Light Bulb Measurements Explained

Measuring Light Bulbs and Luminaires – LED Lighting University by EagleLight LED Lighting

How to Identify and Measure Light Bulbs

Light bulbs are measured in several ways.  This lesson covers how to perform light bulb measurements to aid in selecting LED light bulb replacements. Here are some of the types of measurements for LED light bulbs:
These are just a few of the ways of measuring light bulbs and the light they produce.

Measuring Physical Dimensions of an LED Light Bulb

The physical dimensions of an LED light bulb include:

  • LED bulb shape
  • LED bulb diameter
  • LED bulb length
  • LED bulb base type

These are the discussed in more detail in the specific subjects in EagleLight’s LED University.

LED Light Bulb Shape

Light bulb shapes describe the general physical characteristics of the light bulb. LED light bulbs vary in shape from tube shaped bulbs to funnel or pear shaped light bulbs. These LED light bulb shapes each have a code. For example: T designates a ‘tube’, PAR signifies a ‘parabolic aluminum reflector’ and MR is used for ‘metallic reflective’ typical of many track lights.

LED Bulb Diameter

The diameter of a light bulb is measured in two ways, in metric it is the number of millimeters and in imperial measure if the the number of 1/8 inch increments. The diameter for light bulb shape is measured at the point of maximum size. Examples are: T12 is 12 eights of on inch or 12/8 = 1.5 inches in diameter, PAR20 is 20 eighths of an inch in diameter or 2.5 inches, MR16 is 16 eighths = 16/8 = two inches in diameter.

LED Bulb Length

LED light bulb length is the overall length measured, typically from electrical connection, or base, to the tip of the bulb, or in the case of T bulbs from end to end. This can be in metric or in inches or feet. In some cases the stated length may differ from the actual length like in the case of some four foot T8 florescent replacements, these may actually be slightly less in length.

Bulb Base Types

LED light bulb base types are typically measured by type and diameter in millimeters. For example a typical Edison medium screw-in base like E26 is 26 millimeters in diameter. LED bi-pin bases, like a florescent tube replacement or a bi-pin MR16 track light, are measured between the centers of each pin.