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Bearded Dragon UVB Distance and Placement Guide

6 min read By Bearded Dragon Hub Editorial Team Updated October 5, 2026 Educational guide



This guide focuses on distance and placement for UVB lighting. For choosing which bulb to buy, see our best UVB light roundup. The numbers below are based on manufacturer specifications and current exotic-veterinary consensus, not on individual product reviews.

How far should a UVB light be from a bearded dragon?

For a T5 HO (high-output) UVB fixture — either Arcadia 12% Dragon or Zoo Med ReptiSun 10.0 — the basking spot should be 11 to 14 inches (28–35 cm) below the bulb, with no glass, plastic, or fine mesh between them.

This range comes straight from manufacturer-published UVI (UV Index) charts:

  • Arcadia D3+ Dragon Lamp (T5 HO 12%) provides UVI 4.0–4.5 at 12″ (30 cm) from the basking zone, dropping to UVI 3.0 at 16″ (40 cm) [$TRAE_REF](https://arcadiareptile.com/lighting/lamps/dragon/).
  • Zoo Med ReptiSun T5 HO 10.0 provides a similar effective distance of 8–36″ depending on whether mesh or reflector is used [$TRAE_REF](https://zoomed.com/t5-ho-reptisun-10-0-uvb/).

Target UVI for adult bearded dragons is 4.0–6.0 (Ferguson Zone 3), which both manufacturers engineer their T5 HO products to deliver at 11–14″. Hatchlings and juveniles can sit a little closer to the bulb to stay in the same UVI range.

For older T8 fixtures (now considered legacy), the basking distance doubles to roughly 20–24 inches (50–60 cm), and most welfare-focused keepers have moved to T5 HO. Compact and coil UVB bulbs are no longer recommended for bearded dragons by VCA or ReptiFiles; their effective UVI is too narrow and too variable to hit the basking zone reliably [$TRAE_REF](https://vcahospitals.com/know-your-pet/bearded-dragons-housing)[$TRAE_REF](https://reptifiles.com/bearded-dragon-care-guide/).

[IMAGE: T5 HO UVB fixture mounted inside enclosure with reflector, source-needed, alt=”Diagram showing T5 HO UVB fixture mounted inside enclosure with reflector and basking spot 12 inches below”]

UVB distance chart by setup

Use this table as a starting point. Mesh weakens UVB by roughly 25–35%, so setups with screen tops need to either move the fixture closer or mount it inside the enclosure.

Fixture type Distance without mesh (top of basking spot to bulb) Distance with mesh top Target UVI
T5 HO 12% (Arcadia Dragon / ReptiSun 10.0) 11–14″ (28–35 cm) 6–9″ (15–23 cm), ideally mounted inside enclosure 4.0–6.0
T5 HO 6% (lower-output, used for taller enclosures) 14–18″ (35–46 cm) 9–12″ (23–30 cm) 3.0–4.0 (juvenile baseline)
T8 10.0 (legacy) 20–24″ (50–60 cm) Not recommended 2.5–3.5
Compact / coil UVB Not recommended for bearded dragons

The single biggest variable in this table is the reflector. A high-quality reflector (Arcadia, Vivarium Electronics, or the in-box Zoo Med reflector) recovers roughly 50% of the upward-going UVB and redirects it down toward the basking spot. A bare bulb without a reflector wastes most of its output.

[IMAGE: UVB distance comparison chart, source-needed, alt=”UVB distance chart comparing T5 HO 12 percent T5 HO 10.0 and T8 fixture distances with and without mesh”]

What blocks UVB?

Three common materials and how they impact effective UVB at the basking spot:

  • Glass and acrylic (plexiglass): block virtually 100% of UVB. A UVB fixture mounted under a glass top is delivering essentially no UVB. This is the single most common setup mistake [$TRAE_REF](https://zoomed.com/wp-content/uploads/LF-70_ReptiSun_T5_HO_Terrarium_Hood_Instructions.pdf).
  • Fine screen mesh (aluminum or steel window screen): blocks roughly 25–35% of UVB. A wide-mesh reptile-screen top (e.g. PVC-coated hardware cloth) blocks about 10–15%. Either way, mounting the fixture inside the enclosure below the screen restores the full output [$TRAE_REF](https://reptilewise.com/tools/uvb-distance-calculator/).
  • Plastic diffusers and lamp covers: also block 100% of UVB. Always run UVB fixtures with the manufacturer’s guard removed or with a UVB-transparent guard.

Where to place the UVB fixture relative to the basking spot

The UVB fixture should run parallel to the length of the enclosure, mounted on the same side as the basking bulb but on the opposite side of the screen top. In a typical 4′ × 2′ × 2′ tank:

  1. Mount the T5 HO fixture on the warm side ceiling, inside the enclosure if possible.
  2. Position the basking spot (a flat rock or branch) directly below the center of the bulb, at the distance listed in the table above.
  3. The cool side of the enclosure should fall off naturally to UVI 0–1 at the opposite end — this gradient is normal and lets the dragon self-regulate.

[IMAGE: three common UVB mistakes, source-needed, alt=”Three common UVB setup mistakes showing fixture above mesh glass barrier and no reflector”]

When to replace the UVB bulb

UVB output falls steadily even when the visible light looks normal. Manufacturer and veterinary consensus:

  • T5 HO bulbs: replace every 12 months, regardless of whether the bulb still glows [$TRAE_REF](https://vcahospitals.com/know-your-pet/bearded-dragons-housing).
  • T8 bulbs: replace every 6 months.
  • Compact / coil bulbs: replace every 6 months if used (most welfare-focused keepers have replaced them entirely).

Two practical tips:

  1. Write the install date on the bulb with a permanent marker. When you buy a replacement, the old one goes in the trash — not back in the fixture.
  2. Avoid “high-output” claims without UVI numbers. Reputable manufacturers (Arcadia, Zoo Med, Exo Terra) publish UVI charts. Brands that don’t are selling visible light, not UVB.

How to know if your UVB isn’t strong enough

This is a setup article, not a diagnosis page, but a few practical signs that your UVB output may be too low (and that you should bring the fixture closer or replace the bulb):

  • Your dragon stays in the basking spot for most of the day and rarely moves to the cool side.
  • Your dragon is shaking or twitching, especially in the limbs — see our shaking guide for triage.
  • Your dragon is not eating after ruling out brumation — see our not-eating checklist.
  • You are using a T8, compact, or coil bulb, or your UVB bulb is over 12 months old.

None of these signs is a diagnosis. Metabolic bone disease (MBD) is preventable with correct UVB + calcium, and our MBD guide covers how the two interact.

Quick answers

How far should a UVB light be from a bearded dragon?

For a T5 HO 12% or 10.0 fixture, place the basking spot 11–14″ (28–35 cm) below the bulb, with no mesh between them. With a mesh top, mount the fixture inside the enclosure 6–9″ from the basking spot.

Is 12% or 10.0 UVB better for bearded dragons?

Both are designed for bearded dragons. Arcadia 12% and Zoo Med 10.0 are the most widely used. Arcadia 14% is for taller enclosures (≥24″). Choose based on enclosure height, not bulb strength.

Do I need UVB if my dragon goes outside?

Yes. Outdoor sun exposure is the gold standard and helps, but it cannot reliably substitute for indoor UVB unless your dragon has supervised, unfiltered, direct sun for several hours a week. Most owners use a T5 HO indoors year-round.

Can I put the UVB on a timer?

Yes — pair it with the basking bulb on a 12–14 hour on / 10–12 hour off cycle. UVB does not need to ramp down; abrupt on/off is fine for diurnal species.

What’s the difference between T5 and T8?

T5 HO is a smaller-diameter, higher-output bulb that can sit closer to the basking zone and still deliver UVI 4–6. T8 is older technology with lower output and needs to be roughly twice as far away. For new purchases, T5 HO is the standard.

Sources & review

Last reviewed: 2026-09-01 · Next scheduled review: 2027-03-01 · Editor: Bearded Dragon Hub Editorial Team

Educational content only. Not a substitute for hands-on veterinary care. If your dragon shows signs of illness, contact an ARAV-listed exotics veterinarian.

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