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FurCalc

Reptile UVB Distance Calculator — Ferguson Zone Basking Distance

Calculate the correct distance from a UVB bulb to your reptile's basking spot using the Ferguson zone system (Baines et al., 2016).

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Screens reduce UVB by 30-50 %.

Basking distance
10inches from bulb
Target UVI: 2.6–3.5
For precision, use a Solarmeter 6.5R UV Index meter. Replace bulbs every 12 months (T5/T8) or 6-12 months (mercury vapor) — UV output decays even while visible light remains strong.

Frequently Asked Questions

What are Ferguson zones?

Ferguson zones (Baines et al., 2016) classify reptiles by their natural UV Index exposure. Zone 1: shade-dwellers (ball pythons). Zone 2: partial sun (leopard geckos). Zone 3: occasional baskers (chameleons). Zone 4: full-sun baskers (bearded dragons).

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

Zone 4 target: UVI 2.6-3.5. A ReptiSun 10.0 T5 HO above a screen lid: basking spot at 10 inches. Without screen: 12 inches.

Does a screen lid reduce UVB?

Yes — by 30-50 % depending on mesh density. Always position baskets closer under screens, or mount the bulb through a cutout.

How often should I replace UVB bulbs?

T5 HO and compact bulbs: every 12 months. Mercury vapor: 6-12 months. UV output decays while visible light stays bright — they 'look' fine but provide no UVB.

Do I need a UVI meter?

Yes, ideally. Solarmeter 6.5R is the industry standard ($200). Without one, use manufacturer distance recommendations and replace bulbs on schedule.

Sources & References

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Common mistakes to avoid

  • Placing UVB through glass or plastic — blocks 100% of UVB. Mesh top only.
  • Using UVB bulb past 12 months — UV output degrades even if bulb still lights. Replace annually.
  • Wrong bulb for species — bearded dragon needs 10-12% T5 HO; leopard gecko 5-6% is enough.

Pro tips

  • Verify UVB output with a Solarmeter 6.5 if possible — consumer-grade but reliable.
  • Place basking spot at correct distance per bulb spec (usually 10-15 inches for T5 HO).
  • Mercury vapor combos (heat + UVB in one bulb) have variable UVB output — check at 3 points on basking spot.