How to Measure a Light Bulb

Light bulbs are measured in several ways. Physical dimensions, amount of light output, wattage consumption are just a few. This post will focus on the physical dimensions of light bulbs and the differing types of bases.

There are four ways of measuring a bulbs physical dimensions:


1) Bulb shape: bulb shapes vary, from pear shapes to funnel shapes to tube shapes. Each has a designation. Examples include: PAR for parabolic aluminized reflector or T for tube. Many household halogen lights are a MR16, the MR is for 'metallic reflective'.


2) Bulb diameter: bulb diameter is measured in 1/8 inches increments at the maximum point of diameter. A PAR30 is 30 eighths of an inch in diameter or 3 3/4 inches. A T8 is 8 eights of an inch in diameter or 1 inch. MR16 is 16 eighths or 2 inches in diameter.


3) Bulb length: bulb length is measured in inches as the Maximum Overall Length (MOL). So a 4 foot florescent tube has an MOL of 48 (inches).


4) Bulb base type: Bases are measured in millimeters. So a typical household screw-in base like E27 (E is for Edison Screw-in) is 27 millimeters in diameter. An E11 candelabra base is an 11 millimeter diameter Edison screw in base. For pinned bases like a florescent tube or a pinned MR16 halogen bulb, the measurement is from the center of one pin to the center of the other pin.