How far above the basking spot your UVB lamp should hang — worked from the species' Ferguson zone, the lamp's own published output curve, and the share of it your screen top is taking away.
Last reviewed August 2026 · Free · Nothing you enter is stored
Ultraviolet B light striking a reptile's skin converts a cholesterol precursor into vitamin D3. That D3 is what allows the gut to absorb calcium from food. Without it, calcium passes through the animal largely unused, and the body begins withdrawing what it needs from the skeleton to keep blood levels stable. The result is metabolic bone disease: soft jaws, bowed limbs, spinal kinks, fractures from ordinary movement, and eventually seizures. It is the single most common serious husbandry failure in captive reptiles, and it is entirely preventable with a working lamp at the right distance.
The practical corollary is the one keepers get wrong in the other direction. Dietary D3 substitutes for the D3 the skin would have made, so an animal that is dusted with calcium-plus-D3 at every meal and also kept under a good UVB lamp is being supplied twice. D3 is fat soluble and accumulates. The better arrangement is a working lamp and plain calcium without D3, because cutaneous synthesis is self-limiting — a reptile that has made enough simply stops making more, which is a regulation no supplement pot can offer. Reserve calcium with D3 for animals kept without UVB, and treat that as the fallback it is.
Supplementation schedules follow ReptiFiles' species guides. The rule that matters: calcium with D3 substitutes for UVB, so an animal under a working UVB lamp should get calcium without D3 to avoid oversupplying it.
Almost every sensible UVB recommendation in the hobby traces back to one idea: rather than guessing what a reptile needs, go and measure what it chooses. Ferguson et al. (2010), publishing in Zoo Biology, did exactly that, recording the ultraviolet exposure that wild reptiles voluntarily place themselves in across the course of a day. They sorted species into four bands by behaviour, from shade dwellers through to midday desert baskers.
That work was academic until Baines et al. (2016) turned it into something a keeper could use. Their paper, "How much UVB does my reptile need? The UV-Tool", in the Journal of Zoo and Aquarium Research 4(1): 42-63, assigned hundreds of species to zones and expressed the requirement in UV index, which is a number a handheld meter reads directly. Those are the two references behind every figure in the table below.
Each zone carries two numbers that do different jobs. The mean is the average UV index across the animal's active day, taking shade and movement into account. The basking-site maximum is what the animal will tolerate at the hottest, brightest spot it deliberately occupies. A lamp is aimed at the second of those, because the basking site is the only place in the enclosure where you control the dose.
| Zone | What it describes | Mean UVI | Basking-site max UVI | Example species |
|---|---|---|---|---|
| Zone 1 — Crepuscular or shade dweller | Active at dawn, dusk or night, or living in deep shade. Thermal conformers that take very little sun by choice. | 0–0.7 | 0.6–1.4 | Leopard Gecko, Crested Gecko, Gargoyle Gecko |
| Zone 2 — Partial sun, occasional basker | Basks intermittently, usually in dappled shade or early and late in the day rather than at midday. | 0.7–1 | 1.1–3 | Ball Python, Kingsnake / Milk Snake |
| Zone 3 — Open or partial sun basker | A committed basker that thermoregulates actively in open sun for part of the day. | 1–2.6 | 2.9–7.4 | Bearded Dragon, Blue-Tongue Skink, Green Anole |
| Zone 4 — Mid-day sun basker | Desert and open-country species that bask in full midday sun. The highest UV requirement in captivity. | 2.6–3.5 | 4.5–9.5 | Sulcata Tortoise |
The Ferguson zone system comes from Ferguson et al. (2010) in Zoo Biology, which measured what wild reptiles voluntarily expose themselves to, and was turned into a practical lighting tool by Baines et al. (2016), 'How much UVB does my reptile need? The UV-Tool', Journal of Zoo and Aquarium Research 4(1): 42-63. The mean figure is the average UV index across the animal's active day; the maximum is what it will tolerate at the basking site itself. Target values used here sit inside the published basking-site range for each zone.
Two things follow from reading that table honestly. The first is that no common pet reptile needs zero ultraviolet — even the shade dwellers sit in a band that runs up to a UV index of 1.4 at the basking site, which is a real number requiring a real lamp. The second is that the gap between zone 1 and zone 4 is roughly sevenfold, so a single lamp at a single height cannot possibly be right for a leopard gecko and a sulcata tortoise. Above a UV index of about 7.5 you are into territory that only a zone 4 desert species has any business occupying, and then only at the very top of its basking site.
A UVB tube's output falls away with distance in a curve that is steeper than most people expect. Manufacturers publish these curves, and they are the only defensible basis for a mounting distance. The Arcadia ProT5 range is used here because Arcadia publishes readable, per-strength figures.
| Distance | Arcadia ProT5 6% (Forest) | Arcadia ProT5 12% (Desert) | Arcadia ProT5 14% (Dragon) |
|---|---|---|---|
| 12 inches | UVI 4.5 | UVI 8.5 | UVI 10.5 |
| 15 inches | UVI 3.6 | UVI 6.5 | UVI 7 |
| 20 inches | UVI 2.5 | UVI 3.8 | UVI 4.6 |
| 24 inches | UVI 1.2 | UVI 2.4 | UVI 3.2 |
Read down the middle column and the point makes itself. The same 12% tube delivers a UV index of 8.5 at twelve inches, which suits nothing in captivity, and 2.4 at twenty-four inches, which is a reasonable zone 2 basking figure. The lamp did not change. The height did. This is why arguments about which tube is best are largely beside the point, and why the honest answer to "which bulb should I buy" is "whichever one you can mount at the right distance in the enclosure you actually have".
For the low zones there is a purpose-built option. The Arcadia ShadeDweller Pro (7%) reads a UV index of 0.8 at 10 inches and 0.3 at 18 inches, which is the whole usable band for a zone 1 animal inside a few inches of adjustment. It should not be mounted closer than 8 inches; the ProT5 tubes should not go closer than 10.
Mercury vapour lamps work differently. They emit heat, visible light and UVB from a single envelope, which is convenient in a large enclosure and inflexible everywhere else, because you cannot change the ultraviolet without changing the temperature. Zoo Med publishes mounting distances for the PowerSun UV by Ferguson zone and wattage:
| Ferguson zone | 80 W | 100 W | 160 W |
|---|---|---|---|
| Zone 1 | 16–26 in | 20–32 in | 26–38 in |
| Zone 2 | 16–26 in | 20–32 in | 26–38 in |
| Zone 3 | 6–16 in | 10–20 in | 16–26 in |
| Zone 4 | 4–6 in | 6–10 in | 12–16 in |
Distances published by Zoo Med for the PowerSun UV mercury vapour lamp. Mercury vapour bulbs produce heat and UVB from one fixture, which makes them convenient but means you cannot adjust the two independently.
Whatever the lamp, measure the distance to the animal's back at its highest basking position, not to the substrate and not to the floor of the enclosure. This is the mistake that quietly ruins otherwise careful setups. A branch or a stack of flagstones that raises a bearded dragon four inches closer to a tube mounted at fifteen has moved it into an eleven-inch reading, and on the 12% curve that is the difference between a sensible dose and one well past anything the zone calls for. Set the perch first, then measure from the animal's back upward, then hang the lamp.
Most enclosures put something between the lamp and the animal, and that something is not free. The losses are well documented and large enough that ignoring them will leave a zone 3 animal sitting in zone 2 light without anybody noticing.
| What the light passes through | UVB blocked | UVB reaching the animal |
|---|---|---|
| No mesh — lamp inside the enclosure | 0% | 100% |
| 1/4 inch hardware cloth | 25% | 75% |
| Standard screen top | 35% | 65% |
| Fine mesh screen | 45% | 55% |
| Ordinary window glass | 99.6% at 300 nm | Effectively none |
Mesh blocks a substantial and well documented share of UVB: roughly 25 percent for quarter-inch hardware cloth, 35 percent for a standard screen top, and up to 45 percent for fine mesh. Ordinary window glass blocks essentially all of it — 99.6 percent at 300 nm — which is why a UVB lamp outside a glass lid does nothing at all.
The glass row is the one to take seriously. A UVB lamp resting on a closed glass lid is decorative. It produces light, it produces a little warmth, and it delivers no measurable ultraviolet to the animal underneath it, which means an owner who believes the problem is solved may be watching an animal develop metabolic bone disease under a lamp that is switched on every day. The same is true of acrylic, of the plastic guard sleeve some fixtures ship with, and of a sunny windowsill, which supplies heat and nothing else.
Mesh is recoverable in a way glass is not. Because a standard screen top takes about 35 percent, the lamp above it has to produce roughly one and a half times the target index for the animal to receive the target index, and this calculator does that arithmetic before it looks up a distance. The alternative, where the enclosure allows it, is to mount the fixture inside the enclosure with the animal unable to reach it, which removes the loss entirely.
UVB output decays continuously from the moment a lamp is first switched on, and it decays faster than the visible light does. This is the trap. A tube at fourteen months looks identical to a tube at two months, lights the enclosure just as well, and may be producing a small fraction of the ultraviolet it started with. Nothing about the appearance of the fixture tells you anything at all.
Reputable UVB lamps are replaced every twelve months. UV output falls away long before the visible light does, so a bulb that still looks bright can be producing almost no usable UVB. Budget import brands can drop below useful output in three to six months. Zoo Med ReptiSun T5 HO figures are deliberately absent from this tool: the manufacturer publishes them only as a scanned image that could not be read reliably, and inventing numbers for a lamp that controls an animal's vitamin D synthesis is not acceptable.
Twelve months is the replacement interval for a reputable lamp, and it is a calendar interval rather than a judgement call. Write the date on the end cap with a permanent marker the day you fit it. For budget import brands the honest interval may be three to six months, which is the arithmetic that makes a cheap bulb an expensive one.
Coil and compact UVB bulbs concentrate their output into a narrow, steep cone, so the UV index a few inches from the bulb can be dangerously high while falling away far faster with distance than a linear tube. Photodermatitis and photokeratoconjunctivitis — burned skin and inflamed eyes — were documented in a ball python and a blue-tongue skink housed under compact bulbs in Gardiner, Baines and Pandher (2009), Journal of Zoo and Wildlife Medicine 40(4): 757-766. Use a linear T5 tube or a mercury vapour lamp instead.
The mechanism is worth understanding because it explains why the advice is not simply brand snobbery. A linear tube spreads its output along two or three feet of enclosure, so the dose changes gradually as the animal moves and the animal can select its own exposure the way it would in the wild. A coil concentrates the same output into a small, intense cone. Directly beneath it the index can be very high; a few inches to either side it collapses. An animal that positions itself under the hot spot — which is precisely what a basking reptile is inclined to do — has no gradient to retreat along, and eyes take the damage first because a reptile looking up at a small bright source cannot avert its gaze from it in the way it can from a diffuse one.
Zoo Med ReptiSun T5 HO tubes are conspicuously absent from the lamp list, and their absence is deliberate. They are widely used and by reputation perfectly good lamps. The problem is that Zoo Med publishes their distance-to-index figures only as a scanned image that could not be read reliably at the precision this calculation requires, and putting invented numbers behind a lamp that controls an animal's vitamin D synthesis is not something we are willing to do. If you are running ReptiSun tubes, use the manufacturer's own chart, or better, measure.
Which brings up the instrument that settles all of this. A Solarmeter 6.5 reads UV index directly, costs roughly what two years of replacement bulbs cost, and turns every question on this page into a measurement. Hold it at the basking site facing the lamp, at the height the animal's back occupies, and read the number. It tells you what your particular fixture, reflector, mesh, bulb age and geometry are actually delivering, which no published curve can. Every figure on this page is a defensible starting point derived from published data. A meter reading is the answer.
Everything here is educational and none of it substitutes for a reptile veterinarian. An animal already showing tremors, a soft or rubbery jaw, reluctance to bear weight, or swollen limbs needs a clinical assessment and probably bloodwork, not a new bulb. Correcting the lighting is what stops it getting worse; it is not what treats what has already happened.
It depends on the lamp's output, on the animal's Ferguson zone, and on whether the light passes through mesh on the way down. An Arcadia ProT5 12% reads a UV index of 8.5 at twelve inches and 2.4 at twenty-four, so the same tube can serve a zone 2 snake or a zone 4 tortoise depending only on where you hang it. Work out the target UV index for the species first, then find the distance on the lamp's own published curve, then add for mesh. Measure that distance to the animal's back when it is sitting at its highest basking position, not to the substrate.
They are not zone 4 desert baskers, but they are not zone 0 either. The Ferguson system places leopard geckos and corn snakes in zone 1 and ball pythons in zone 2, which means low but real exposure — a UV index of roughly 1.0 to 2.0 at the basking site. Animals in these zones emerge at dawn and dusk and take small amounts of sun deliberately. A low-output tube such as the Arcadia ShadeDweller Pro, which reads 0.8 at ten inches, is designed for exactly this. The old claim that these species need no UVB at all came from the fact that they survive without it on dietary D3, which is not the same thing as thriving.
No. Ordinary window glass blocks 99.6 percent of UVB at 300 nm, which is the part of the spectrum that drives vitamin D3 synthesis. A UVB lamp sitting on a closed glass lid delivers nothing to the animal underneath it. The same applies to acrylic, to the plastic sleeve some fixtures ship with, and to a window — a reptile sunning itself on a windowsill is getting warmth and light and no usable ultraviolet at all. Mesh is different: it blocks a large share but not all of it, between 25 and 45 percent depending on the weave.
Every twelve months for a reputable brand, regardless of how it looks. UV output decays long before the visible light does, so a bulb that is still bright can be producing almost no usable UVB. Budget import brands can fall below useful output in three to six months. Write the date on the end cap in permanent marker when you fit it, because nobody remembers otherwise, and a bulb that has quietly stopped working looks exactly like one that is working.
They are riskier than a linear tube and there is a documented reason to say so. Coil bulbs concentrate their output into a narrow, steep cone, so the UV index a few inches from the glass can be dangerously high while falling away far faster with distance than a tube does. Gardiner, Baines and Pandher (2009), in the Journal of Zoo and Wildlife Medicine 40(4): 757-766, documented photodermatitis and photokeratoconjunctivitis — burned skin and inflamed, painful eyes — in a ball python and a blue-tongue skink kept under compact bulbs. A linear T5 tube spreads its output over the length of the enclosure and is the better choice in essentially every setup.
Without D3 if the animal is under a working UVB lamp, with D3 if it is not. Dietary D3 substitutes for the D3 the skin would have made under ultraviolet light, so supplying both routes at full strength oversupplies a vitamin that is stored in fat and is toxic in excess. The better arrangement is UVB plus plain calcium, because the animal regulates its own synthesis and simply stops making more when it has enough — a self-limiting system that a supplement pot cannot replicate.
