than a shop label — it is verifiable, it is published, and it is the same for everyone. What it is not is a description of you, a judgement, or a quantity comparable to the tag in the jumper you are wearing.
It is worth saying plainly, because this is where most people stall: a pattern number several steps above your shop size means the drafting system has more sizes and different spacing. It does not mean anything else.
The evidence is in the charts themselves
The clearest demonstration is that pattern brands do not agree with each other either. Take one body — 38-inch bust, 30-inch waist, 40-inch hip — and read it against the published charts held in this site's calculator:
The same body, four charts.
- Simplicity, McCall's, Butterick and Vogue — a size 16, and exactly 16 at all three points, since those four brands share one Misses chart.
- Closet Core, sizes 0–20 — a size 12.
- Cashmerette — a size 10, reading the bust against their C/D cup line.
- Merchant & Mills, current grade — a size 12, with the bust falling between their 12 and 14.
Four numbers, one body, no contradiction. Each is correct for its own chart and meaningless outside it.
If pattern companies cannot produce a common number between themselves, a shop label was never going to translate into one.
Cup assumptions make it worse still
Pattern sizes are keyed to a bust circumference at an assumed cup — most commonly a B, meaning a two-inch difference between the high bust and the full bust. Two people with an identical 38-inch bust and different cup sizes are different shapes above the waist, and the chart cannot see the difference.
Some brands address it directly. Cashmerette prints separate bust cutting lines for C/D, E/F and G/H. Closet Core's larger block is drafted for a D cup and its smaller one for a B. Sew House Seven's Curvy range is fit for a DD. Where the chart does not offer that choice, the full bust adjustment is how you make it yourself — and the measurement that tells you whether you need one is the high bust, not the shop size you usually buy.
What to do instead
- Measure yourself properly. Six measurements, taken as described in measuring yourself accurately. Not remembered numbers, not last year's.
- Choose the size from the chart that pattern actually uses. Not the brand's newest chart, not a generic one — the chart on the envelope in your hand.
- Expect three different answers at bust, waist and hip, and blend between them. That is normal, not a problem.
- Check the finished measurements before deciding, because the size that matches your body is not necessarily the garment you want.
Two practical traps
Buying by remembered size. Envelopes are printed in size ranges, and a paper pattern bought in the wrong range does not contain the lines you need. Check that the range covers your measurements before you buy — particularly with brands that split into separate blocks, such as Closet Core's 0–20 and 14–32, or Sew House Seven's Standard and Curvy, where the ranges overlap numerically but are drafted differently.
Assuming a brand is consistent with itself. Merchant & Mills publishes no single site-wide guide — the chart lives on each pattern, and more than one grade is in circulation, so the same nominal size is not the same garment across their range. Whatever chart is printed on the pattern in your hand is the one that governs.
The short version
The number on a shop label describes a garment. The number on a pattern envelope describes a row in a table. Neither describes you — and the six measurements you take with a tape do. Start there and the rest of the process becomes arithmetic.
Brand figures are taken from each company's own published body-measurement charts, reproduced in this site's calculator and linked to their source; the worked comparison is read directly from those charts. Ready-to-wear sizing is set per retailer and is not standardised. Not professional fitting advice.