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Exo Terra’s T5 tubes (Reptile UVB100, UVB200 and UVB300) aren’t listed because we haven’t found published UV readings by distance for them. If you use one, set the distance with a UV meter.
How the calculator works
Every distance recommendation is derived from three factors multiplied together:
- Base distance range — the distance at which each bulb gives the basking spot a UV index of about 4 to 6 under a quality reflector with no mesh. For the Arcadia Desert 12% this comes from Arcadia’s published readings (6.5 at 15 inches, 4.4 at 18 inches, measured under a ProT5), and for the Zoo Med Reptisun 10.0 from ReptiFiles’ distance for this tube in an Arcadia fixture (16–17 inches for a UV index of 4.0–4.5). Zoo Med’s own ReptiSun T5 HO Terrarium Hood charts show a UV index above 7 closer than 10 inches to its 22-inch tube and 14 inches to its 34- and 46-inch tubes. Check your own setup with a Solarmeter 6.5 UV index meter.
- Fixture efficiency multiplier — a quality reflector fixture redirects scattered UVB back toward the basking zone, measurably increasing output at distance. Dr Frances Baines notes that a high-quality aluminium reflector can boost the output under a T5 tube up to threefold compared with a bare tube (LafeberVet). The calculator assumes a thin reflector gives about 70% and a bare strip about 40% of a quality reflector at the same distance, and moves the tube closer to match. These fixture figures are estimates, so measure if you can.
- Mesh attenuation factor — screen tops block roughly 25–45% of UVB. Dr Frances Baines lists about 25% for ¼-inch wire mesh, about 35% for Exo Terra and Zoo Med screens and Arcadia’s LampGuardPro, and about 45% for Zen Habitats and Dubia screens (LafeberVet). This is the most commonly missed variable in UVB setups.
The calculator multiplies the fixture and mesh factors together and moves the tube closer to make up for the lost UVB, using the inverse-square rule (UV falls off roughly with the square of the distance). The estimated UVI shown is what the basking spot should get at that adjusted distance.
What Are Ferguson Zones?
Ferguson zones come from field research by Gary Ferguson and colleagues, who measured the UV levels that wild lizards and snakes chose to spend time in (Zoo Biology, 2010). The UV-Tool, a lighting guide for zoos built on that work (Baines et al., Journal of Zoo and Aquarium Research, 2016), sorts species into four zones. For each zone it gives the average UV index the animals were found in and a maximum, which belongs at the spot closest to the lamp.
The UV-Tool puts the inland bearded dragon in zones 3–4: an open-habitat to mid-day sun basker. Even mid-day baskers spend most of their basking time at about UVI 3–5, and the UV-Tool treats UVI 7–8 as the absolute maximum for zone 4 reptiles under artificial lamps. This calculator aims for UVI 4.0–6.0 at your dragon’s back on the basking spot, with a gradient down to zero in the shade. ReptiFiles gives UVI 4.0–4.5 for the basking area, and Arcadia gives UVI 6.0 as the upper basking level for a bearded dragon. Much below 3.0 at the basking spot is under the lowest maximum the UV-Tool gives for zones 3–4 (2.9), so a normally pigmented dragon may not get enough UVB. Reduced-pigment morphs are the exception, covered below the table.
| Zone | Typical habits | UVI: average / maximum | Examples (UV-Tool) |
|---|---|---|---|
| Zone 1 | Crepuscular or shade dweller | 0–0.7 / 0.6–1.4 | Leopard gecko, crested gecko |
| Zone 2 | Partial sun or occasional basker | 0.7–1.0 / 1.1–3.0 | Ball python |
| Zone 3 | Open or partial sun basker | 1.0–2.6 / 2.9–7.4 | Bearded dragon (zones 3–4), veiled and panther chameleons |
| Zone 4 | Mid-day sun basker | 2.6–3.5 / 4.5–9.5 | Bearded dragon (zones 3–4), Uromastyx |
The mesh obstruction problem
Mesh obstruction is the most common silent failure mode in bearded dragon UVB setups. An owner follows the distance recommendation printed on the bulb box — but that recommendation assumes the tube is inside the enclosure with no barrier between it and the animal. When the tube is mounted above a mesh lid, a significant percentage of UVB never reaches the basking spot.
Measured attenuation values for common mesh types (Baines, LafeberVet):
- ¼-inch wire mesh: about 25% UVB reduction
- Exo Terra and Zoo Med screen tops: about 35% reduction
- Zen Habitats and Dubia screens: about 45% reduction
- No mesh (tube inside enclosure): 0% reduction — maximum efficiency
The fix is straightforward: either mount the UVB tube inside the enclosure (eliminating the mesh barrier entirely), or reduce the tube-to-surface distance by enough to compensate for the attenuation. The calculator handles this adjustment automatically when you select your mesh type.
T5 HO vs. T8: why it matters
T5 HO (high-output) linear fluorescent tubes are the current veterinary standard for bearded dragon UVB. T8 tubes operate on lower current, produce lower UVB output at distance, and degrade faster — needing replacement every 6 months vs. 12 months for T5 HO. For an adult bearded dragon in a 4x2x2 (120-gallon) or larger enclosure, T8 tubes generally cannot reach UVI 4.0+ at the mounting distances that fit standard enclosure heights.
The two T5 HO bulbs with the strongest track record and most consistent output data:
- Arcadia Desert 12% T5 HO — 12% UVB output; Arcadia lists its Desert T5 for Ferguson zones 3–4. Pairs with the Arcadia ProT5 kit (fixture + reflector) for a complete, calibrated system. Mount it 16–18 inches from your dragon’s back with no mesh (Arcadia measures a UV index of 6.5 at 15 inches and 4.4 at 18).
- Zoo Med Reptisun 10.0 T5 HO — 10% UVB output, the most widely recommended and available bulb in the US. Mount it 16–18 inches from your dragon’s back with no mesh, the same as the Arcadia (ReptiFiles). Widely available in pet stores.
Frequently asked questions
Can I use the same distance for a T8 and T5 version of the same bulb?
No. T8 versions of the same brand and percentage produce significantly lower UVB output at distance than T5 HO tubes. The Zoo Med Reptisun 10.0 T8, for example, reaches UVI 4.0 only within 6–8 inches — a distance that works in small setups but won't reach a basking platform in a standard 40-gallon enclosure. The calculator adjusts distances for each bulb type independently.
My enclosure is 36 inches tall — is that a problem?
For a T5 HO tube mounted inside without mesh, a 36-inch enclosure gives you significant flexibility — especially with the Arcadia 12%, which gives the target UV index at about 16–18 inches. Use the basking platform to bring your beardie’s back to that distance from the tube. Use the enclosure height slider to reflect the actual tube-to-basking-surface distance, not the total enclosure depth.
How do I measure the distance correctly?
Measure from the bottom of the tube (the light-emitting surface) to the top of your beardie’s back while it sits in its normal basking posture on the basking platform. Do not measure to the floor of the enclosure — this will give you a number that's several inches too long and result in under-exposure. A 4-inch platform in a 16-inch-tall enclosure leaves roughly 12 inches of effective distance, not 16.
Does enclosure size affect UVB output?
Indirectly. Larger enclosures let you create proper UVI gradients — a high-UVI basking zone on one end and a low-UVI retreat on the other. Bearded dragons self-regulate UV exposure by moving between zones, just as they would in the wild. This gradient is healthy and intentional. The calculator gives you the basking-spot distance; your beardie handles the gradient itself.
What's a Solarmeter 6.5 and do I need one?
The Solarmeter 6.5 is a handheld UV index meter that gives a direct UVI reading anywhere in the enclosure. It's the meter Arcadia and ReptiFiles use for their UVI figures, and the way to confirm that a calculated distance actually delivers the target UVI with your specific bulb, fixture, and enclosure. At ~$120, it's a one-time investment that eliminates all guesswork. Not mandatory, but strongly recommended for new setups or when troubleshooting suspected UVB deficiency.
How often should I replace my UVB bulb?
T5 HO bulbs: every 12 months. T8 bulbs: every 6 months. UVB output degrades significantly before visible light output does — a tube can still look bright at 18 months while delivering 30–40% of its original UVB. The 12-month rule for T5 HO bulbs reflects the point where output typically falls too low to keep the basking spot in the target range. Write the installation date on the tube with a permanent marker and set a calendar reminder.
More free tools: Cost Calculator · Tank Size Calculator · Ca:P Ratio Calculator
Sources & methodology notes
- Baines, F., Chattell, J., Dale, J., et al. (2016). How much UV-B does my reptile need? The UV-Tool, a guide to the selection of UV lighting for reptiles and amphibians in captivity. Journal of Zoo and Aquarium Research, 4(1), 42–63. — zone table, species zones (inland bearded dragon: zones 3–4), basking levels and the UVI 7–8 safety maximum for zone 4
- Ferguson, G.W., Brinker, A.M., Gehrmann, W.H., et al. (2010). Voluntary exposure of some western-hemisphere snake and lizard species to ultraviolet-B radiation in the field: how much ultraviolet-B should a lizard or snake receive in captivity? Zoo Biology, 29(3), 317–334. — the original Ferguson zones
- Arcadia Reptile — Desert 12% T5 lamp, UV index by distance (measured under a ProT5) — rated for Ferguson zones 3–4
- Arcadia Reptile — DragonLamp 14% T5 — UVI 6.0 as the upper basking level for a bearded dragon
- Zoo Med — T5 HO ReptiSun 10.0 UVB distance chart
- Zoo Med — ReptiSun T5 HO Terrarium Hood UV index charts (22-, 34- and 46-inch tubes)
- ReptiFiles — Bearded Dragon Lighting, Heating & Humidity Requirements — basking UVI 4.0–4.5, the UVI 3.0 cap for hypomelanistic, translucent and silkback morphs, and the ReptiSun 10.0 distance in an Arcadia fixture
- Baines, F. — UVB Lighting for Reptiles, LafeberVet (2024) — mesh and reflector figures, and why compact lamps are not ideal
- UV Guide UK — Special report: eye problems with some high UVB fluorescent compact lamps and tubes (first published 2007)
- VCA Animal Hospitals — Bearded Dragon Husbandry
- Association of Reptile and Amphibian Veterinarians (ARAV)
Related Reading
- Bearded Dragon Habitat Setup: The Complete Tank Guide
- Bearded Dragon Night Heat: Do They Need It? (CHE vs DHP Guide)
- Bearded Dragon Enclosure Materials: Glass, PVC, Wood, and DIY Guide
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