Heating & Lighting
How to Read a UVB Bulb: T5, T8, and Percentages Explained
Learn what T5 and T8 UVB bulbs mean, what the output percentages tell you, and how often to replace them for your reptile.
Standing in the lighting aisle of a reptile shop or scrolling through product listings, you will see labels like "T5 10.0" or "T8 5.0 UVB" with little explanation. The numbers are not arbitrary choices. They describe the physical size of the tube, how much ultraviolet-B radiation the lamp produces, and whether that output will suit your animal's needs.
This guide breaks down each part of a UVB bulb label so you can choose the right lamp, position it correctly, and know when to swap it out before your reptile's health starts to suffer.
What T5 and T8 Mean
The T in T5 and T8 stands for tubular, and the number refers to the tube's diameter measured in eighths of an inch. A T8 lamp is 8/8 of an inch (1 inch or 25 mm) across. A T5 lamp is 5/8 of an inch, noticeably slimmer.
That size difference has real practical effects. T5 tubes, particularly T5 HO (high-output) versions, push more UV energy per unit of length than T8 tubes at the same rated output. A T5 HO 10.0 lamp positioned 12 to 18 inches (30 to 46 cm) from a basking spot will deliver more usable UV at that distance than a T8 10.0 at the same placement. T8 lamps work well in smaller enclosures where the tube sits closer to the animal, typically 6 to 10 inches (15 to 25 cm) away. T5 HO is the better fit for tall enclosures or species that need strong UV output, such as adult bearded dragons or uromastyx.
Compact coil UVB bulbs, the screw-in type, use neither designation. They do produce UVB, but output drops off quickly with distance and the beam is narrow. Most keepers switch to linear T5 or T8 tubes for consistent coverage across the entire basking zone.
What the UVB Percentage Number Actually Means
The number after the tube type (2.0, 5.0, 10.0, 12.0) is a rough guide to what proportion of the bulb's total light output falls in the UVB range. A 5.0 lamp puts out roughly 5 percent of its energy as UVB wavelengths (around 290 to 320 nm). A 10.0 lamp puts out roughly 10 percent, and so on.
These figures are not standardised across manufacturers. Two lamps both labelled "T5 10.0" from different brands may not deliver identical UV Index readings at the same mounting distance. Treat the percentage as a useful starting category, not a precise engineering specification.
Here is a general matching guide by species type:
| Output label | Suited to |
|---|---|
| 2.0 | Shade-dwelling species: dart frogs, crested geckos, some skinks |
| 5.0 | Semi-arboreal or moderate-UV species: corn snakes, blue-tongue skinks, juvenile bearded dragons |
| 10.0 | High-UV desert and savanna species: adult bearded dragons, uromastyx, tortoises |
| 12.0 | Intense desert baskers kept in large, deep enclosures |
For more context on why UV radiation matters in the first place, see What Is UVB and Why Does Your Reptile Need It.
The Ferguson Zone System and Bulb Selection
Wildlife researchers developed the Ferguson Zone classification to describe how much UV exposure reptile species actually seek in the wild. The zones run from 1 (shade dwellers) to 4 (open, full-sun baskers). Knowing your animal's zone helps you match both the bulb label and the mounting distance so the UV Index at the basking spot falls in the right range.
Zone 1 animals, such as crepuscular or forest species, need a UV Index of roughly 0.6 to 1.4 at the basking surface. Zone 3 to 4 animals, the open-country baskers, need a UV Index of 2.9 or higher. A T5 HO 10.0 tube mounted inside a mesh-topped enclosure at 10 to 12 inches (25 to 30 cm) generally reaches UV Index readings in the 3 to 5 range, which suits most zone 3 species. Move the lamp to 6 inches (15 cm) above mesh and readings climb. Raise it to 18 inches (46 cm) and they drop.
Mesh lids absorb a meaningful share of UVB. Measurements through standard aluminium mesh commonly show reductions of 30 to 50 percent compared to readings taken with no lid. If your enclosure has a solid glass or plastic top, UVB transmission falls to near zero, so the lamp must go inside the enclosure entirely.
Placement, Distance, and Mounting Position
The right bulb installed poorly will still leave your animal under-served. These placement principles apply across most setups:
Tube length relative to enclosure length. Run the UVB tube along at least two thirds of the enclosure's length. This creates a gradient, giving the animal a high-UV zone near the basking spot and a lower-UV retreat at the cooler end. Reptiles use that gradient naturally.
Distance from the basking surface. For T5 HO 10.0 bulbs through a mesh lid, a starting distance of 10 to 14 inches (25 to 36 cm) above the basking surface works for most zone 3 species. For zone 2 species with a 5.0 bulb, 8 to 12 inches (20 to 30 cm) is a reasonable range. Always verify with the manufacturer's distance chart for your specific lamp, because output curves vary by brand.
No glass or plastic between lamp and animal. Both materials filter out the majority of UVB. The lamp needs a clear line of sight to the basking area, which is why most setups use open mesh tops or mount the fixture inside the enclosure.
Use a reflective hood. A good aluminium reflector can nearly double the UV output reaching the basking surface compared to a bare tube mounted without one. Most linear fixture kits include a reflector; it is worth using.
For help setting up your full lighting and heating arrangement, Reptile Heating and Lighting Explained for Beginners covers the complete picture. Once your lighting is in place, How to Set the Right Basking Temperature walks through calibrating your heat gradient.
How Often to Replace UVB Bulbs
This is one of the most common missteps in reptile keeping. A UVB fluorescent tube continues emitting visible light for well over a year, sometimes two, but its UVB output degrades long before the bulb goes dark.
Most manufacturers rate T5 HO UVB tubes for 6 to 12 months of effective UV production. After that point, the bulb may look normal and illuminate the enclosure fine, but it is no longer delivering meaningful UVB. An animal under a spent bulb starts developing deficiency signs: reduced appetite, lethargy, softening bones, abnormal shedding. By the time those signs appear, the deficiency has been building quietly for weeks or months.
A workable replacement schedule:
- T5 HO tubes: every 6 months in high-demand setups, 12 months at the outside
- T8 tubes: every 6 months (T8 UV output typically degrades faster than T5 HO)
- Compact coil bulbs: every 6 months
Mark the installation date on the fixture or set a phone reminder. Some keepers use a handheld UV meter to measure output at the basking surface when the bulb is new, then again at 6 months, so they can see the actual drop rather than guessing. That reading is the only reliable way to know whether the lamp is still doing its job.
Frequently Asked Questions
Can I use a T5 tube in a T8 fixture? No. T5 and T8 lamps require different ballasts and have different pin configurations. They are not interchangeable. A mismatched combination will prevent the lamp from starting or shorten its working life significantly.
My UVB bulb is still bright. Does it really need replacing on schedule? Yes. Visible brightness and UVB output degrade at different rates. A tube that looks fully bright may be producing well under 20 percent of its original UVB after 12 months of daily use. Replace on a calendar schedule rather than judging by eye.
How do I know if a bulb is producing enough UVB without a meter? You cannot tell reliably without one. Reptile UV meters such as the Solarmeter 6.5 are the only accurate way to confirm UV Index levels at the basking surface. They are a one-time purchase that removes the guesswork from both initial setup and replacement timing.
Does a higher percentage UVB bulb let me mount it further away? Generally yes, within limits. A T5 HO 12.0 can sit higher than a 5.0 and still deliver useful UV at the basking surface. The relationship is not perfectly linear, though, and it varies by brand, so check the specific lamp's data sheet for recommended mounting distances at each Ferguson Zone.
Is UVB the same as UVA? No. UVA (320 to 400 nm) affects colour perception and behaviour in reptiles. UVB (290 to 320 nm) is what drives vitamin D3 synthesis in the skin. Most reptile lighting produces both, but they are distinct wavelengths with separate biological roles. The percentage label on a reptile bulb refers specifically to UVB output.