T style bulbs are named by the diameter of the tubular bulb in ⅛" units.
| T 1½ | 3/16" | .1875" | 4.76mm |
| T 1¾ | 7/32" | .2187" | 5.55mm |
| T 2¼ | 9/32" | .2812" | 7.14mm |
| T 2¾ | 11/32" | .3437" | 8.73mm |
| T 3¼ | 13/32" | .4062" | 10.32mm |
| T 5 | 5/8" | .625" | 15.84mm |
| T 6 | 3/4" | .75" | 19.05mm |
Wedge based is measured by the thickness and the width.
e.g.: a W2x4.6d is 2mm thick and 4.6mm wide