Terrarium UVB Distance Calculator

UVB Distance Planning for Reptiles

UVB enables reptiles to synthesize vitamin D3, which is essential for calcium absorption. Too little UVB causes metabolic bone disease. Too much causes eye damage and burns.

Ferguson Zone system (UV Index required):

UVB bulb output at 30 cm (reference point):

Mercury vapour is the one row on that table you should not trust. Output between brands and wattages varies more than the whole rest of the list put together, the beam is narrow so a step sideways changes the reading, and the lamp is also the heat source, so the mounting distance is fighting two requirements at once. Take 7 as a placeholder and then measure it.

The inverse square law: UV intensity drops rapidly with distance. Doubling the distance cuts UV to 1/4 the intensity. UV at distance d = UV_at_30cm x (30 / d)^2

This means:

• T5 HO 12% at 30 cm = UV index 5.5 (Zone 4)
• Same bulb at 45 cm = UV index 2.4 (Zone 3)
• Same bulb at 60 cm = UV index 1.4 (Zone 2-3)

And it works in both directions. A weak lamp is not disqualified from a high-UV species, it simply has to sit closer than the 30 cm reference. A compact coil reading 1.5 at 30 cm reads 3.4 at 20 cm, which lands inside Zone 4. Whether that is a good idea is a separate question, and usually the answer is no: coil bulbs throw a narrow, uneven beam, so the animal gets a hot spot of UV rather than a gradient it can move through. But the arithmetic is not the thing stopping you, and a calculator that refuses to answer is hiding the trade-off rather than showing it.

Mesh vs glass: UVB does NOT penetrate glass, which filters virtually 100% of it. Fine mesh (window screen grade) blocks 30-50% of UVB depending on weave. Coarse reptile screen blocks 15-30%. Measure UV at the basking level through any mesh you use.

UVI meter: A Solarmeter 6.5 is the standard reptile UVI meter (~$120). Measure at the basking level, not at the bulb. Replace bulbs every 6-12 months, because output drops long before the visible light dims.