Terrarium UVB Distance Calculator
Find the correct UVB bulb distance for your reptile.
Enter species UV index requirement and bulb type to get the safe basking distance in inches and cm.
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):
| Zone | UV Index | Species Examples |
|---|---|---|
| Zone 1 | 0-0.7 | Nocturnal geckos (leopard, crested), pythons |
| Zone 2 | 0.7-1.0 | Corn snakes, blue-tongued skinks, day geckos |
| Zone 3 | 1.0-2.6 | Water dragons, chameleons, red-eyed tree frogs |
| Zone 4 | 2.6-6.0 | Bearded dragons, tortoises, iguanas |
| Zone 5 | 6.0-9.7 | Uromastyx, desert-dwelling lizards |
UVB bulb output at 30 cm (reference point):
| Bulb Type | UV Index at 30 cm |
|---|---|
| Compact coil (10%) | ~1.5 |
| T8 fluorescent (10%) | ~2.0 |
| T5 HO (6%) | ~3.5 |
| T5 HO (10-12%) | ~5.5 |
| T5 HO (14%) | ~8.0 |
| Mercury vapor (160W) | ~9.5 at 45 cm |
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)
Mesh vs glass: UVB does NOT penetrate glass — glass filters virtually 100% of UVB. 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 — output drops before the visible light dims.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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