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Light bulb shapes and bases

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One of the famous light bulb shapes is called A60 but what does that mean? And what does the screw size E27 mean? This document tries to explain this and links to relevant sources.

Light bulb shapes

Different light bulbs, different shapes. There is a light bulb for every occasion and lamp holder. But not every light will fit every lamp holder. Some lampshades are just to small to fit a big light bulb in it. Other lamp holders are that big /tall that it easily could fit a bigger light bulb.

To help you find the right lamp, they have been given codes that are divided into categories. The letter(s) does mention the shape series. E.g. round shape, candle shape, reflector, etc. The most famous one is a regular round lightbulb as shown below. This is an A series light bulb.

Regular light bulb - A60 E27
Regular light bulb – A60 E27 – Source: Wikipedia

The numbers after the letter or letters do say something about the size of the light bulb but it is quite difficult to tell how this number is calculated so here is a quote of How-To Geek:

A bulb’s size is measured in 1/8-inch increments, so an A19 bulb would be 2-3/8″ in diameter because there are nineteen 1/8-inch increments. An easier way to determine the diameter, though, is to take the number and divide it by 8. So 19 divided by 8 is 2.375, or 2-3/8 as a fraction.

Source: How-To Geek, What Do Those Confusing Ligh Bulb Codes Mean?

Another example are Tx coils where the “x” stands for 2, 3 or 4. These are CFL light bulbs as shown below.

CFL light bulb
CFL light bulb – Source: Pixabay / nuzree

Specific for the A-series light bulbs you can find more information on Wikipedia:

To get to know more different light bulb shapes you can visit this page of with an overview of a few of the most common known series:

Light bulb bases

Screws, 2 connectors, small connector, big connector… There are many ways how a light fits in a fitting. A few commonly known ones are E27 and GU10 but what do these terms mean?

Again the letter or letters do stand for the shape category of the light bulb base. The E-series are screw connectors to fit in a screw fitting. The GU10 is a “twist & lock” base. The numbers after the letter(s) refers to the diameter of the base in millimeters. So the example of E27 above means a light bulb with screw connector with a base diameter of 27 millimeters. The GU10 is a twist & lock base with a diameter of 10 millimeters.

To get to know more different light bulb bases you can visit this page of with an overview of a few of the most common known series:

More information

How-To Geek have written a nice article about light bulb shapes and bases on which this Knowledge Base article is partly based. Have a read on the article to get to to know some more detailed information on this topic:

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