Reptile UVB Calculator
UV index at the basking spot drives every reptile's ability to synthesize vitamin D3 and absorb calcium. Pick the species, bulb, basking distance, and bulb age — the gauge below shows the UVI at your reptile's back and whether it sits inside the species-specific Ferguson zone safe band.
Quick Conversion
Formula: cm = in x 2.54
Species
Open-basking desert lizard. Needs strong UV at basking; cool side near 0 UVI.
Bulb
Industry favourite for Zone 3-4 species. Replace at 12 months.
Distance + bulb age
Ferguson Zones explained
UV index by distance — Arcadia ProT5 12% (new)
| Distance (in) | Distance (cm) | UVI new | UVI at 12 mo | Suits |
|---|---|---|---|---|
| 4 | 10 | 63.00 | 34.65 | Zone 4 |
| 6 | 15 | 28.00 | 15.40 | Zone 4 |
| 8 | 20 | 15.75 | 8.66 | Zone 4 |
| 10 | 25 | 10.08 | 5.54 | Zone 4 |
| 12 | 30 | 7.00 | 3.85 | Zone 4 |
| 14 | 36 | 5.14 | 2.83 | Zone 4 |
| 16 | 41 | 3.94 | 2.17 | Zone 3 |
| 18 | 46 | 3.11 | 1.71 | Zone 3 |
| 20 | 51 | 2.52 | 1.39 | Zone 2 |
| 24 | 61 | 1.75 | 0.96 | Zone 2 |
Need to size the rest of the setup? See bearded dragon tank size and reptile heating calculator.
Formula
UVI = baseline x (12 / distance_in)^2 x age_factorWorked: Arcadia ProT5 12 percent (baseline 7.0 UVI at 12 inches new), basking at 10 inches, 6 months in (age_factor ~ 0.72) gives UVI = 7.0 x (12/10)^2 x 0.72 = 7.26 — a Zone 4 reading suitable for sulcata and uromastyx, too hot for a leopard gecko.
Recent readings
Saved readings appear here — kept in your browser only.
How to dial in UVB in 5 steps
- Identify Ferguson zone. Use the species selector — the gauge highlights the safe band.
- Pick a matching bulb. 6 percent T5 for Zone 1-2; 12 percent T5 for Zone 3-4; MVB for very large enclosures.
- Set basking distance. Adjust the slider until the gauge points inside the cyan band.
- Log install date. The replacement card calculates the next-due date based on bulb type.
- Verify with a Solarmeter. Check actual UVI at month 0, 6, 12 — replace if it drops below the lower target band.
From incandescent guesswork to Ferguson zones — the history of reptile UVB
Until the 1990s captive reptile keepers relied on plain incandescent bulbs and sunlamps designed for human tanning. Metabolic bone disease was so common it was effectively the unofficial cause of death for captive iguanas, chameleons, and bearded dragons. The first dedicated reptile UV fluorescent — Zoo Med's Reptisun — launched in 1990 and reframed the conversation around UVB-specific output.
The breakthrough came in 2010 when Gary W. Ferguson, William H. Gehrmann, Kristopher B. Karsten, Andrew M. Hammack and others published a peer-reviewed framework in Zoo Biology that catalogued wild reptile basking behaviour against Solarmeter UVI readings. The four-zone system — shade dwellers, partial sun, open basking, mid-day sun — is the foundation every modern husbandry guide cites, including the temperature gradient model this site uses.
Hardware caught up next. Arcadia released T5 HO Desert 12 percent in 2012, Zoo Med followed with Reptisun T5 HO 10.0 in 2013. T5 HO became the hobby default because the narrower 16 mm tube allows the phosphor to sit closer to the arc, delivering 3-4x the UVB output of older T8 tubes at the same length.
The Solarmeter 6.5 (1999) and 6.5R (2018) democratised verification — for around 250 USD any keeper can measure exactly what the calculator above predicts. The Association of Reptile and Amphibian Veterinarians (ARAV) now lists UVI measurement as a standard of care, and the British Veterinary Zoological Society includes Ferguson zone checks in their reptile triage flowcharts.
What remains contested is how much UV crepuscular and nocturnal species need. The 2016 update to Ferguson's tables placed Eublepharis macularius (leopard gecko) and Python regius (ball python) firmly in Zone 1, contradicting decades of pet-shop advice that they need no UVB at all. This calculator follows the updated framework and exposes those species' quiet but real D3 needs.
Trusted by reptile keepers and vets
“I rotate residents through this calculator before they design a quarantine setup. The Ferguson zone gauge mirrors how we actually plan lighting and the bulb-age slider catches the most common mistake I see in private collections.”
“We swapped from telling customers a bulb lasts 18 months to using this tool at checkout. Return rate for MBD-treated dragons dropped within three months.”
“Honest, source-cited UV guidance is rare. The species-by-Ferguson-zone match is exactly what we recommend in our 2025 husbandry guide.”
“My panther chameleon Kiwi was lethargic and I thought it was the heat. The calculator showed my 18-month-old bulb was producing under 1.0 UVI. New tube and he was climbing in three days.”
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