What UVB Bulb Does My Reptile Need?

UVB selection is one of the most consequential decisions in reptile keeping — and one of the most commonly misunderstood. The right UVB setup depends on your species, enclosure size, bulb type, fixture, distance from the basking site, and whether mesh is obstructing the output. Tank size alone doesn't determine what you need.

Already know what you need? → Best Reptile UVB Lights

Quick Reference: UVB Need by Species

The table below gives a general starting point. The sections that follow explain how to dial in the right setup for your specific enclosure, distance, and mesh configuration. These are starting points, not universal prescriptions — actual UVI at the basking site is what matters.

Start Here: Ferguson Zones

The most useful framework for UVB selection is the Ferguson Zone system, which classifies reptiles by their natural UV exposure behavior in the wild. The goal in captivity is to provide a UVI gradient that reflects the animal's natural exposure — a basking spot toward the higher end of the appropriate range, with lower UVI available in shaded areas so the animal can self-regulate.

Ferguson zones describe observed exposure behavior in the wild and are a useful guide for setting captive UVI targets — but species assignments aren't always clean-cut, and the zones aren't automatic captive prescriptions. Use them as a framework, not a formula.

The only way to verify what your animal is actually receiving is to measure UVI at the basking site with a UVI meter. Bulb percentage, distance, and fixture type all affect output, and the interaction between them varies enough that measurement is the most reliable approach.

T5 vs T8: Which Do You Need?

UVB bulbs come in two main tube formats: T5 HO (high output) and T8. T5 HO produces higher UVB output and generally maintains useful UVI at greater distances from the basking site than T8. T8 produces lower output and is typically more appropriate for smaller enclosures or lower-zone species where the basking site is close to the bulb.

The practical difference between them depends heavily on the fixture, reflector quality, bulb strength, and whether mesh is between the bulb and the animal — so these are relative differences, not fixed effective distances.

For enclosures with a standard mesh top, T5 HO's higher output can make it easier to achieve the target UVI through the mesh — but appropriate distance and output still need to be dialed in for your specific setup. Mesh reduces UVB transmission meaningfully, and the degree of reduction varies by mesh type and weave density.

UVB Percentage: What It Means and What It Doesn't

UVB bulbs are commonly sold as 5%, 6%, 10%, 12%, and similar percentages. These numbers refer to the proportion of UV output that falls in the UVB range — they are a rough guide to intensity, not a precise prescription.

The percentage alone does not tell you what UVI the animal receives. The actual UVI at the basking site depends on:

• Distance from the bulb to the basking surface
• Whether mesh is between the bulb and the animal, and what type
• The fixture's reflector quality
• Bulb age (UVB output degrades over time even when the bulb still produces visible light)
• Bulb brand and manufacturing variation

Use percentage as a starting point for selecting a bulb in the right output range for your species. Then adjust distance and confirm with measurement where possible.

Species-Specific Starting Points

Bearded Dragon (Zone 4 — high UVB)

Bearded Dragons have among the highest UVB requirements of commonly kept reptiles. A higher-output T5 HO bulb is the standard starting point for adult setups. Distance from the basking site, mesh type, and fixture reflector all affect the UVI delivered — the right combination depends on your specific enclosure. Shade areas must be available so the animal can avoid UVB when it chooses. See the Bearded Dragon Care Guide.

Leopard Gecko (Zone 2 — low UVB)

Current evidence supports providing low-level UVB rather than none. A lower-output linear UVB bulb at appropriate distance is the standard approach. Extensive shade must be available. See the Leopard Gecko Care Guide.

Corn Snake (Zone 2 — low UVB)

Crepuscular/low-exposure species. Low-level UVB is increasingly recommended based on current husbandry research, though corn snakes have been kept without it for decades. If providing UVB, a low-output linear bulb at appropriate distance is suitable. Shade must be available. See the Corn Snake Care Guide.

Hognose Snake (Zone 2–3 — low to moderate UVB)

Low to moderate UVB. A lower/moderate-output linear UVB bulb with shade available is appropriate. See the Hognose Snake Care Guide.

Crested Gecko (Zone 1 — very low UVB)

Shade dweller with very low natural UV exposure. If providing UVB, use a low-output bulb at significant distance with extensive shaded areas. Many keepers maintain crested geckos without UVB successfully with appropriate dietary supplementation. See the Crested Gecko Care Guide.

African Fat-Tailed Gecko (Zone 2 — low UVB)

Similar requirements to Leopard Gecko. Low-level UVB with shade available. See the African Fat-Tailed Gecko Care Guide.

Mesh and UVB Transmission

Standard aluminum window screen and most reptile enclosure mesh reduces UVB transmission — the degree varies by mesh type and weave density, but reductions are meaningful enough to factor into your setup. This means the UVI your animal receives through a mesh top will be lower than what the bulb produces at the same distance in open air.

• If your fixture sits inside the enclosure without mesh between bulb and animal, you may need to increase distance or use a lower-output bulb compared to an above-mesh setup
• Some UV-transparent mesh products reduce transmission less than standard metal mesh
• When in doubt, measure with a UVI meter rather than assuming the setup is correct

Bulb Replacement

UVB output degrades over time even when the bulb continues to produce visible light. A bulb that looks fine may be delivering little or no useful UVB. Follow manufacturer guidance for replacement intervals — these vary by bulb type and brand, and blanket rules can be misleading in either direction. Measuring UVI periodically is the most reliable way to know when output has dropped below a useful level.

Mark the installation date on the bulb or fixture so you don't lose track of how long it has been running.

Ready to Choose a UVB Fixture?

Once you know your species' UVB requirement, enclosure size, and whether mesh is a factor, the next step is choosing a specific fixture and bulb.

→ Best Reptile UVB Lights — recommendations by species and enclosure size

Related Guides

• Best Reptile UVB Lights
• Complete Reptile Setup Checklist
• Reptile Species Guides Hub
• Bearded Dragon Care Guide
• Leopard Gecko Care Guide
• Corn Snake Care Guide
• Hognose Snake Care Guide
• Crested Gecko Care Guide
• African Fat-Tailed Gecko Care Guide