Most people buy shoes half a size too small, because they measured in the morning, sitting down, in thin socks.
Do it in the evening, standing, with the socks you will actually wear. Put your heel against a wall, mark the longest toe on a sheet of paper, and measure from the wall to the mark. Do both feet: one is usually longer, and that is the one that decides.
Then add about a centimetre. That gap is what stops your toes hitting the front going downhill, and it is the difference between a shoe you wear and one you keep meaning to sell.