UVB lighting is the most expensive single husbandry mistake new reptile keepers make. The bulbs cost $30 to $100 each. The species that needs it dies without it. And every keeper assumes that “the light still works” because it lights up, when in fact UVB output decays months before the visible spectrum fades.
This guide covers the actual replacement schedules for every common reptile UVB bulb type, how to verify output without a meter, and why the date schedule matters more than how the bulb looks. The schedules in this guide come from the manufacturer specs cross-checked against the data collected by reptile lighting groups using Solarmeter 6.5 measurements over time.
How UV output decays
A reptile UVB bulb has two outputs: visible light, which lasts the life of the bulb until it stops emitting any light at all, and UVB, which decays steadily from day one. By the time a UVB bulb dims visibly, the UVB has been at zero for months.
The decay curve for a typical T5 HO tube looks roughly like this:
- Day 0 to 30: 100 percent UV output (peak performance after initial burn-in)
- Month 1 to 9: Gradual decay to about 70 percent of original
- Month 10 to 12: Faster decay to about 50 percent
- Month 13 onward: Decay accelerates, output drops below useful threshold within 1 to 3 months
Replacing at month 12 keeps the animal in the useful range. Replacing at month 18 means the animal has been UVB-deficient for 6 months without you knowing.
Replacement schedules by bulb type
The dates below are conservative replacement intervals. Many keepers stretch them by 1 to 2 months in low-need species, but the conservative approach prevents subclinical MBD that you cannot see until it is too late.
| Bulb type | Brand examples | Replacement interval |
|---|---|---|
| T5 HO linear tube | Arcadia ProT5, Zoo Med Reptisun T5 HO, Exo Terra Linear UVB T5 HO | 12 months |
| T8 linear tube | Zoo Med Reptisun T8, Exo Terra Repti Glo T8 | 6 to 9 months |
| Compact coil UVB | Zoo Med Reptisun Compact, Exo Terra Reptile UVB 100 Compact | 4 to 6 months |
| Mercury vapor combo (heat plus UVB) | Mega-Ray, Powersun, Solar Glo | 12 months |
| Metal halide UVB | Arcadia Lumen-Ize, SolarRaptor HID | 12 to 14 months |
Buy the next bulb when you install the current one. Tape the install date on the bulb itself with a permanent marker. When the calendar reminder fires, you have the replacement already on hand.
Verifying output: meter vs cards vs guessing
Three ways to check if a UVB bulb is still emitting useful output:
Solarmeter 6.5 ($260): The professional tool. Reads UV index directly at the basking spot. The only method that produces a real measurement.
UV test cards ($5 to $15): Cards that change color when exposed to UVB. Useful for “is there any UVB at all” but cannot distinguish UV index 1.5 (too low) from UV index 4.0 (correct). Unreliable for actual husbandry decisions.
Visual inspection: Useless. A bulb that lights up brightly can have zero UVB.
If you keep multiple reptiles requiring UVB, the Solarmeter pays for itself across 4 to 5 bulb cycles by letting you confirm exactly when a bulb has decayed past useful output rather than replacing on a fixed date.
Why glass and screen filter UVB
UVB does not pass through standard glass. If your enclosure has a glass top and the UVB tube sits on top of the glass, almost no UVB reaches the basking spot. The bulb is essentially decorative.
Recommended mounting:
- Glass tank: Replace the glass top with a screen top. Mount UVB tube on top of the screen, directly over the basking spot.
- PVC enclosure: Mount UVB tube inside the enclosure or through a cutout in the top.
- Open-top wood enclosure: Mount UVB tube under the top edge of the enclosure interior.
Screen mesh also filters some UVB. Fine fly screen blocks about 50 percent of UVB output. PVC-coated aluminum mesh blocks about 30 percent. If you have a screen between the bulb and the animal, increase the bulb output rating one step or move the bulb closer to compensate.
Bulb output and distance pairings by species
The required UV index at the basking spot varies by species. The bulb output strength and the distance from basking spot must both be set correctly.
| Species | Target UV index | Recommended bulb | Distance from basking |
|---|---|---|---|
| Leopard gecko | 0.5 to 1.5 | T5 5.0 or T5 7% | 12 to 15 inches |
| Crested gecko | 0 to 1.5 | T5 5.0 or skip | 12 to 18 inches |
| Bearded dragon | 4.0 to 6.0 | T5 12% or T5 10.0 | 10 to 12 inches |
| Veiled chameleon | 3.0 to 5.0 | T5 12% or T5 10.0 | 6 to 9 inches |
| Russian tortoise | 4.0 to 6.0 | T5 12% or T5 10.0 | 10 to 12 inches |
| Sulcata tortoise | 4.0 to 8.0 | T5 14% or mercury vapor | 10 to 12 inches |
| Red-eared slider | 3.0 to 5.0 | T5 10.0 | 10 to 14 inches |
| Day gecko | 3.0 to 5.0 | T5 12% or T5 10.0 | 8 to 12 inches |
| Corn snake | 0 to 1.5 | T5 5.0 or skip | 12 to 18 inches |
| Ball python | 0 to 1.5 | T5 5.0 or skip | 12 to 18 inches |
The skip-able species (leopard gecko, crested gecko, corn snake, ball python) can be kept without UVB if calcium-with-D3 supplementation is provided. The species marked 3.0+ cannot be kept without UVB and dietary D3 cannot fully substitute.
Common UVB mistakes
The frequent failures keepers make with UVB:
- Replacing only when the bulb visibly dies: UV output has been gone for months by then.
- Putting the bulb above a glass top: Glass filters out almost all UVB.
- Using a compact coil bulb for a high-need species: Compacts produce a narrow cone, not enough coverage for chameleons or bearded dragons.
- Skipping the meter and skipping the date schedule: You are guessing, and guessing wrong is invisible until MBD appears.
- Buying the cheapest bulb on Amazon: Off-brand UVB tubes often produce no measurable UVB out of the box.
A reptile UVB program is a recurring cost, like dog food or cat litter. Budget $50 to $100 per year per enclosure for bulb replacements. Mark the install date on the bulb. Verify with a meter if you can afford one, follow the schedule strictly if you cannot. Most reptile MBD cases come from keepers who installed UVB correctly two years ago and never replaced it.
Frequently asked questions
How often should I replace a reptile UVB bulb?+
It depends on the bulb type. T5 HO linear tubes last 12 months. T8 linear tubes last 6 to 9 months. Compact coil UVB bulbs last 4 to 6 months. Mercury vapor combo bulbs last 12 months. Always replace on a date schedule, not when the bulb stops emitting visible light. UV output decays well before the bulb fails visibly and a dim-looking bulb can be producing zero usable UVB for months.
How do I know if my UVB bulb is still working?+
The only reliable way is a UV index meter, specifically a Solarmeter 6.5. It reads UV index at the basking spot in real time and costs about $260. Stick-on UVB cards (the ones that change color) are unreliable and only confirm presence, not strength. If you cannot afford a meter, follow the date-based replacement schedule strictly. Replace the bulb on its anniversary regardless of how bright it looks.
Are T5 UVB tubes better than T8 tubes?+
T5 HO tubes produce significantly higher UV output and last twice as long. A T5 HO Arcadia ProT5 12% produces about 3 to 4 times the UV index of a T8 Reptisun 10.0 at the same distance. For chameleons, day geckos, bearded dragons, tegus, and any species needing UV index 3.0 or higher at the basking spot, T5 HO is the modern standard. T8 tubes are acceptable for low-UVB species like leopard geckos and corn snakes if used at all.
Do compact coil UVB bulbs cause eye problems?+
Older compact UVB bulbs (pre-2014) caused photo-kerato-conjunctivitis in many species, but modern compact bulbs from reputable brands (Arcadia, Zoo Med Reptisun, Exo Terra Reptile UVB 100) have largely solved this issue. The bigger problem with compact bulbs is their narrow UV cone, which forces the animal to bask in exactly one spot to get any UVB. Linear T5 tubes are better for almost all species because they spread UV across a wider area.
Why does my reptile still have MBD after I installed UVB?+
Several common reasons. Bulb installed too far from basking spot (UVB drops sharply with distance). Bulb blocked by glass or fine mesh (both filter most UVB out). Bulb too old, often past 12 months and decayed below useful output. Wrong bulb output strength for the species. No calcium supplementation paired with UVB. MBD reversal requires fixing all the husbandry simultaneously, not just adding a bulb.
