T5 vs T8 vs T12: Tube Diameters and LED Replacement Options

If you've shopped for tubes, you've seen the codes T5, T8, and T12. They confuse a lot of people because they don't refer to length or brightness — they describe the tube's diameter. Understanding them makes choosing an LED replacement easy.

The "T" Number Is Diameter in Eighths of an Inch

The number after the T is the tube's diameter measured in eighths of an inch:

  • T5 = 5/8" (about 16 mm) — the slimmest, modern high-efficiency tube.
  • T8 = 8/8" = 1" (about 26 mm) — today's standard, by far the most common.
  • T12 = 12/8" = 1.5" (about 38 mm) — the old, fat tube being phased out.

So a "4-foot T8" is a 1-inch-diameter tube that's four feet long. Length and diameter are separate specs — you need both.

T12 — The Old Standard

T12 tubes are the thick, legacy fluorescents found in older shops, garages, and basements, almost always on bulky magnetic ballasts. They're the least efficient and are disappearing fast. LED replacement: because magnetic ballasts aren't LED-compatible, T12 fixtures are best converted with Type B ballast-bypass LED tubes — you remove the old ballast and wire the sockets to line voltage. A 4-ft T8-style LED tube fits the same G13 bi-pin sockets.

T8 — Today's Workhorse

T8 is the modern standard for offices, shops, kitchens, and troffers, typically on electronic ballasts. LED replacement: the easiest path — use a Type A (plug-and-play) LED tube if the electronic ballast is healthy, or Type B / Type A+B for maximum efficiency. Most LED tubes on the market are designed around the T8 G13 bi-pin form factor.

T5 — Slim and High-Output

T5 tubes are thinner and a bit shorter than their T8 counterparts (a 4-ft T5 is about 45.8"), with a smaller G5 mini bi-pin base. They come in standard and High Output (T5HO) versions for bright commercial and high-bay use. LED replacement: use an LED tube made specifically for T5/G5 — a T8 LED won't fit the smaller base or shorter length. We stock LED T5 tubes for direct swaps.

Can You Mix Them?

No — the diameter and base differ, so a T5, T8, and T12 are not interchangeable as lamps. The good news: when converting to LED, you generally keep your fixture and choose an LED tube matched to its base and length, with the right type (A/B/C) for the ballast situation. See our Type A/B/C guide.

Quick Reference

  • T12 (1.5") → old/magnetic ballast → LED Type B ballast-bypass.
  • T8 (1") → standard/electronic ballast → LED Type A, B, or A+B.
  • T5 (5/8") → slim, G5 base → dedicated LED T5 tube.

Find Your Tube

Browse our LED Tubes collection by diameter, length, and type — in singles and money-saving 6- and 12-packs. Unsure what you have? Measure the diameter and length, note the base, and we'll match it. The glossary defines every term.

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