Open your hand and place a tape measure around your base knuckles (do not include the thumb). Now make a loose fist and measure the circumference:
If you do not have a tape measure at your disposal, you can use a thread to take the measurement and then determine the length of the thread using a ruler.
I have wide feet and this were a little snug the first few days of wearing but not too tight. Now after a bit of wear, they are exactly what I was hoping for. Perfect fit and so comfortable, no blisters and great durable leather