Short answer: the number after the "T" is the tube's diameter in eighths of an inch, so a T5 is 5/8 inch (about 15.9 mm) and a T8 is 1 inch (about 25.4 mm). They use different pin bases — T5 uses a G5 miniature bi-pin, T8 uses a G13 medium bi-pin — and different lengths, so a T5 and a T8 are not interchangeable. T8 LED tubes are the common drop-in retrofit for older fluorescent fixtures; T5 (and especially T5HO) is a compact, efficient choice for commercial high-bay and grow lighting. Which is "better" depends on your existing fixture and the job, not on the form factor alone.
What the "T" and the number mean
The code always starts with T and ends with a number. The T means the bulb body is tubular, and the number is the tube's diameter in eighths of an inch. So T5 = 5 eighths = 5/8 inch, and T8 = 8 eighths = 8/8 = 1 inch. The same naming covers both fluorescent tubes and their LED replacements — an "LED T8" simply has the same tube diameter and pin base as a fluorescent T8, so it can fit the matching fixture.
T5 vs. T8 at a glance
| Spec | T5 | T8 |
|---|---|---|
| Tube diameter | 5/8 in (15.9 mm) | 1 in (25.4 mm) |
| Pin base | G5 miniature bi-pin (5 mm pin spacing) | G13 medium bi-pin (13 mm pin spacing) |
| Typical lengths | Metric — e.g. ~1149 mm ("4 ft" nominal) | 2 ft, 4 ft, 8 ft (4 ft most common) |
| Typical LED wattage (4 ft class) | ~14–25 W standard, T5HO higher | ~12–18 W |
| Efficiency (lumens per watt) | Generally high; T5/T5HO are efficient | High for modern LED T8 |
| Common use | Commercial high-bay, grow lights, slim fixtures | General/office/home retrofit, huge installed base |
| Interchangeable? | No — different base and length | No |
Figures are typical ranges for LED tubes and vary by brand and model; always check the product spec sheet.
Diameter and base: the differences that decide fit
The most practical difference is physical. A T8 is 1 inch across and uses the G13 base with pins 13 mm apart; a T5 is 5/8 inch and uses the smaller G5 base with pins 5 mm apart. Because the bases and the tube lengths differ, you cannot drop a T5 into a T8 fixture or vice versa — the pins will not line up and the length is wrong. This is the first thing to check: identify what your fixture already accepts before comparing anything else.
Efficiency and brightness
Brightness is set by the tube's rated lumens, not by whether it is a T5 or T8, so compare the lumen figure on the box rather than assuming one form factor is brighter. That said, T5 and especially T5HO (High Output) tubes pack high output into a slim tube and are efficient per watt, which is why they are popular in high-bay and horticultural fixtures. Modern LED T8 tubes are also very efficient and typically replace a 32 W fluorescent T8 with roughly 12–18 W for similar light. To estimate the running-cost difference for your own fixtures, use the LED savings calculator and the electricity cost calculator.
Heat
A slim T5 concentrates its output and can run warmer at the tube surface than a T8, so in tight or touchable installations a T8 is the safer, cooler-running choice. LED tubes of either size run cooler than the fluorescent versions they replace, but a densely packed T5HO fixture still generates real heat that the fixture must dissipate.
Lifespan and cost
For LED tubes, rated life is commonly in the tens of thousands of hours (fluorescent T8 was often rated around 36,000 hours), and frequent on/off switching wears fluorescent tubes faster than LED. On upfront cost, T8 hardware is usually cheaper and more widely stocked because of its huge installed base; T5/T5HO fixtures can cost more but suit specific commercial layouts. Compare total cost over the fixture's life — not just the sticker — with the LED lifespan calculator.
T5 vs. T8 vs. T12
T12 is the oldest of the three: a 1.5 inch (38 mm) tube that also uses the G13 base (the same as T8). Because T8 and T12 share the G13 base, a T8 tube often fits a T12 fixture physically, though the ballast may need to match or be bypassed for LED. T12 fluorescents are the least efficient and have been largely phased out under energy-efficiency rules, which is why most retrofits move T12 fixtures to LED T8 tubes. In short: T12 → LED T8 is the common upgrade path, while T5 is a separate ecosystem with its own G5 base and fixtures.
Which should you use?
- Retrofitting an existing fluorescent fixture? Match what it already takes — most home and office fixtures are T8 (or T12 that accepts T8), so an LED T8 tube is usually the straightforward drop-in.
- Building a new commercial, high-bay, or grow setup? T5 / T5HO fixtures are compact and efficient and are a strong choice where they are specified.
- Either way, compare rated lumens and watts, confirm the base (G5 vs G13) and length against your fixture, and model the running cost with our free LED calculators before you buy.
For related choices, see how bulb types stack up in the bulb comparison tool.
FAQs
Are T8 and T5 LED tubes interchangeable?
No. They use different pin bases (T8 = G13, T5 = G5) and different lengths, so a T5 will not fit a T8 fixture or vice versa. Check the base and length your fixture requires first.
What's brighter, T5 or T8?
Neither is inherently brighter — brightness depends on the tube's rated lumens. A T5HO (High Output) tube can be very bright in a slim package, but a high-lumen LED T8 can match or exceed a standard T5. Compare the lumen rating, not the form factor.
Can a T8 tube work with a T5 ballast?
No. The G5 base and shorter length of a T5 fixture do not accept a T8 tube, and a T5 ballast is not designed for T8. For LED tubes, also check whether the tube is ballast-compatible, ballast-bypass (direct wire), or hybrid.
Is T5 or T8 more energy efficient?
Both modern LED versions are efficient. T5/T5HO tubes are known for high output per watt in a slim form, while LED T8 tubes efficiently replace 32 W fluorescent T8s at roughly 12–18 W. Compare the lumens-per-watt figure of the specific tubes you are considering.
Conclusion
T5, T8, and T12 differ first in physical size and base — T5 (5/8 in, G5), T8 (1 in, G13), and T12 (1.5 in, G13) — which decides what fits your fixture before efficiency or price enters the picture. For most home and office retrofits, an LED T8 tube is the practical drop-in; T5/T5HO is the efficient choice for compact commercial and grow fixtures. Confirm the base and length, compare rated lumens and watts, and estimate the running cost with the LED savings calculator before deciding.
Jack Shi
Founder & editor, LEDaskJack Shi builds and writes LEDask, an independent LED-lighting tools project operated by clooms. He designs the calculators, checks their formulas and reference values against published engineering data, and writes the guides across the site.
