Reptile UVB Distance Calculator
How far the lamp should sit from the basking spot to hit your species' Ferguson zone.
https://calculators.nirajiitr.com/pets/reptile-uvb-calculator
How it works
A fluorescent tube is a line source rather than a point source, so its output falls off closer to linearly with distance than the inverse-square law you might expect — doubling the distance roughly halves the UV rather than quartering it. Lamp factor scales for the tube type, and the difference is stark: a T5 high-output tube delivers several times what a compact coil does at the same distance. Mesh matters more than most people expect, with fine insect screen absorbing around 40% of the UV passing through it.
UVI ≈ base output × lamp factor × (30 / distance) × mesh factor- Ferguson zone
- How much UV a species seeks out in the wild, 1 to 4
- UVI
- UV index at the basking surface
- lamp factor
- Relative output: T5 12% is the reference
- mesh factor
- 0.6 for fine mesh, 0.8 for coarse, 1.0 for none
Worked example
A bearded dragon (zone 4, target UVI 2.6–3.5) under a T5 HO 12% tube through fine mesh.
- 1Lamp factor 1.0 for T5 12%, mesh factor 0.6 for fine mesh
- 2At 30 cm: 5.5 × 1.0 × (30/30) × 0.6 = 3.3 UVI
- 3That sits inside the 2.6–3.5 target band
- 4At 45 cm it would fall to 2.2 — below the zone
Around 30 cm from tube to basking surface. The mesh alone costs nearly half the output, which is why a mesh-top setup needs the lamp far closer than a tube mounted inside.
Frequently asked questions
What is a Ferguson zone?
It is a classification of how much UV a species actually exposes itself to in the wild, measured by researchers following animals in their native habitat. Zone 1 covers shade dwellers and nocturnal species such as crested and leopard geckos; zone 4 covers open mid-day baskers such as bearded dragons and uromastyx. Matching the zone matters more than matching a lamp wattage, because a species that evolved in forest shade does not benefit from desert-level UV and can be harmed by it.
Why do compact and coil UVB bulbs get such a bad reputation?
Partly history and partly physics. Early compact bulbs from the 2000s produced UV at the wrong wavelengths and caused widespread eye damage, and although modern ones are much improved, the format is still poor: output is low, it falls away very quickly with distance, and it lights only a small spot rather than a gradient. A linear T5 high-output tube running most of the enclosure length gives a usable gradient the animal can move through, which is what you actually want.
How often do UVB tubes need replacing?
Every six to twelve months depending on the brand, and this is the part people miss most often. The tube keeps producing perfectly good visible light long after the UVB output has decayed below anything useful — so it looks fine while the animal is effectively getting nothing. Write the installation date on the tube, and replace on schedule whether or not it still lights.
Does mesh really block that much?
Yes, and it is the commonest reason a setup that looks right on paper fails. Standard fine insect screen absorbs roughly 40% of UVB, and finer or painted mesh can take more. Mounting the tube inside the enclosure removes the problem entirely, and is what most keepers move to eventually; failing that, the lamp simply has to sit proportionally closer to compensate.
Can a reptile get too much UVB?
It can. Excess UV causes photo-kerato-conjunctivitis — painful inflammation of the eyes — and skin damage, and the animal cannot always avoid it in a small enclosure. This is why a gradient matters: there must be a properly shaded area the animal can retreat to, so it can self-regulate the way it would in the wild. A correct basking UVI with nowhere to escape it is still a bad setup.