For fail proof measure please take the time to measure your foot.
Put a piece of paper on the ground next to a wall
Standing firmly on the piece of paper with your heel slightly touching the wall, mark the paper where your foot is the longest (most likely at the tip of your big toe)
With a ruler, measure the distance from the wall to the mark on the paper (indicating the longest part of your foot)
Take that measurement and match it with a measurement in the “Inches” column in the size chart.
Wear socks when measuring your feet
Measure your feet at the end of the day, as your feet tend to expand throughout the day
Measure both of your feet, as one may be bigger than the other