Learning Center

Learn How To Measure

Measuring Incandescent, Halogen, CFL and HID Lamps
Letters designate the shape of the glass bulb and numbers indicate the diameter of the bulb in eighths of an inch


For Example:
   "A19" indicates a standard bulb having a diameter of 19/8 or 2-3/8 inches. A19 Lamp
   "T10" indicates a tubular shaped bulb having a diameter of 10/8 or 1-1/4 inches. T10 Lamp
   "MR16" indicates a mini reflecter having a diameter of 16/8 or 2 inches. MR16 Lamp
   "ED37" indicates a large HID bulb having a diameter of 37/8 or 4-5/8 inches. ED37 Lamp

To determine the length of a fluorescent lamp, you do not measure the bulb. The nominal length of the bulb is the measurement from the back of the socket on the fixture

Normal Length

To determine the type of lamp you need, measure the endcap and use the illustration below as a guide

Type of Lamp