What size UVB bulb for your tank

Why gallons cannot answer this

A gallon rating is a volume, and UVB is delivered over a length at a distance. Two 40-gallon tanks, one tall and one long, need different tube lengths and different mount heights, and a 120-gallon enclosure tells you nothing about whether the lamp can sit 11 inches above the basking surface. So the sizing is done from length and height, with the gallon figure only used to translate a listing.

Question one: how long should the tube be?

The published guidance for a bearded dragon is a tube about half the length of the enclosure, mounted on the same side as the heat lamps. That gives a lit basking zone at one end and a genuinely shaded end at the other, which is what lets the animal regulate its own exposure. A tube that runs the full length removes the choice.

Question two: what output class?

This is set by the animal, never by the enclosure. The species Ferguson zone gives a basking UVI target, and the lamp has to be able to reach that target at a distance your enclosure actually allows.

Wattage is not a UVB size

Wattage describes power draw, not ultraviolet output, and the two do not track. A higher wattage T5 does not mean a higher UVI at your basking surface; the percentage rating, the fixture reflector and the distance do. Wattage is a heat-lamp question instead, and it has its own page - what wattage heat bulb for a reptile.

The fixture is part of the size

Every published distance figure assumes a reflector, and a reflector roughly doubles usable output. Run the same tube bare and you are not at a known UVI at all. The fixture also locks your tube length, so buy the two together - or buy a matched kit, which is why the ShadeDweller kit is our usual gecko recommendation. Fixtures are judged on their own page.

Where the lamp sits along the enclosure

A half-length tube has to go somewhere, and the correct side is the warm side - the same end as the heat lamps. Lighting and heat belong together because that is how the animal reads an enclosure: one end is sun, the other end is shade, and both the ultraviolet and the warmth should be in agreement about which is which. Splitting them puts the UVB over the cool end and asks the animal to choose between basking and ultraviolet.

The shaded end is a requirement rather than leftover space. A shade-dwelling species in particular regulates its own exposure if you give it somewhere to retreat to, which is why a 12 inch lamp over a tall crested gecko enclosure is the right size even though it covers only part of the height.

Height interacts with this. In a tall enclosure the lamp usually has to come down from the ceiling on a hanging kit rather than sit on the lid, because a ceiling-mounted tube in a 36 inch tall terrarium is far outside the published 8 to 12 inch distance for a crested gecko. Measure from the lamp to the branch or platform the animal actually uses, not to the floor.

Three things that make a correctly sized lamp useless

• A compact coil instead of a linear tube. A coil lights a point; a basking area is a zone, and every distance figure on this page assumes a tube.
• Glass or plastic over the lamp. Both block UVB completely while the visible light carries on looking perfectly normal.
• An expired tube. UVB output falls long before visible output does. Twelve months, on the calendar - the detail is here.

Then verify it

A distance chart plus a mesh assumption is an estimate, and published mesh corrections span roughly a 30% to 45% block - wide enough that two keepers with identical lamps at identical heights can be delivering meaningfully different UVI. A Solarmeter 6.5 or 6.5R turns the estimate into a reading, and UVB distance and Ferguson zones explains how to use it.