Halogen Bulb Sizing Guide: T3 vs T4

When seeking a replacement for light bulbs in a home or commercial environment, investing in either a T3 or T4 light bulb is one of the best ways to illuminate the home, office, car and any other dark space. 

Usually, many people do not realize the shape or size of light bulbs that correctly fit their light fixtures until the bulb gets burnt out and it’s time to do some replacement. That is where a sizing guide comes into play. 

The T-series bulbs come in a cylindrical tube form and can effectively transfer power from one end to another. Are you looking to purchase the T-series halogen bulbs but unsure how to go about the sizing? Then, here’s a comprehensive sizing guide for the T3 and T4 sizes that will enable you to make faster buying decisions. 

Halogen Bulb Sizing Guide: T3 vs T4

Sizing Chart for T3 and T4 Bulbs

Tube TypeT3T4
Diameter (mm)9.512
Diameter (inches)3/81/2
Size chart for T3 and T4 halogen bulbs

What Size is a T3 Bulb?

The T3 halogen bulb belongs to the T-series halogen bulb family. This bulb is tabular in shape, and like the T4, it is also known as the peanut bulb. However, it is smaller than the T4 halogen bulb. The diameter of the t3 halogen bulb is 3/8 inch, and it’s measured in 1/8  inch.

What Size is a T4 Bulb?

Due to their compact design, T4 halogen bulbs are sometimes known as peanut bulbs. With a diameter of 1/2 inch, it is easy to fix during installation. It is a beautiful choice for confined places that require a clear focus without much applicability to the surroundings, such as a work area or a storage cabinet. This tabular halogen bulb’s portability also makes it cost-effective.

How Long is a T3 Halogen Bulb? 

A t3 halogen bulb’s usual length is 3.071 inches. It’s compact yet adaptable for illumination applications such as lamps and floodlights.

How Long is a T4 Halogen Bulb?

Although these halogen bulbs can come in various styles, specifications, and even base types, their standard length is 1.25 inches.

Are All T3 Bulbs the Same Length?

Looking at its various types, you will find that not all T3 bulbs are the same length. However, there’s a noteworthy detail: their Maximum Overall Length (MOL) falls between 1.05″ and 1.72″ inches. 

When it comes to T3 halogen bulbs, another feature to consider is the filament design. They also come in C-6, C-8, C-2V, C-2F, C-2R, and 2C-2R filament designs.

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Are all T4 halogen bulbs the same length?

T4 halogen bulbs are available in various lengths, wattages, and bulb bases, with the latter distinguishing them more than the former.  Some T4 halogen bulbs are built for specific applications, such as fitting into a bulb or being suitable for a desk lamp.

As you may already know, T4 halogen bulbs are mostly tabular in shape, but they do not all come in the same length.

A 500-watt T4 halogen bulb, for example, has a Maximum Overall Length of 3.50 millimeters and a center length of 2.37 millimeters.

How do I Identify a T3/T4 Halogen light bulb?

Identifying a bulb is quite simple. All you need to do is read the label on the base, as that’s where all information regarding bulbs is shown. 

Unfortunately, this is not always the case with every bulb, so knowing the characteristics and properties of the bulb you are looking for can be so helpful. 

Here are some details that can be used to identify T3 and T4 halogen light bulbs quickly.

The Bulb Base Type 

Knowing the base type of the T3/T4 halogen bulb you are looking for is the first step in the right approach.

As mentioned earlier, it’s not usually easy to spot T3/T4 halogen bulbs, but you will find all the information you need near their base area. 

One thing you must note about T3/T4 halogen light bulbs is that they come in a variety of base types, ranging from pins to plugins and a host of other bases.

A common T4 halogen bulb base type is the candelabra, whereas the T3 base type is the RSC base. So, while searching for a T3/T4 halogen light bulb, always look out for the base.

The Pin Spacing 

This part is tricky because you might need to measure the distance between the pins to figure it out. 

For example, if the bulb has a G base and an 8-pin spacing, the T4 halogen bulb transforms into a G8 T4 halogen bulb. If the pins’ diameter turns out to be 8 millimeters, the letter identifying the bulb’s base will have that number behind it.

Aside from these two factors, it would be best if you also considered the voltage and wattage of your T3/T4 halogen bulbs. You also need to check these factors match the fixture in which your bulb will be used.

How are T3/T4 Halogen Bulbs Measured?

A T3/T4 halogen bulb is simple to measure because it is already upright in a tabular configuration, similar to conventional light bulbs. 

To measure t3/t4 halogen bulbs, a ruler or a measuring tape may be all you need. The only downside is that certain locations must be calculated before making a final selection.

Let’s look at the three most essential parts of the T3/T4 halogen bulb that you should be aware of.

1. Bulb Length

This is the length of your T3/T4 halogen bulb from top to bottom. In essence, while measuring the size of your bulb, start at the top and work your way down to the tip of the base. 

Any number you arrive at after measuring the exact length of your bulb.

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2. Bulb Width

To measure your halogen bulbs, place your tape or ruler in the center of the bulb, near the surface, and make sure it reaches from one side to the other. The result you get is the width of your light bulb.

3. Bulb Base

Most T4 halogen bulbs have pin bases. To ascertain the size of the base, you can measure from the tip of one pin to the other.

That is not the case with T3 halogen bulbs, especially if you measure in opposite directions. This means that to get the actual base of the T3 bulb, you might need to measure it separately.

What is the Difference Between a T3 and T4 Halogen Light Bulb?

These two halogen bulbs are identical in that they both belong to the T-series. They are both tabular-shaped bulbs. Both halogen light bulbs, however, have variances.

Bulb Diameter

The first noticeable difference is the diameter, with the t3 halogen light bulb having a diameter of 3mm and the T4 halogen light bulb having a diameter of 4mm.

Bulb Wattage 

In addition, the wattage of these two light bulbs is different. A standard T3 halogen light bulb has a wattage of 5 to 20 watts, while a T4 halogen light bulb has 35 to 100 watts.

Base Type

While the T3 halogen bulb functions on the G4 base size, the T4 halogen light bulb uses the GY6 base size.

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Can I Replace my T3 and T4 Halogen Bulbs With LED?

Of course, you can separately replace t3 and t4 halogen bulbs in many cases. Doing this allows you to save energy and enjoy light performance much better. 

How do I know what size tube light I need?

It is unlikely that you will get the chance to measure the light bulb you are about to buy to figure this out. This is why most bulbs are designed to display the measurement at the bulb’s base, so all you have to do is look at the bulb’s base.

The brand and diameter of the light bulb are always listed next to each other, but if the bulb you want to buy or replace does not have this information, you should ask the vendor. If you have the opportunity to measure the bulb yourself, use a measuring tool, such as a ruler or tape to determine the diameter. It is measured in eighths of an inch.

Aside from the diameter, the base size is another aspect of the bulb that must be measured. This can be done by first determining the width of the base, then the voltage and wattage that fits your existing or new fixture. 

When you have determined all of this, you will be able to deduce the size of tube light you will need for your home.


With this guide, you now know everything there is to know about type T halogen bulb led replacements for your home as well as how to find them in the proper sizes and fittings for your T-series halogen bulb installation.

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If you would like to recommend your favorite type T halogen bulb to others, please leave a comment below.

We hope you found this information helpful.

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