I am a EUR 42 in shoes ( US 8.5 ) I have to take a half size up in my adidas 42 2/3 or US 9 for soccer shoes and one size up 43 1/3 or US9.5 on sneakers.
The reason is simple.
Sports shoes are sized right. so a 42 is 26cm long in the inside and not like my dress shoes that are longer, as you have often designs with an enlongated ( or square
) toe box
in fact most dress shoes in size 42 are made to fit a foot 26 cm long like mine, so they are made wider and longer than that, so the 26cm foot is comfortable inside.
Sports shoes are sized techically. So a size 42 are made 26.5 cm long on the inside, so I can put them on, but I could not walk on them as my big toe would be touching the end of the shoe.
Normally you wear thicker socks when performing sports so you'd need to size up the shoe, some sports like soccer or cycling need an snug fit at the toes so you size up 1/2 size ( so I have to take a 27cm or 42 2/3, US9 shoe ) to be able to play well, for other sports like basketball or tennis in wich you have to jump a lot I need another 1/2 size up or my big toe would be hitting the end of the shoe when I jump forward for example, so I take a 27.5cm shoe or EUR 43/3 US 9.5