The heel height is the first sizing decision
The J'Adior slingback ships as a flat, a 6.5 cm comma heel, an 8.5 cm open-toe slingback sandal and a 10 cm comma heel, and the four are not one reference. Published weights move with the material as much as with the height: 168 g for the houndstooth tweed 6.5 cm, 170 g for the black technical fabric 6.5 cm and 189 g for the nude technical fabric 10 cm. Heel height changes where the foot sits inside the shoe, which is why the same labelled size can feel different across the heights — a 10 cm comma heel pushes the foot forward, so the toe box is the part that reports the size first, and a flat leaves the heel to do the reporting.
The strap does not adjust
The slingback strap is a fixed embroidered cotton ribbon finished with a flat knot bow. It does not buckle and it does not stretch, so the strap sitting flush against the heel is the fit test: if it hangs loose at the labelled size, going down is the answer and going up will not help. This is the most common reason a Dior slingback is returned at a nominal size that was technically correct, and it is why the printed half-size-larger note is attached to foot width rather than to length — a wider foot needs volume, and the ribbon cannot give any.
B30: true to size, with charts that disagree
Dior's B30 Countdown product pages state that the style fits true to size and recommend your normal shoe size, and the model is sold from IT/EU 35 to 49. The published foot-length column is where the agreement stops: at EU 45 the figure runs from 28.6 cm to 30 cm depending on which B30 chart you read, and at EU 46 the gap reaches 1.7 cm — wider than one full size step. A measured EU 42 pair gives a 28 cm insole and a 31.5 cm outsole, which is the clearest demonstration that insole length and foot length are two different measurements. Size from your foot in centimetres against the EU column rather than from a US label.
What to send with a Dior shoe question
Three things settle it: the reference or style name, because the fit line is published per model rather than per house; your foot length in centimetres, measured from heel to longest toe against a wall; and whether you want a close fit or room for a thicker sock. On the J'Adior that is enough, because the fixed ribbon strap makes the decision binary — it either sits flush or it does not. On the B30 it also tells us which published chart the piece was made against, which is the only way to reconcile the differing centimetre columns.