How Often to Replace a UVB Bulb for Turtles and Tortoises

Most turtle and tortoise UVB bulbs need replacing every 6 to 12 months, even if they still light up. The Merck Veterinary Manual gives 9–12 months for fluorescent UVB tubes and 12 months for mercury vapor bulbs, while a reptile vets’ guide from the Association of Reptile and Amphibian Veterinarians (ARAV) says to change every 6 months if you can’t measure the output. Follow the date on your bulb’s packaging, and if you don’t know it, 6 months is the safe choice.

The tricky part is that an old UVB bulb looks exactly like a new one. Here’s why, what the guides say for each bulb type, and how to know for sure.

Why do UVB bulbs need replacing if they still work?

Because the part you can see and the part your turtle needs wear out at different speeds. The ARAV guide notes that many UVB bulbs keep giving off normal visible light long after their UV output has dropped off, and VCA’s vets say UV output falls as a bulb ages. Your turtle uses UVB to make vitamin D3, which it needs to absorb calcium from its food. A bulb that glows but no longer gives off much UVB is doing half its job.

The drop starts sooner than most people expect. In a six-month study at Lincoln Park Zoo, researchers ran five UVB-emitting bulbs on a 12-hours-a-day timer and measured them every month. UVB from every bulb dropped noticeably during the first four months, starting after the first month. The bulbs in that study are older models, so the numbers won’t match a modern tube exactly, but the pattern is the reason every guide puts a date on replacement.

How often to replace a UVB bulb, by type

Reputable guides don’t all give the same number, so here’s what each one says:

Put together, that gives you a simple rule:

1. Your bulb’s packaging comes first. It’s written for that exact bulb. VCA’s advice is to replace on schedule or sooner if the manufacturer says so.
2. No packaging, no meter? Replace every 6 months. That’s the most cautious guidance, and it’s what two separate vet sources recommend when you can’t check the output.
3. Have a UV meter? You can go by the reading instead of the calendar (more on that below), which can let a good bulb run longer.

How long do T5 UVB bulbs last?

T5 high-output (HO) tubes are the long, thin bulbs that ReptiFiles recommends for red-eared sliders, box turtles and Russian tortoises. As fluorescent tubes, they fall under Merck’s 9–12 months. If the box for your tube gives a different interval, use that one.

What about compact (coil) UVB bulbs?

Compact and coil bulbs are fluorescent too, so the same 6–12 month range applies. The ARAV guide notes they give off less UVB overall than T5 HO tubes, but the UVB is very strong right up close to the bulb. Keep them at the distance on the packaging so your turtle can’t climb or bask too close.

Do mercury vapor bulbs last longer?

Merck gives mercury vapor bulbs 12 months, the top of its fluorescent range. These bulbs give off heat as well as UVB, but the UVB still fades with age, so they need a replacement date too.

Do UVB bulbs expire if you never use them?

The ARAV guide describes UV output falling “over time and with use,” and the zoo study measured bulbs that were switched on every day. None of the sources we checked measured bulbs sitting unused in a box, so there’s no reliable answer. The practical approach is to start the clock on the day you install the bulb, not the day you bought it.

How to check a UVB bulb with a UV meter

You can’t see UVB, so the only way to know what a bulb is putting out is to measure it. A handheld UV index meter reads the UV index (UVI) at the spot where you hold it.

• How often: the ARAV guide suggests checking every 1–2 months.
• Where: at your turtle’s basking spot, at about the height of its back.
• When to replace: once the reading has dropped below the target for your species.

Here are the basking-spot targets from the guides above:

“Ferguson zones” group reptiles by how much sun they get in the wild, from shade-dwellers in zone 1 to midday baskers in zone 4. The ARAV guide puts red-eared sliders in zone 3. More UVB isn’t automatically better, so aim for your species’ range rather than the highest reading you can get.

Things that make a new bulb look old

Before you blame the bulb, check that nothing is blocking it. A healthy bulb can still deliver very little UVB if:

• There’s glass or plastic in the way. VCA, Merck and the ARAV guide all point out that glass and most clear plastics filter out UVB. A bulb sitting on top of a glass lid, or behind a clear plastic cover in the fixture, isn’t reaching your turtle.
• It’s behind a mesh screen. Mesh lets some UVB through but not all of it. ReptiFiles’ box turtle care sheet, for example, puts the basking spot about 13–14 inches (33–36 cm) from the lamp through mesh, and 17–18 inches (43–46 cm) without.
• It’s too far away. UVB gets weaker the farther your turtle is from the bulb. The ARAV guide gives 12–18 inches (30–45 cm) from the basking spot as a general range, and ReptiFiles lists distances by bulb model in its red-eared slider guide.
• The fixture has no reflector. The ARAV guide notes a metal reflector can double the UV a fluorescent tube delivers.

If you’re setting up for the first time, how long turtle lights should be on covers the daily schedule, placement over the basking spot and why the lights go off at night.

How to change a UVB bulb

1. Switch the light off and let it cool before you touch it. Mercury vapor bulbs in particular get very hot.
2. Fit the new bulb at the same height and in the same spot, unless your new bulb’s packaging gives a different distance.
3. Write the date on the fixture. The Unusual Pet Vets suggest this, or a phone reminder, so you don’t have to remember.
4. Recycle the old one. Fluorescent tubes and coils contain mercury, and the US EPA recommends taking them to a local collection point or a store that recycles bulbs rather than putting them in the trash. Some states require it.

A quick UVB checklist

• Replace on the date on the packaging, or every 6 months if you don’t know it.
• Start counting from the day the bulb goes in.
• Nothing between the bulb and your turtle except, at most, mesh you’ve adjusted the distance for.
• Bulb over the basking spot, at the distance on the packaging.
• If you have a UV meter, check the basking spot every month or two.

Bulb changes are easy to forget because nothing looks wrong. A date on the fixture works, and if you’d rather get a nudge, ShellPal can remind you when the UVB bulb is due, alongside your water changes or tortoise soaks.

Sources

• Management and husbandry of reptiles (Merck Veterinary Manual)
• Lighting requirements for reptiles (VCA Animal Hospitals)
• Ultraviolet light, Companion Reptile Husbandry Series (NAVC and the Association of Reptile and Amphibian Veterinarians)
• Seeing the light: reptile UV lighting explained (The Unusual Pet Vets)
• Preliminary report on the quantification of ultraviolet-B radiation from artificial light sources over time (Schmidt et al., Lincoln Park Zoo)
• Red-eared slider UVB lighting (ReptiFiles)
• Russian tortoise care sheet (ReptiFiles)
• Eastern box turtle care sheet (ReptiFiles)
• Recycling and disposal of CFLs and other bulbs that contain mercury (US EPA)