Bearded Dragon UVB Distance & Replacement Schedule
Exact UVB distance and replacement guidance for bearded dragons: 12 to 18 inch mounting, 6 to 12 month replacement, target UV index, and why a glowing bulb can still fail.
You can own the perfect UVB tube and still fail your bearded dragon if the distance is wrong or the bulb is past its life. UVB is invisible, fades silently, and is blocked by glass and mesh, so the two questions that matter most are: how far should the bulb sit from my dragon, and how often must I replace it? This guide gives precise answers and shows how a UV index meter removes the guesswork entirely.
Quick Answer
- Distance: T5 HO 10.0 mounted on screen, 12 to 18 inches from bulb to your dragon's back at the basking spot.
- Replacement: every 6 to 12 months per the manufacturer, even if it still glows.
- Target UV index: roughly 3 to 4.5 at the basking spot, near zero in the shade.
- Verify with: a reptile UV index meter, the only way to know the real number.
- Remember: glass blocks UVB entirely; mesh blocks a third or more.
Tools for Getting Distance and Timing Right
Solarmeter Reptile UV Index Meter (Solarmeter 6.5R)
$249.00 on Amazon
The reference handheld meter for measuring reptile UV index precisely.
Giangarden Reptile UVI Radiometer (data logging)
$88.98 on Amazon
More affordable UV index meter with data logging for tracking decline.
REPTI ZOO T5 HO 10.0 UVB Replacement Tube (24W)
$37.99 on Amazon
Fresh desert tube for your scheduled 6 to 12 month swap.
Why distance is so critical
UVB strength falls off rapidly with distance. A T5 HO 10.0 is a powerful desert-strength tube, so a basking spot too close to the bulb can overexpose your dragon and irritate its eyes and skin, while one too far gives too little UV to support vitamin D3 production. The sweet spot is a band where the UV is strong enough to be useful but not so strong it causes harm. Your job is to put the basking platform at the right height within that band.
Distance by mounting method
| Mounting | Recommended distance to dragon's back |
|---|---|
| On top of mesh screen (T5 HO 10.0) | 12 to 18 in |
| Inside enclosure, no mesh (T5 HO 10.0) | 14 to 20 in |
| Mercury vapor bulb | follow maker's chart, often 12 to 18 in |
These are starting points. Brand, reflector design, tube age, and mesh density all shift the real number, which is why a meter is the gold standard. Set your basking platform height so your dragon's back lands in the right band when it is up and basking, not the floor of the tank.
Glass and mesh: the hidden blockers
Two materials quietly sabotage UVB setups. Glass and clear plastic block UVB almost completely, so a tube shining through a glass lid delivers essentially nothing. Mesh screen blocks a meaningful fraction, often a third or more with fine, dense mesh. This is why a screen-mounted tube must sit closer than an open-mounted one, and why mounting the fixture inside the enclosure gives stronger UV at the same distance. Use the widest-gap safe screen you can.
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The replacement schedule
Here is the rule that prevents the most metabolic bone disease cases in otherwise good setups: replace your UVB tube every 6 to 12 months, following the manufacturer's interval, regardless of how bright it still looks. UVB output and visible light are produced by different mechanisms, and the UVB fades long before the glow does. A tube that is two years old can light up the tank perfectly while delivering almost no usable UV.
To stay on schedule:
- Write the install date directly on the tube with a permanent marker.
- Set a recurring calendar reminder for the replacement date.
- Keep a spare tube on hand so you can swap immediately.
- If you own a meter, log the UV index every couple of months and replace when it drops below your target.
How a UV index meter removes the guesswork
A reptile UV index meter reads the actual UV index at any point in the enclosure. Hold it at the basking height, through the mesh if your tube is screen-mounted, and you see the real number your dragon receives. With a meter you can:
- Set the basking platform to hit a target UV index of roughly 3 to 4.5.
- Confirm the cool end tapers toward zero, proving you have a real gradient.
- Measure how much your specific mesh reduces UVB.
- Track the decline over months and replace the bulb exactly when output drops, not by a rough guess.
A meter is a high-ticket tool, but for keepers who want certainty, or who keep several reptiles, it pays for itself by preventing both underexposure and overexposure. If a meter is out of budget, follow the distance and replacement defaults above closely, and consider asking an experienced local keeper or reptile vet to check your setup.
Putting it together
- Mount the T5 HO 10.0 on or inside the screen, never under glass.
- Set the basking spot 12 to 18 inches from the tube.
- Replace every 6 to 12 months and log the date.
- Verify with a UV index meter when possible, targeting 3 to 4.5 at the basking spot.
- Pair correct UVB with regular calcium dusting to keep bones strong.
Distance and replacement are where good intentions go wrong. Get both right and your dragon receives the UV it needs, year after year, with no silent slide toward bone disease.
Frequently Asked Questions
How far should a T5 HO UVB bulb be from a bearded dragon?
For a T5 HO 10.0 tube mounted on the mesh screen, position the basking spot so your dragon's back sits about 12 to 18 inches from the bulb. If the tube is mounted inside the enclosure with no mesh in the way, increase that distance slightly because more UV reaches your dragon. Brand and reflector affect the exact figure, so the safest approach is to confirm the UV index with a meter at the basking height.
How often should I replace a bearded dragon's UVB bulb?
Replace a T5 HO UVB tube every 6 to 12 months, following the manufacturer's stated interval, even though it still produces visible light. UVB output declines steadily and invisibly, so a bulb that looks fine may no longer give your dragon usable UV. Mark the install date on the tube or a calendar, and replace on schedule. A UV index meter lets you track the real decline and replace exactly when needed.
Can a UVB bulb still work if it is glowing?
A glowing tube does not mean working UVB. Visible light and UVB are produced differently, and the UVB fades long before the glow does. This is the single most common reason dragons develop metabolic bone disease in otherwise good setups: the keeper sees light and assumes the bulb is fine. Always replace on schedule by date, regardless of how bright the tube looks, or verify output with a meter.
Do I need a UV index meter?
A UV index meter is not strictly required, but it is the only way to know the actual UV your dragon receives. It removes guesswork about distance, reflector efficiency, mesh blockage, and bulb decline. Serious keepers use one to dial in the basking zone to a target UV index and to time replacements precisely. If you keep multiple reptiles or want certainty, a meter is a worthwhile high-ticket investment.
What UV index should my bearded dragon's basking spot be?
Bearded dragons are a high-UV desert species, so keepers commonly target a UV index of roughly 3 to 4.5 at the basking spot, tapering to near zero at the shaded cool end to create a gradient. This is measured with a reptile UV index meter at the height of your dragon's back. Without a meter, follow the bulb maker's distance chart and keep the tube in the 12 to 18 inch range as a safe default.
Does mesh screen reduce UVB?
Yes. A mesh screen between the bulb and your dragon blocks a meaningful portion of UVB, often a third or more depending on how fine and dense the mesh is. That is why a screen-mounted tube needs to be slightly closer than one mounted inside the enclosure. Use the widest-gap safe screen you can, and if you use a meter, measure through the actual mesh to see the real number reaching the basking spot.
Should I replace a mercury vapor bulb on the same schedule?
Mercury vapor bulbs also lose UVB output over time and should be replaced on the manufacturer's recommended schedule, often around 12 months, even while they still produce heat and light. Because mercury vapor bulbs combine heat and UVB, a fading bulb can keep your dragon warm while quietly giving too little UV. Track the install date and replace on schedule, or verify output with a UV index meter.
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