Light Bulb Buying Guides

How to Measure Light Bulbs

how to measure light bulbs

Measuring your light bulbs is a simple process, and it’s one that’s important if you’re trying to replace a bulb and have lost the original packaging or are in the process of upgrading to energy efficient LEDs. After all, you need to be sure that the new bulb you select is going to fit into your fixture.

If you’ve never had to measure a light bulb, you may be wondering what measurements you need to take, and where exactly to start your measurements from. Let’s take a look:

Measuring the Bulb

The glass or plastic upper part of the light bulb is measured and labelled in millimetres or eighths of an inch.

There are two types of measurements you’ll need from this area:


To correctly measure the length of your light bulb, measure from the top of the glass or plastic bulb to the very tip of the base using a ruler or measuring tape. If you are using a soft measuring tape, keep it taut rather than following the edge of the bulb—you want the measurement of the bulb’s height, not its surface length.

PRO TIP: When measuring the length of light bulbs such as fluorescent tubes, take your measurement from the tips of the pins in either end.


Each light bulb has a code that indicates its shape and diameter at the widest point. For instance, an A19 code indicates that it’s a classic A-series light bulb with a diameter of 19 eighths of an inch (2.38 inches or 60.33mm). To get the correct width measurement for your bulb, you also need to measure it at the widest point, which will vary based on the type of bulb you choose. Here are some examples:

Measuring the Base

Regardless of the type of base a light bulb has, such as a bayonet or screw, the base is measured in millimetres. Light bulb base measurements will be labelled using a code similar to the one used for the bulb. For example, a standard light bulb base is an E27, which means that the base is an Edison screw base that measures 27mm.

Screw and bayonet bases are measured right across the bottom. Pin bases, including fluorescent tubes, measure the space between the pins.

Measure Twice

Some lamps can be difficult to fit simply because they don’t have much space allotted for the bulb, but so long as your new bulb is close in size to your old one, you shouldn’t have any problems. Just make sure you keep these light bulb measurements handy:

  • Length
  • Diameter
  • Base measurements

Whether you need help figuring out the proper size, colour, or type, if you need assistance selecting a light bulb, we can help! Get in touch with us through live chat, email ([email protected]), or over the phone (+44 (01869) 362222).

Learn more about selecting the perfect bulb for your lamp in our light bulb buying guides.


About Nicky Walker

Nicola is a lighting specialist at The Lightbulb Company with a keen awareness of the important role lighting plays in our everyday lives. She enjoys discovering new ways in which lighting can affect science, mental health and of course, most of all, reduce our carbon footprints.