General Facts

1) Bulbs are measured in eights of an inch (1/8") at their widest diameter. For example, this 75PAR30 is 30 eighths of an inch in diameter, or 3.75 inches.


2) Maximum overall length (MOL) of the bulb is measured in inches from end-to-end. For example, this M400/U has an MOL of 11.5 inches.

3) A bulb's base is measured in millimeters. For example, this 35W 120V JDR Halogen Lamp has an E26 medium screw-in base, which measures 26mm in diameter.

4) A fluorescent bulb's base is measured from the center of one pin to the center of the other pin. For example, this G30T8 Germicidal Fluorescent Tube has a G13 bi-pin base, which measures 13mm from pin to pin.

EX39 Mogul Base

Fluorescent Tube Diameter