Reptile UVB Calculator - Ferguson Zone, UVI and Timer

A UVB lamp that still glows can be doing half its job

The visible light of a UVB tube tells you nothing about its UV output. In tests published with the UV-Tool, tubes had lost 39.9% of their UV index on average after 4,000 hours, and one brand lost 64% after only 1,000. This reptile UVB calculator starts from what the animal needs instead: the Ferguson zone of the species, the UV index range across the enclosure and at the basking spot, the photoperiod for summer and winter turned into a month-by-month timer schedule, and where your lamp stands on its decay curve. A reading from a UV index meter is checked against the zone, and lamp wattages give the yearly electricity.

The problem: a bearded dragon basking under a tube bought last year may get a fraction of the UV it got when the tube was new, with nothing visible to show it. The same mistake works the other way: a strong lamp mounted too low can push a shade-dwelling gecko far above anything it meets in the wild.

The fix: know the target range for the species, measure the basking spot with a UV index meter, and log the lamp hours. The calculator puts all three side by side, so a falling reading is caught before the animal shows it.

What the calculator asks for

Species - 18 common pet reptiles from the UV-Tool appendix, or Other species with a zone and climate you choose.
Lights-on time - optional, like 7:00 AM; without it the day is centered on noon.
Measured UV index - optional, at the basking spot at animal height.
Hours the UVB lamp has been used - optional, for the decay check.
Wattage of the UVB, heat and daylight lamps and the price per kWh - optional, for the yearly electricity.

Ferguson zones and the UV index range for each

Ferguson and colleagues measured the UV exposure of free-living reptiles and sorted species into four groups by how they use the sun. The UV-Tool authors summarized the zones in their first table, with one range for the general gradient and one for the maximum recorded, which they suggest as the ceiling for zones 1 to 3.

Zone	Behavior	Zone range UVI	Maximum UVI	Lamp method
1	Crepuscular or shade dweller, thermal conformer	0-0.7	0.6-1.4	shade method
2	Partial sun or occasional basker	0.7-1.0	1.1-3.0	shade method
3	Open or partial sun basker	1.0-2.6	2.9-7.4	sunbeam method
4	Mid-day sun basker	2.6-3.5	4.5-9.5	sunbeam method

The shade method spreads a gentle gradient from the zone maximum near the lamp down to zero in the shade; the sunbeam method lights a bright basking zone, like a patch of sun. Zone 4 is the one exception to the ceiling, because some of those species bask at UVI 9.5 or more in the wild.

UVB needs by species: zone, photoperiod and winter

The species rows below come from the UV-Tool appendix. Photoperiod is given as summer:winter hours of light, and the winter column is the treatment the appendix lists. Where a species spans two zones, the ranges run from the lower zone to the upper one.

Species	Zone	Gradient UVI	Basking max UVI	Photoperiod	Winter
Bearded dragon	3-4	1.0-3.5	2.9-9.5	13:11 (or 14:10)	none listed
Leopard gecko	1	0-0.7	0.6-1.4	14:10	cooling
Crested gecko	1	0-0.7	0.6-1.4	14:10	cooling
Veiled (Yemen) chameleon	3	1.0-2.6	2.9-7.4	13:11	none listed
Panther chameleon	3	1.0-2.6	2.9-7.4	12:12 (or 13:11)	none listed
Blue-tongued skink	2-3	0.7-2.6	1.1-7.4	13:11 (or 14:10)	cooling
Egyptian uromastyx	4	2.6-3.5	4.5-9.5	14:10	cooling
Savannah monitor	3-4	1.0-3.5	2.9-9.5	13:11	cooling
Asian water dragon	2-3	0.7-2.6	1.1-7.4	13:11	cooling
Giant day gecko	3	1.0-2.6	2.9-7.4	13:11	none listed
Green anole	2	0.7-1.0	1.1-3.0	14:10	cooling
Ball python	2	0.7-1.0	1.1-3.0	12:12	cooling
Corn snake	1-2	0-1.0	0.6-3.0	13:11	none listed
Boa constrictor	2	0.7-1.0	1.1-3.0	13:11	cooling
Red-eared slider	3-4	1.0-3.5	2.9-9.5	13:11	hibernation
Hermann's tortoise	3	1.0-2.6	2.9-7.4	14:10	hibernation
Spur-thighed (Greek) tortoise	3	1.0-2.6	2.9-7.4	14:10	hibernation
Russian (Horsfield's) tortoise	3	1.0-2.6	2.9-7.4	14:10	hibernation

Decide by the reading, not by the calendar

What you see	What it means	What to do
Lamp under 105 h	Still burning in, output dropping fast (12.6% on average, 6-23%)	Measure again after burning-in
Basking reading below the zone maximum	Lamp too far, behind mesh, or decayed	Lower it or remove mesh; if that fails, replace
Reading within the maximum range	Basking spot on target	Check that the far end drops toward shade
Reading above the maximum, zones 1-3	More than recorded in the wild	Raise the lamp until it reads in range
Lamp past 4,000 h	Tested tubes were down 30-48%	Measure; replace when below the range

Six enclosures, six results

Bearded dragon, lights at 7:00 AM

Zones 3-4, gradient UVI 1.0-3.5, basking 2.9-9.5. Lights off at 8:00 PM in June (13 h) and 6:00 PM in December (11 h); 4,381 lamp hours a year.

Same dragon, meter reads 5.2

Within the 2.9-9.5 maximum. At 2,500 h the lamp is past burning-in and about 4.1 months from the 4,000 h test mark on this schedule.

Ball python under a strong tube

Zone 2. A basking reading of 3.5 is above the 3.0 ceiling, so the lamp goes higher or further to one side.

Leopard gecko, no switch-on time

Zone 1, UVI 0-0.7. Day centered on noon: 5:00 AM to 7:00 PM in June, 7:00 AM to 5:00 PM in December.

Lamp at 60 hours

Still burning in. Measure the basking spot again once it passes 105 hours; the early drop averaged 12.6%.

Electricity for three lamps

24 W UVB plus 75 W heat on the dragon schedule: 434 kWh a year, $73.73 at $0.17 per kWh.

Questions about UVB lighting

How long should a UVB light be on each day?

As long as the daylight period for the species: 12 hours all year for tropical species, 13 in summer and 11 in winter for subtropical, 14 and 10 for temperate. The table above lists each species; the calculator spreads the change over the months.

Should the UVB light be on at night?

No. UVB is part of the daylight period and goes off with the other lights. The night is dark.

Does a leopard gecko need UVB?

The UV-Tool puts it in zone 1, a crepuscular species with a low gradient of UVI 0-0.7 and a maximum of 0.6-1.4. That is low, but it is not zero, and a shade method gradient gives it the choice.

What is a Ferguson zone?

One of four groups of reptiles sorted by how much sun they take in the wild, from shade dwellers in zone 1 to mid-day baskers in zone 4. Each has a UV index range measured in the field with a UV index meter.

Why does the meter matter?

The zones were measured with a Solarmeter 6.5 UV index meter, which closely matches the wavelengths that make vitamin D3. A broadband UVB meter reads different numbers from the same lamp, so its readings cannot be compared with the zone ranges.

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