Free tool · the number, not a guess
How much UVB does my reptile need?
The honest answer is a number — the UV Index (UVI) at the basking spot — set by your species' Ferguson Zone. Pick your reptile for its fact-checked target, the right bulb, and how to place it.
- Use a linear T5 HO tube (or a matched UVB kit), not a coil/compact bulb.
- Distance sets the UVI — mount it at the height the bulb's chart gives for your target, and remember mesh cuts UVB.
- Always build a gradient down to shade so the animal can move out of the UV.
- A Solarmeter (UV index meter) is the only way to know your number rather than guess.
- Replace the bulb every ~12 months — UVB fades long before the light dies.
- Light-sensitive morphs (pale, albino, silkback/scaleless) sit at the low end, with extra shade.
UVI targets are fact-checked by The Baskden Team against each species' care sheet and ourFerguson-zones guide — nothing is saved or sent anywhere.
The four Ferguson Zones
A reptile's zone is set by how much sun it seeks in the wild — and that sets its target UVB. Higher zones need a real gradient: a bright, high-UV basking spot and shaded low-UV retreats to move between.
| Zone | Type | Basking UVI target | What it means |
|---|---|---|---|
| Zone 1 | Crepuscular / shade-dweller | ~0.5–2.0 | Active at dawn, dusk, or night and mostly in cover — needs only gentle, low-level UVB, not a bright basking beam. |
| Zone 2 | Occasional basker | ~2.0–3.0 | Basks now and then but spends a lot of time hidden — modern husbandry provides moderate, low-Zone UVB even though it was long kept without any. |
| Zone 3 | Open / partial-sun basker | ~3.0–4.5 | A committed daytime basker — needs strong UVB at the basking spot, always with shade to move into. |
| Zone 4 | Extreme sun-worshipper | ~4.5–6.0 | Out in the open desert sun — needs the highest UVB of any pet reptile, delivered over a strong gradient down to shade. |
Quick answers
How much UVB does my reptile need?
It’s measured as UV Index (UVI) at the basking spot, and it depends on the species’ Ferguson Zone. Pick your reptile above for its fact-checked target. As a guide: crepuscular geckos (Zone 1) want roughly UVI 0.5–2.0; occasional-basking snakes (Zone 2) about 2.0–3.0; open-sun baskers like bearded dragons, skinks, tortoises, chameleons, tegus and monitors (Zone 3) around 3.0–4.5; and extreme desert baskers like uromastyx (Zone 4) the most, about 4.5–6.0. Whatever the target, always give a gradient down to shade — never a single blanket of UV.
What is a Ferguson Zone?
A Ferguson Zone groups reptiles by how much sun — and therefore UVB — they naturally expose themselves to in the wild. Based on Dr Gary Ferguson’s field research and expanded by the peer-reviewed Baines UV-Tool, reptiles fall into four zones, from Zone 1 (crepuscular, shade-dwelling species that get very little UV) up to Zone 4 (extreme baskers out in the desert midday sun). Your species’ zone sets the target UVI to provide at its basking spot.
Do leopard geckos and snakes really need UVB?
Yes — modern husbandry provides low-level UVB for them even though they’re crepuscular and were long kept without it. Leopard geckos, crested geckos and most beginner snakes sit in the low zones (Ferguson Zone 1–2), so they need only gentle UVB — roughly UVI 0.5–1.5 for a leo, 2.0–3.0 for a colubrid snake — not the intense lighting a bearded dragon needs. Pale, albino and “silkback”/scaleless morphs are extra light-sensitive and belong at the very low end.
What UVB bulb should I use, and how often do I replace it?
Use a linear T5 HO fluorescent tube (or a matched UVB kit), not a coil/compact bulb — linear tubes give even, reliable coverage. Mount it the correct distance above the basking spot to hit your species’ target UVI (follow the bulb maker’s distance chart, and remember mesh cuts UVB), and provide shade so the animal can move out of the UV. Replace the bulb roughly every 12 months: its UVB output fades long before the visible light goes out, so a bulb that still glows can be producing almost no useful UVB. A Solarmeter (UV index meter) is the only way to truly know your number rather than guess.
Can a reptile get too much UVB?
Yes — excess UV (or the unnaturally short-wavelength UV from some cheap lamps) can damage eyes and skin, so the goal is never “as much as possible.” Provide the right UVI for the species with a gradient down to shade, so the animal can photoregulate — move in and out of the UV as it needs. Too little UVB, on the other hand, causes metabolic bone disease (MBD): soft or bent bones, a rubbery jaw, tremors and weakness. The right number with shade avoids both.
The full UVB & Ferguson-zones guide → ·Does my reptile need UVB & heat? ·All tools