How to Measure a Fluorescent Tube

Replacing or ordering new fluorescent tubes may seem simple – until you discover that there are many different sizes, sockets, and variations. To avoid ordering the wrong product, it’s important to know how to measure your fluorescent tube correctly. The dimensions affect not only the fit in the fixture but also brightness and energy efficiency. In this article, we go through step by step how to measure a fluorescent tube and what you need to consider before buying a new one.

Why Is It Important to Measure a Fluorescent Tube?

When replacing a fluorescent tube, it’s crucial to measure the correct length and diameter, as different fixtures are designed for specific standards. A tube that is too long or too short simply won’t fit, and even small differences can cause problems.

In addition to the dimensions, identifying the correct socket type is also essential, as different connections require different fittings. Forcing a tube that doesn’t fit may damage both the fixture and the tube, which in the worst case can cause electrical faults or break the glass. By measuring carefully, you avoid unnecessary returns, downtime, and you ensure a safer environment where the lighting works as it should.

Different Fluorescent Tubes in Different Sizes

Fluorescent tubes come in various lengths and thicknesses. Here are some of the most common types on the market today:

T8 Fluorescent Tubes

T8 is the “classic” variant that has been around for a long time and is still widely used in offices, schools, warehouses, and retail spaces. They are 26 mm thick and come in many lengths and color temperatures (e.g., warm white, neutral white, cool white). They are relatively inexpensive and provide even light, but they use more energy than newer options and have a shorter lifespan than LED. Many T8 fixtures use magnetic ballasts, which can cause slight flickering.

T5 Fluorescent Tubes

T5 is a more modern version of T8 and slimmer at only 16 mm in diameter. They are more energy-efficient and can produce higher light output at the same length compared with T8. T5 tubes are common in newer buildings focusing on energy efficiency. They require specific fixtures designed for T5 and cannot be used as a direct replacement for T8 in older holders. T5 tubes also often perform best at a specific temperature (around 35°C), making them better suited for indoor use rather than cold environments.

Compact Fluorescent Lamps (CFL)

Compact fluorescent lamps are a “mini version” of long fluorescent tubes, often bent or spiral-shaped to fit smaller fixtures. They became popular as replacements for incandescent bulbs due to their low energy consumption and long lifespan. They come in many socket types and can be used in everything from small lamps to spotlights. A disadvantage is that they can take a few seconds to reach full brightness and contain a small amount of mercury, requiring proper recycling.

LED Tubes

LED tubes are not traditional fluorescent tubes but use LED technology, shaped to fit fluorescent tube fixtures. They are the most modern option, consuming 50–70% less energy than T8 and T5. They switch on instantly without delay or flicker and have a much longer lifespan (often 30,000–50,000 hours). They also contain no mercury. LED tubes are available as retrofit versions that fit in certain existing fixtures or versions that require re-wiring.

How to Measure the Length and Diameter of a Fluorescent Tube

Follow these steps before ordering a new tube:

1. Turn off the power and let the tube cool
Always switch off the light and preferably the circuit breaker to ensure no current is present. Fluorescent tubes can get warm, so let them cool before handling.

2. Remove the fluorescent tube
Grip it gently at both ends. Rotate it a quarter turn until the pins are vertical. Then pull it out from the holders. Be careful, as the glass is fragile.

3. Measure the length of the tube
Place the tube on a flat surface and measure from one metal end to the other. You don’t need to include the pins—measure the tube’s body. Common T8 lengths include around 60 cm, 120 cm, or 150 cm.

4. Measure the diameter
This helps you determine the tube type. A T5 tube is about 1.6 cm in diameter, while T8 is about 2.6 cm. A simple tape measure or visual comparison is usually enough.

5. Check the socket
The socket is where the pins sit. If the distance between the pins is 5 mm, it is usually a T5 tube (G5 socket). If it is 13 mm, it is typically a T8 tube (G13 socket).

6. Read the markings on the tube
Most fluorescent tubes have printed text showing wattage, color temperature (e.g., 830 = warm white, 840 = neutral white), and sometimes lumen output.

7. Note the fixture type (if switching to LED)
If switching to LED, check whether your fixture uses a starter. A small cylindrical starter usually means an older magnetic ballast; no starter often means an electronic ballast. This determines which LED tube is compatible.

How to Measure a Compact Fluorescent Lamp (CFL)

1. Turn off the power
Switch off the lamp and circuit breaker. CFLs can be warm, so wait before touching them.

2. Remove the lamp
Pull straight out of the socket (no twisting needed). Hold the plastic or metal base, not the glass.

3. Identify the shape
CFLs may be U-shaped, quad-tube, or 2D-shaped. The shape helps identify the correct replacement.

4. Count the pins and check the socket
For example:

2 pins (e.g., G24d)
4 pins (e.g., G24q, 2G11, GR10q)

Write down the pin count and layout.

5. Measure the length and width
Measure from the base to the top. For 2D tubes, also measure the width.

Common Mistakes When Measuring Fluorescent Tubes

Including the pins in the measurement: Only measure the glass tube plus metal ends.

Forgetting to measure the diameter: Length alone isn’t enough; T5 vs. T8 matters.

Ignoring the socket type: Pin spacing must match the fixture.

Not reading the tube’s markings: Wattage and color code are important.

Assuming LED replacements always fit: Some require LED starters or rewiring.

Not prioritizing safety: Always turn off power and handle the tube by the ends.

We Help You Find the Right Size

Need help identifying the correct tube for your facility?
Our experts at Aura Light can guide you to the best solution based on your needs, space, and energy goals.

Feel free to contact us for advice — we’ll help you choose the right tube that combines quality, efficiency, and sustainability.

Frequently Asked Questions (FAQ)

How do I measure the length of a fluorescent tube?

Measure from the outermost pin on one side to the outermost pin on the other. This gives the standard length, e.g., 120 cm.

Should I measure only the glass or the entire tube?

Include both the glass and the end caps. Otherwise, you might order the wrong size.

Do I need to measure if the markings are readable?

If the marking is clear, it’s usually enough. If worn or uncertain, measuring manually is safest.