T5 vs T8 UVB Lamps for Reptiles: Which Should You Buy?

Verdict: buy a T5 HO (high output) UVB tube in a reflector fixture for almost every reptile setup. Zoo Med calls its T5 HO lamps twice as strong as its standard tubes, and on Arcadia's charts the T5 version of the same lamp gives about three times the UV index of the T8 version at 12 to 15 inches. That lets you mount it higher, run it over a mesh lid and still reach the basking-zone targets for desert species. A T8 tube still makes sense in one case: a small, low enclosure for a shade-dwelling (Ferguson zone 1 or 2) species, where its gentle, broad output is the point. The Ferguson zones guide explains which group your animal is in.

T5 vs T8 at a glance

Arcadia D3+ Desert 12% and D3 Forest 6% lamps, T5 vs T8, from Arcadia's product pages

Spec	T5 HO	T8
Tube diameter	16 mm (about 5/8 in)	26 mm (about 1 in)
Desert 12%: UVI at 12 in	8.50 (in the ProT5 reflector)	2.95 (lamp with reflector)
Desert 12%: UVI at 15 in	6.50	1.90
Forest 6%: UVI at 12 in	4.50	1.45
Forest 6%: UVI at 15 in	3.60	0.95
Desert 12%, about 2 ft long	24W, 22 in, 850 lumens	18W, 24 in, 420 lumens
Desert 12%, about 3 ft long	39W, 34 in, 1,600 lumens	30W, 36 in, 950 lumens
Rated life	12 months	One year

Zoo Med describes its T5 HO ReptiSun lamps the same way: "twice as strong as standard ReptiSun 10.0 lamps." Its T8 ReptiSun 10.0 is sold for UVB reaching about 20 inches from the lamp, against 12 inches for the T8 5.0.

UV output and distance

This is the difference that decides most purchases. At 15 inches, Arcadia's T5 Desert 12% gives UVI 6.50, inside the range Baines et al. (2016) recommend for mid-day baskers such as bearded dragons, while the T8 version of the same lamp gives UVI 1.90, closer to a zone 3 average than a basking peak. To reach a basking-spot reading of 4 to 6 with a T8, the animal has to get within about 10 inches of the tube (Arcadia lists the T8 Desert at UVI 4.00 at 10 inches), which leaves little headroom in a tall enclosure.

The paper behind the UV-Tool reaches the same conclusion from the other direction. Baines et al. describe T8 tubes as producing "low levels of UV irradiance, similar to those found in outdoor shade," suited to the shade method in small enclosures where the maximum needed is UVI 0.7 to 1.0. For larger enclosures, they say T5 HO tubes can be mounted further from the animal to give the same low UVI, and they list high-output T5 tubes, with mercury vapour and metal halide lamps, as able to produce "much higher levels of UV-B than T8 fluorescent tubes, up to levels typical of natural sunlight" for the basking zone.

Coverage: how wide the UV patch is

Output at one point is only half the story; the UV-Tool authors say the UV patch should be at least as wide as the animal's whole body. Linear tubes of both types spread UV along their length, which is why Baines et al. describe T8 tubes as giving low UV "over a relatively large area close to the tube." A T5 HO in a reflector does the same from further away: in the paper's charts, an Arcadia T5 HO 12% tube in an aluminium reflector gave UVI 0.5 to 1.0 across an area more than 130 cm (about 51 in) in diameter at 85 cm (about 33 in), while a metal halide lamp reached that level at 45 cm over a patch under 25 cm across. Arcadia says its ProT5 reflector concentrates UV under the lamp at around 12 inches for a basking zone, then spreads it into a wide flood with distance. In practice, choose the tube length to cover the basking area plus some margin, and leave the far end of the enclosure as shade.

Mesh lids make the gap bigger

A lamp resting on a screen top loses part of its output before it reaches the animal. Zoo Med says screen covers can filter out up to 50% of UVB, and Arcadia says stainless steel mesh can reduce the UV index by 30% or more. Arcadia markets its 14% Dragon T5 partly on this point: the extra output lets light penetrate further through a mesh-top terrarium. Halving a T8's 1.90 at 15 inches leaves under 1, which is shade-dweller territory. If your enclosure is a glass tank with a mesh lid and you keep a basking species, that alone points to a T5 HO.

Fixtures and ballasts are not interchangeable

Zoo Med is explicit that T8 lamps must be used in a T8 fixture that supplies the correct power, and HO T5 lamps in high-output T5 fixtures. It also warns that not every T5 is high output: there are normal-output and very-high-output T5s too, so check that the fixture and ballast match the lamp. Arcadia sells T5 electronic controllers and T8 magnetic controllers as separate parts and lists its T5 lamps as flicker free. In practice, if you already own a T8 fixture, a T5 HO lamp will not fit it; you are buying a new fixture either way.

Light, energy and size

For a similar length, the T5 HO draws more watts and gives much more light. In Arcadia's Desert range, the 22-inch T5 draws 24W for 850 lumens and the 24-inch T8 draws 18W for 420 lumens. If you are budgeting running costs, the difference is small: 6 extra watts for 12 hours a day is about 2.2 kWh a month. The reptile electricity cost calculator works it out for your rate. The slimmer T5 also takes less room inside a wooden or PVC enclosure.

Lifespan and disposal

Arcadia rates both its T5 and T8 UVB lamps for a year. Independent measurements in the UV-Tool study found good fluorescent tubes lost about 40% of their UV index on average after 4,000 hours, and one lamp from another maker lost 64% in 1,000 hours, so the replacement schedule matters more than the tube type. Zoo Med labels its T8 ReptiSun as containing mercury and directs owners to lamprecycle.org for disposal; treat any spent fluorescent UVB tube the same way.

Who should buy which

Your setup	Better choice	Why
Bearded dragon or other zone 3 to 4 basker, any enclosure	T5 HO 12% to 14% (or Zoo Med 10.0)	Reaches UVI 4 to 7 at 15 to 20 in on the makers' charts
Glass tank with a mesh lid	T5 HO	Mesh can remove 30 to 50% of the UV
Tall enclosure (24 in or more)	T5 HO	T8 output falls below 1 UVI at 15 in for a 6% lamp
Small, low enclosure for a zone 1 to 2 species	T8, or a low-output T5 kit such as Arcadia's ShadeDweller	Low UVI over a wide area is the goal
You already own a working T8 fixture and keep a shade dweller	T8	No new fixture needed; replace the lamp yearly

For a basking species, the kit to start with is a T5 HO lamp in a reflector, such as the Arcadia ProT5 Dragon 14% kit for a 4-foot enclosure, or Zoo Med's 24-inch ReptiSun T5 HO hood for a smaller one.

For a leopard gecko or another zone 1 species, a low-output T5 such as Arcadia's ShadeDwellerPro gives UVI 0.65 at 12 inches per Arcadia, which fits the zone 1 maximum without a T8 fixture. The full lamp shortlist is on the best UVB lights for reptiles page, and the species pages for bearded dragons and leopard geckos show where the lamp goes.