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The right replacement fluorescent tube is identified by more than its T-number. Check the lamp’s marked type, nominal length, base and wattage, then confirm the fixture or ballast supports it. T5, T8 and T12 are well-documented families; the other two formats below are included as common examples, not as a complete or universal size standard.
What does the T-number mean?
The letter T means tubular. The number gives the tube’s diameter in eighths of an inch: T5 is 5/8 inch, T8 is 1 inch, and T12 is 1 1/2 inches. Approximate metric equivalents are 16 mm, 26 mm and 38 mm. Metric figures are rounded, and the code tells you diameter—not length, base or electrical compatibility. Feit Electric explains the T-number system; the Hong Kong EMSD guide also gives approximate metric diameters.
Five tube formats you may encounter
These are useful examples for identification, not a definitive worldwide list of exactly five standardized sizes. The cited technical and catalog material establishes T5, T8 and T12; it does not establish universal dimensions or bases for T2 and T4, so check their actual markings and product documentation rather than assuming a fit.
| Format | Diameter | What to know |
|---|---|---|
| T2 | Not stated in cited sources | Confirm the exact lamp marking, length and base from the lamp or manufacturer documentation. |
| T4 | Not stated in cited sources | Confirm the exact lamp marking, length and base from the lamp or manufacturer documentation. |
| T5 | 5/8 in (about 16 mm) | The EMSD guide identifies a G5 base and notes that advanced T5 lengths and end fittings differ from corresponding T8/T12 models. |
| T8 | 1 in (about 26 mm) | Philips catalog examples use a medium bi-pin base. Some T8-for-T12 substitutions are described for particular switch-start circuits, but this is not a general compatibility guarantee. |
| T12 | 1 1/2 in (about 38 mm) | Philips catalog examples use a medium bi-pin base. Match the lamp to the fixture and ballast rather than relying on diameter alone. |
The T5, T8 and T12 dimensions and compatibility cautions are described in the EMSD guide; the dated Philips/Signify catalog contains product examples. Check the original lamp and fixture documentation, since bases, lengths and electrical requirements can vary within a family and by region.
#1 Best Overall
- Directly replaces traditional F32T8 or F40T12 fluorescent light bulb if compatible with existing ballast; simple installation
- If your ballast is incompatible or outdated, it can be easily bypassed (direct-wire for single/double-end power). Note: Ballast bypass requires basic electrical wiring; professional installation by a electrician is recommended if needed.
- Clear cover offers high light transmittance for maximum brightness; aluminum substrate ensures efficient heat dissipation; ideal for industrial or commercial applications
- Constructed with aluminum and polycarbonate for superior heat dissipation; backed by a 5-year manufacturer warranty
- 24W LED equivalent to 64W fluorescent light; reduces power consumption by 60%; saves up to 70% on electricity costs
How to identify the replacement tube
- Read the existing lamp. Look for its family code, nominal length, wattage and base designation. Feit’s linear-tube selection guide advises checking shape, size, length and diameter.
- Measure nominal length correctly. Philips/Signify says to measure from the back of one fixture socket to the back of the other, not simply the glass tube. See Measuring Lamps and Understanding Ordering Codes.
- Match the base and pins. A T-number does not identify the end fitting. T5 G5 fittings, for example, are not interchangeable just because a tube appears close in diameter or length.
- Check the fixture and ballast. Confirm the fixture documentation supports the exact lamp type and wattage. Do not treat a physical fit as proof of electrical compatibility.
- For an LED replacement, check wiring requirements. Establish whether the LED tube is compatible with the existing ballast or requires a wiring change. Feit lists a four-foot T8/T12 LED replacement with a G13 base; it is an LED product, not a fluorescent tube.
Can you replace one tube family with another?
Not by matching diameter alone. The EMSD guide discusses T8 replacing T12 in certain T12 switch-start circuits, while also noting different T5 end fittings and possible adapter needs for conversion. That older, Hong Kong-specific guidance is not a blanket rule for fixtures elsewhere. If the ballast type or fixture support is unclear, use the fixture or ballast manufacturer’s documentation before buying.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Before you buy
- Tube family and technology: fluorescent or LED retrofit.
- Nominal length, measured socket-back to socket-back.
- Base and pin arrangement.
- Wattage and any other markings on the existing lamp.
- Fixture and ballast compatibility, including any required wiring changes for an LED tube.
A search such as “four-foot T8 tube” narrows the options, but choose a replacement fluorescent tube only after matching the remaining specifications. The T-size by itself does not establish compatibility.
Quick Recap
Best Value
- 【WARM TIPS】- Please pay attention to the title information in our link before placing an order to select the lights that suits you. The t8 led bulbs 4 foot are compatible with both Type A + B and single-ended & double-ended power supply.Their installation methods are different. Improper operation may damage your lights and fixtures. If you have any questions, please contact us directly through Amazon.
- 【PLUG & PLAY OR BALLAST BYPASS】- JESLED T8 4Foot Type A+B LED light bulbs can be installed with a ballast( Plug & Play) or without a ballast (BALLAST BYPASS), making LED conversion fast, simple and secure. Check ballast list for compatibility.(Note: Our 4 foot t8 led bulbs don't compatible with dimming ballast, please do not use it with our lights.)
- 【SINGLE OR DOUBLE END POWERED】- JESLED 48inch LED bulbs are made for installation with a single-ended or double-ended connection. You can install this tubes directly into your fixture’s original sockets (tombstones), and will not need to retrofit your fixture, faster for install.
- 【BRIGHT, ENERGY SAVING】- JESLED dual row 4ft T8 LED tube can illuminate your space with 3200+ lumens (>133lm/w) and outstanding life rating of 50,000+ hours! Instantly economize 65% on your electricity bill when you replace 65watt old fluorescent tubes with JESLED high output 24watt 4ft LED light bulbs.
- 【HASSLE-FREE WARRANTY】 -JESLED offers 24/7 after-sale support, 60 days refund policy and 3 years hassel-free warranty. Please contact us directly if you meet any issue.
Rank #4
- 【 Fit Models 】: T8 10W replacement bulbs compatible with most 20W machine, such as Asp-ectek, Li-Ba, Micn-aron, etc.
- 【 Packing 】: We will get 6 pcs bulb. Bulb size: 13.5 x 1 x 1 inch (L x W x H) , 13.5 inches is the overall length with cap pin(without pin size is 13 inches)
- 【 365-nm wavelength】Professional 365-nm wavelength
- 【 Note: 】The light tube is fragile, if it's broken or doesn't work caused by shipping, please contact us to get a replacement.
- 【 Warranty 】 We provide high quality bulb, 1 year and 6000 hours warranty , if have any question, just contact our customer support
Rank #3
- Specification: T8(Tube diameter is 1-inch, not 1.5-inches) Fluorescent tube 48 Inches, 32 Watts, cool white 4100K light, 2850 Lumens, G13 Medium Bi-Pin
- Easy to install: Remove the old bulb and put in the new one, plug and play, we do not need re-wire the ballast
- Energy Efficient: Each bulb gives you an average long life of 15,000 hours, Using 75% less energy than incandescent bulbs with high energy savings
- Widely use: Ideal solution for commercial, industrial applications
- Customer support: in case of any broken during transit, please contact us for replacement
Rank #2
- F8T5/Cool White Pack of 2
- 12" Straight T5 Fluorescent Tube
- 12,000 Life Hours, G5 Mini Bi-Pin Base
- 4100K
- 8 Watt
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
