Find which Dior sizing system is printed
Dior ready-to-wear uses more than one regional convention. Women's clothing commonly appears with French sizes, while men's pieces can use letters with regional equivalents. The product page or inner label should be read before any conversion takes place. A letter is not automatically an Italian or US number, and a French 38 is not an Italian 38. Once the system is known, compare the finished garment. The number alone describes the label; the chest and length describe what happens when the cotton jersey is laid out.
Relaxed jersey can be wide and short
A Dior relaxed jersey piece can add most of its extra room across the chest while keeping the body length close to the previous size. The published Oblique relaxed-fit sweatshirt rows show a 36 cm chest increase from the smallest to largest size against only 7 cm of length. A T-shirt may use a different pattern, but the principle is the same: width and length do not move at the same rate. Do not choose a larger size solely to gain 1 cm of hem. Ask whether the T-shirt is cut straight, boxy or tapered, and compare the finished chest with a shirt whose volume you like.
Neckline and shoulder seams complete the picture
A crew neck can appear close even on a wide body if the shoulder seam lands on the natural point. A dropped shoulder lets the sleeve hang farther out and can make the armhole feel roomier without changing the chest. Ribbed neckline depth also matters: a narrow binding holds the opening, while a deeper rib changes how the shirt sits under a jacket. Two Dior T-shirts with the same chest can therefore be useful for different combinations. The front photograph shows the neckline; the back shows shoulder and upper-back construction; the label confirms the size.
A flat-check request that removes the guess
Provide a T-shirt you already wear with its flat chest doubled, shoulder width, centre-back length, sleeve and hem width. Add the Dior marked size, the fit word and a clear note about whether you want a close or relaxed result. If the shirt will be worn under a jacket, include the jacket's shoulder measurement so the layers are judged together. This is more precise than converting between regional letters, and it produces a specific answer even when Dior's size table is not published for the exact style.
The cleanest decision rule
Choose the size by shoulder first when the T-shirt will be worn alone, then by chest when it will be layered. Check length before committing because a hem that is clearly too short or too long changes the look every time you wear it. The neck opening and sleeve width can be adjusted less easily than a slightly loose body. When two Dior shirts sit close on paper, compare those four locations in the same order, and ask for a second chest measurement if the jersey surface makes the first reading uncertain.