The LV scale is a UK scale
An LV size is a UK number wearing a different name. LV 4 is a UK 4 with an EU 38 and a US 5; LV 8 is a UK 8 with a French/EU 43 and a US 9; LV 11 is a UK 11 with an EU 46.5 and a US 12. The men's menu runs from LV 4.0 to 16.0 with half steps through the middle, and the women's menu is published the other way round, on EU 34.0 to 42.0 with half steps to 39.5. That means the same shoe can be described by two different numbering systems depending on which menu it sits in, and a listing that quotes "size 43" without saying which ladder produced it is ambiguous by a full step. Read across the published rows and the US column is the LV number plus one throughout, while the EU column takes alternating one- and one-and-a-half-centimetre steps.
"Runs large" is printed per reference
Louis Vuitton writes the fit into the product copy, and the instruction is attached to the colourway rather than to the model. Most LV Trainer references carry one size down; reference 1A9JGN, a Monogram denim and grained calf pair, carries one and a half sizes down; the LV Trainer Maxi carries half a size down; and the LV Skate and LV Tatic Runner carry one size down. The reason is the material and the last: a stiff grained calf pair and a softer denim pair are cut on the same pattern but do not sit the same way on the foot, and the deliberately oversized Maxi is built with extra volume the brand treats as a separate fit question. Two references in one family can therefore carry advice a full size apart. Within one family the printed advice runs from half a size to a size and a half — a full size of swing between references that share a name.
The published foot-length tables disagree
Two widely republished LV foot-length tables differ by up to a centimetre at the same LV size: LV 8 is 26.9 cm on one and 27.5 cm on the other, LV 9 is 27.7 cm against 28.5 cm, and LV 11 is 29.4 cm against 30.5 cm. One table is sourced to the brand's own guide, the other to a competitor's Trainer sheet, and both are live. The EU column is stable across them; the centimetre column is not. That is why the useful number is a measured foot rather than a converted label — measure your foot against a wall, find it in the column you trust, and treat the size-down instruction as a label adjustment on top of that. Set side by side, one column climbs exactly 1 cm per LV size while the other climbs roughly 0.8 to 0.9 cm, so the two are identical at LV 4 and the difference accumulates with size.
What an outsole can and cannot tell you
A measured LV Trainer shows an outsole roughly 3.5 cm longer than the foot its size is meant to carry, which is normal for a sneaker with a thick midsole and a rubber bumper: the outsole length is the mould, not the fit. So a listing that measures the shoe from heel to toe will always overstate what it fits, and a listing that measures the insole is closer to useful. Ask for the insole length and your own foot length in centimetres, plus the reference or colourway, and the number falls out of the table without a second round of questions.