It is recommended to measure the foot at the end of the day instead of in the morning. This is because the activity you perform throughout the day usually widens your foot slightly.
MIDE DEL TALÓN A LA PUNTA DEL DEDO: for this you can place a piece of paper on the floor with one of the edges attached to the wall. Pour the paper keeping your heel glued to the wall. With a pencil, make a mark at the end of your thumb. Now measure the distance from the mark you have traced to the end of the edge of the paper and compare it to the official guide you will see on the table below.
What if you're right between two sizes? In that case it is recommended that you choose the largest one.