A pattern block is the base set of measurements a house drafts every garment from — chest, waist, shoulder, sleeve and length, in fixed proportion to one another. Two jackets can carry the same size label and be drafted from different blocks, which is why a correct numeric conversion still leaves room to be wrong about the fit.
Start by separating two questions that get asked as one. The first is arithmetic: what US size does this Italian number correspond to? That is settled, it does not vary by brand, and our size guide has the tables for jackets, dresses and shirts. The second question is the one that actually causes returns: given that the conversion is correct, will the garment fit the way you expect? Italian labels do not, as a rule, run small by the numbers. They run differently in shape, and the difference is concentrated in three places — the chest, the shoulder, and the length of the body.
Italian tailoring cuts closer through the chest and higher under the arm than most US ready-to-wear, which buys mobility but reads as tight to someone used to a roomier armhole. It also tends to run shorter in the jacket body, a proportion that suits the Italian silhouette and surprises a taller buyer who converted the size correctly and still finds the hem sitting above where they expect. Shoulder is the third variable and the least forgiving: a shoulder that is too narrow cannot be let out, and a shoulder that is too wide is expensive to correct. Chest and waist are comparatively easy for a tailor. Judge a secondhand jacket on the shoulder first.
The useful mental model is that "Italian sizing" is not one standard. Canali and Corneliani both draft to a fairly generous block by Italian standards — closer to what a US buyer expects, which is part of why both travel well into the American market. Kiton and the Neapolitan houses cut leaner through the waist and shorter in the body. Zegna sits between the two, and has moved over the years, so a jacket from a decade ago and a current one at the same marked size are not reliably interchangeable. Spread across those houses, the variation is larger than the gap between an Italian block and a US one.
Everything above concerns tailoring. Italian knitwear behaves differently, because a sweater has no structure to fight you and the fit tolerance is wide. The question worth asking of a secondhand Italian knit is not whether it runs small but whether it has been washed — a cashmere sweater that has been through a machine cycle has felted slightly and will measure shorter in the body and tighter in the sleeve than the same piece unwashed. Read the measurements the seller gives rather than the size on the label. We cover what to look at in how to inspect a secondhand cashmere sweater, and the brand-level cases in does Loro Piana run small.
Three measurements settle nearly every case, and any seller who has the garment can take them in a minute. Shoulder seam to shoulder seam across the back. Chest, measured flat across the garment an inch below the armhole and doubled. Length from the base of the collar to the hem. Compare those three against a jacket you already own and like, not against a size chart. A converted number tells you which listings to open; the measurements tell you which one to buy.
