UVB Bulb Replacement Calculator
UVB bulbs lose the majority of their UV output long before they stop glowing. Your tortoise or lizard may be basking under a “working” bulb that provides virtually no UV. Track your last replacement and get reminded when it's time.
Why UVB replacement timing matters
UVB fluorescent tubes use phosphors that degrade over time. The phosphors responsible for UV emission degrade significantly faster than those producing visible light — which is why a bulb can appear perfectly bright while delivering almost no usable UVB to your animal.
Consequences of expired UVB
Without UVB, reptiles cannot photosynthesize vitamin D3 in their skin. D3 is essential for calcium absorption. Chronic deficiency causes metabolic bone disease (MBD) — soft or deformed bones, shell pyramiding in tortoises, and in severe cases, organ failure. MBD is preventable; replacing bulbs on schedule is the primary prevention.
Replacement intervals by bulb type
- T5 HO linear (Arcadia D3+, Zoo Med Reptisun 10.0 T5): 6 months
- T8 linear: 6 months
- Compact fluorescent (screw-in): 6 months — degrade faster than linear tubes, check at 4–5 months
- Mercury vapor bulbs: 12 months — combined heat and UVB, higher initial cost but longer lifespan
Tips for accurate tracking
- Write the installation date on the bulb with a marker when you install it
- Set a phone calendar reminder — or use the email reminder above
- Replace both bulbs at the same time if you have a dual-lamp fixture, even if one was installed later
Frequently asked questions
Can I tell when a UVB bulb is expired by looking at it?
No — UV output degrades invisibly. The visible light stays normal long after UV levels have dropped to ineffective levels. Track the installation date; it's the only reliable method.
Do I need UVB for my tortoise?
Yes. All tortoise species require UVB when kept indoors. Without natural sunlight, a quality UVB bulb is essential for vitamin D3 synthesis and calcium metabolism.
What is the best UVB bulb for tortoises?
T5 HO linear bulbs from Arcadia (D3+ 12%) or Zoo Med (Reptisun 10.0 T5) are widely regarded as the best options. Pair with a suitable reflector and mount at the correct distance per manufacturer specs.
Also caring about diet? Check food safety verdicts specific to your species — all citations included. Browse species →
This tool is for planning purposes only and does not constitute veterinary advice. Consult a reptile veterinarian for individual care recommendations.