Do you have a size chart that shows Inches versus Metric measurements?

Yes, to help you select the correct size lamp shade, pls check below the Inches to Metric conversions:

5 inch   = 127 mm
6 inch   = 153 mm
7 inch   = 178 mm
8 inch   = 204 mm
10 inch = 254 mm
11 inch = 279 mm
12 inch = 305 mm
14 inch = 356 mm
16 inch = 407 mm
18 inch = 457 mm
20 inch = 508 mm