# How often to replace a UVB bulb

> Set the bulb type and install date to get a replacement countdown for your UVB tube: 10–12 months for T5 HO, about 6 months for T8. The cycles come from the Arcadia ShadeDweller PRO specification and ReptiFiles guidance.

## Direct answer

Replace T5 HO tubes every 10–12 months and T8 tubes about every 6 months, per the Arcadia ShadeDweller PRO specification and ReptiFiles. Fluorescent UVB output decays with use while the tube still looks bright, so replace on schedule rather than judging by brightness.

## Replacement countdown

Use the interactive calculator on the page: https://petguidecalc.com/reptile/uvb-bulb-replacement/ — pick a bulb type and install date to get a replacement countdown, status, and due date. For T8, set the replacement cycle from 6–12 months.

## Replacement cycles at a glance

| Bulb type | Replacement cycle | Basis |
|---|---|---|
| T5 HO | 10–12 months | ShadeDweller PRO specification |
| T8 | ~6 months (adjustable 6–12) | Faster output decay |

## How the countdown is calculated

Fluorescent UVB tubes lose output as they run, but the visible light stays roughly constant, so a worn tube can look fine while producing much less UVB. This calculator treats replacement as a countdown from the install date instead of a guess based on brightness.

The cycles come from the Arcadia ShadeDweller PRO specification sold by Light Your Reptiles: replace T5 HO tubes every 10–12 months. T8 tubes decay faster, so they default to about 6 months, and the calculator lets you set 6–12 months to match your tube and fixture.

The countdown subtracts the time in service from the recommended cycle, then reports OK, DUE SOON, or OVERDUE with a replacement due date. A tube about half the enclosure length suits most enclosures, and a UV index meter reading at the basking surface is the final check for real output.

## UVB output questions, answered

**Q: How often should I replace a UVB bulb?**

Arcadia's ShadeDweller PRO specification, sold through Light Your Reptiles, sets T5 HO replacement at every 10–12 months. T8 tubes decay faster, so plan on about 6 months and adjust up to 12 months with this calculator. Fluorescent UVB output drops with use while the tube keeps looking bright, so the calendar matters more than appearance.

**Q: Why does my UVB tube look bright but still need replacing?**

Fluorescent UVB output decays with running hours while visible light stays roughly the same. A tube can light the enclosure brightly and still deliver a fraction of its original UVB. Replace T5 HO at 10–12 months and T8 at about 6 months, then measure the basking UV index with a UV index meter, as ReptiFiles recommends.

**Q: How is the replacement countdown calculated?**

The calculator counts months from your install date to the recommended cycle, 10 months for T5 HO or 6 months for T8 by default, and reports OK, DUE SOON, or OVERDUE. For example, a T5 HO installed 11 months ago and run at the 12-month end of the window shows DUE SOON with about 1 month left.

**Q: What happens if I use a UVB tube past its replacement date?**

Past the replacement point the tube can still look bright while delivering far less UVB, so the enclosure looks lit but the animal gets less UV. ReptiFiles targets a bearded dragon basking UV index of 3.0–6.0; after swapping the tube, confirm the reading at the basking surface.

**Q: How long should the UVB tube be for my enclosure?**

A tube roughly half the enclosure length is a practical starting point, so a 48-inch enclosure suits a 24-inch (60 cm) tube. Arcadia's ShadeDweller PRO T5 line covers common sizes. Judge by the delivered output at the basking surface, not by length alone.

## Next steps for your setup

- [Reptile UVB Calculator](/reptile/uvb-calculator/) — Bulb distance and expected UV index by species, the setup half of UVB lighting.
- [Bearded Dragon Temperature Checker](/reptile/bearded-dragon-temperature/) — Check basking and cool-side temperatures against bearded dragon targets.
- [All Reptile Care Tools](/reptile/) — The reptile calculator hub, including heating, humidity, and feeding tools.

## References

The replacement cycles and UV verification guidance come from the sources below, accessed September 2026.

- [Quantifying the vitamin D3 synthesizing potential of UVB lamps at specific distances over time](https://doi.org/10.1002/zoo.20345) — Zoo Biol, 2010.
- [Light Your Reptiles — Arcadia ShadeDweller PRO T5 Bulb](https://www.lightyourreptiles.com/products/arcadia-shadedweller-pro-t5-bulb-12.html) — Light Your Reptiles. Product page for the Arcadia ShadeDweller PRO T5 that specifies replacement every 10–12 months.
- [ReptiFiles — Bearded Dragon Temperatures & UVB](https://reptifiles.com/bearded-dragon-care/bearded-dragon-temperatures-uvb/) — ReptiFiles. Bearded dragon UVB guidance, including the 3.0–6.0 basking UV index target and meter-verification advice.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_