String Gauges

Below are the Tennis Industry Association's gauge standardization specifications. The string gauge is usually measured with the string under tension. For example, a string that measures 1.26 mm unstrung would appear to be either a 16 or 16L. When strung, however, the diameter might become 1.21, which would make it a 17-gauge string.

U.S. Int'l. Diameter
13 12 1.65-1.80 mm
14 11 1.50-1.65 mm
15 9.5 1.41-1.49 mm
15L 9 1.33-1.41 mm
16 8.5 1.26-1.34 mm
16L 8 1.22-1.30 mm
17 7.5 1.16-1.24 mm
18 7 1.06-1.16 mm
19 4 0.90-1.06 mm
20 3.5 0.80-0.90 mm
21 3 0.70-0.80 mm
22 2.5 0.60-0.70 mm