Reptile UVB Calculator
The reptile UVB calculator helps you choose the correct bulb distance and daily photoperiod for your reptile's UVB setup. UVB light enables reptiles to synthesise vitamin D3, which is essential for calcium metabolism. Without correct UVB exposure, reptiles develop metabolic bone disease. This calculator uses the Ferguson Zone system to categorise your species by natural UV exposure and recommend a target UVI (UV Index) at the basking spot. Select your species category and bulb type to get recommended basking distance and daily photoperiod in hours.
Ferguson Zone UVI targets
Zone 1: 0 - 0.60 UVI (shade dwellers)
Zone 2: 0.60 - 1.0 UVI (partial shade)
Zone 3: 1.0 - 2.6 UVI (part-time baskers)
Zone 4: 2.6 - 7.4 UVI (intense baskers)
Distance guidance for T5 HO 10%:
30 cm: 4-8 UVI, 45 cm: 2-4 UVI, 60 cm: 1-2 UVI
Frequently asked questions
Why do reptiles need UVB light?
Most diurnal (day-active) reptiles need UVB radiation to synthesise vitamin D3 in their skin, which is essential for calcium absorption. Without adequate UVB, reptiles develop metabolic bone disease (MBD), characterised by soft, deformed bones, lethargy, and muscle tremors. Nocturnal species like leopard geckos need little or no UVB.
What is UVI and how does it relate to UVB bulb placement?
UVI (UV Index) is a standardised scale for measuring UV intensity. The Ferguson Zone system, developed by Gary Ferguson et al., classifies reptiles into zones 1 to 4 based on their natural UV exposure. Zone 1 (crepuscular, 0-0.6 UVI) to Zone 4 (basking in high UV, 4.0+ UVI). Bulb placement determines the UVI at the basking spot.
How far should I place a UVB bulb from the basking area?
Distance depends on bulb wattage and type. A 10% T5 HO tube at 30 cm typically delivers 4 to 8 UVI. At 45 cm, it delivers 2 to 4 UVI. At 60 cm, it delivers 1 to 2 UVI. Linear T5 bulbs provide wider coverage than compact bulbs. Always check the manufacturer's distance chart for your specific bulb.
How many hours of UVB light should I provide?
Photoperiod should mimic the natural daylength for the species' region of origin, typically 10 to 14 hours in summer and 8 to 10 hours in winter. A consistent 12-hour photoperiod works for many tropical species. Use a timer so the animal experiences a predictable day/night cycle.
When should I replace a UVB bulb?
UVB output degrades over time even if the bulb still emits visible light. Most T5 HO fluorescent UVB bulbs should be replaced every 12 months. Some manufacturers rate bulbs for up to 18 months. LED and mercury vapour bulbs have different schedules. Test with a UV meter (Solarmeter 6.5 is widely used) rather than relying on appearance.
Sources
- Ferguson, Gary W. et al. (2010): UV Exposure and Vitamin D3 in Free-Living Reptiles. Published in Copeia, American Society of Ichthyologists and Herpetologists.
- USGS: USGS - Solar Radiation and the Water Cycle.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.