Wrap a thin strip of paper or piece of string around the base of the finger you want to measure. With a pen, mark the point on the paper or string where it overlaps, forming a circle on the finger. Measure the length of the paper or string using a flat ruler, from the starting point to the pen mark. Then find the matching size in the chart below. 
Don't measure cold fingers, as this is when fingers are the smallest. The best time to measure a finger is at the end of the day when the finger is the largest. Take at least two measurements for accuracy.
 
Size 4 = 1.840 Inches, 46.8 Millimeters
Size 5 = 1.940 Inches, 49.3 Millimeters
Size 6 = 2.041 Inches, 51.8 Millimeters
Size 7 = 2.141 Inches, 54.4 Millimeters
Size 8 = 2.242 Inches, 56.9 Millimeters
Size 9 = 2.342 Inches, 59.5 Millimeters
Size 10 = 2.443 Inches, 62.1 Millimeters
Size 11 = 2.543 Inches, 64.6 Millimeters
Size 12 = 2.644 Inches, 67.2 Millimeters
Size 13 = 2.744 Inches, 69.7 Millimeters