T5 vs T8 UVB for reptiles

What the two numbers actually mean

T5 and T8 describe tube diameter in eighths of an inch: a T8 is one inch across, a T5 is five eighths. That is the whole of the naming. The difference that matters to a reptile is that the thinner tube runs a higher output per inch of length, and "HO" on a T5 lamp means high output. Zoo Med puts the comparison in writing on its own product page for the T5 HO ReptiSun 10.0: twice as strong as standard ReptiSun 10.0 lamps in Zoo Med's product information. The standard lamp in that sentence is the T8.

Twice as strong is not twice as good. UVB is a dose, and the dose a species needs is published: UVI 4.0-4.5 at the basking surface for a bearded dragon, UVI 2.0-3.0 at the basking area for a ball python, UVI 0.5-1.5 at the basking spot for a leopard gecko per the care references. A stronger lamp reaches those figures from further away. It does not make them larger.

The published figures, side by side

Two things fall out of that table. First, the T8 range and the T5 HO range are not the same lengths - a 22 in T5 HO tube and a 24 in T8 tube are both sold for a 4 ft enclosure, and the fixtures are not interchangeable. Second, the lamp with the clearest published reach figure is the T8 10.0, at 20 in from the lamp surface as Zoo Med states it. Zoo Med's own T5 HO text points you at the distance chart on the package instead, which is why every UVB recommendation on this site comes with a measured distance rather than a product name alone.

The question that actually decides it: how high is your lamp?

Measure from where the lamp will sit to the animal's back at the basking spot. Not the floor, and not the top of the enclosure - the animal's back. Then subtract for anything in between: a mesh top costs roughly a third of the UVB passing through it, and a solid glass or plastic lens blocks UVB completely. Our UVB distance and Ferguson zones page has the mount-distance arithmetic in full.

Cost, honestly

A T8 tube is the cheaper lamp and the cheaper fixture, and that gap is real rather than marginal. A T5 HO setup costs more at both ends - tube and reflector fixture - and the tube is on the same 12-month replacement clock, so the difference recurs annually rather than once. For a leopard gecko in a 36 x 18 x 18 in enclosure, where both formats land in range, that makes T8 a defensible choice and not a compromise. For a bearded dragon, where the target is UVI 4.0-4.5 at the basking surface at the basking surface, the T5 HO is the only format that reaches it at a mount distance a real enclosure allows, and the saving on a T8 buys you a lamp that does not do the job.

Running costs follow wattage rather than format: a 24 in T8 draws 17 W, a 22 in T5 HO draws 24 W. Both are rounding errors next to a basking lamp, as reptile lighting running costs works through.

Replacement is the same discipline either way

Zoo Med states that the T8 ReptiSun 5.0 provides optimum levels of UVB for 12 months and should be replaced once a year in its product information, and the same annual interval applies to the T5 HO lamps and the Arcadia tubes. UVB output decays long before the tube stops making visible light, so a lamp that still looks fine can be delivering a fraction of its rated output. When to replace UVB bulbs covers how to date a lamp you have already installed, and why a calendar reminder beats judgement by eye.

So which one should you buy?

Buy T5 HO if you keep a high-UVI species, if the lamp has to sit more than about a foot from the animal, or if the enclosure has a mesh top taking its cut on the way through. That covers most bearded dragon setups and every tall enclosure. Buy T8 if the lamp sits low over a shallow enclosure, if the species target is low, or if you already own a working T8 fixture and the honest upgrade path is a fresh tube plus a meter reading rather than a new fixture. Our Arcadia vs Zoo Med comparison is the next decision after this one, and best T5 fixtures covers the reflector that a T5 HO tube needs to deliver what it claims.