What this calculates and what it deliberately does not
This page does two separate jobs. The first is an ordinary building-physics heat loss calculation applied to a box: surface area times a U value times the temperature difference, with an allowance for air leaking through the screen. That gives a continuous wattage to hold an ambient temperature, which is a useful sanity check on whether a 50 watt fitting is plausible or whether you need three times that.
The second is a distance comparison for UVB, expressed as a percentage of whatever a lamp manufacturer chart quotes at its reference distance. That is a ratio, not an index.
What this page does not do, and will not: supply any temperature, any basking figure, or any UV index for any species. Those numbers are specific to the species, sometimes to the life stage, and getting them from a generic calculator is how animals get hurt. They belong in a current care sheet for your exact species and in a conversation with a veterinarian who treats exotics.
The thermostat is not optional
An unregulated heat source is a burn hazard to the animal and a fire hazard to the building. Heat mats and cables in particular can reach temperatures that cause contact burns, and reptiles do not reliably move away from a heat source that is damaging them. Every heat source in an enclosure belongs on a thermostat sized for it, with the probe located where the animal can actually reach, secured so it cannot be dislodged.
Then check the thermostat with something independent. A thermostat reports what its own probe reads, and a probe that has slipped behind a hide reports a temperature nothing in the enclosure experiences. A second thermometer at the basking site and a third at the cool end costs very little and is how you catch that failure.
Measure the surface, not the lamp
The wattage figure is the number people ask for and the least informative one in the whole setup. What a basking animal experiences is a surface temperature at a particular spot, and that depends on the lamp, the reflector, the distance, the colour and material of the basking surface, the ambient temperature and the airflow past it. Two identical lamps over different basking rocks produce different surface temperatures.
So measure. An infrared thermometer aimed at the basking surface reads the surface; a probe taped to it reads it more reliably. A digital thermometer at the cool end tells you whether a gradient exists at all. Then adjust by moving the lamp height or changing the thermostat setting, and measure again after the enclosure has had hours to stabilise rather than minutes.
Distance, lamp type and why UVB falls off differently
Light from a point source follows an inverse square law, so doubling the distance quarters the intensity. A compact or mercury vapour lamp is close enough to a point source that the square law is a fair approximation. A long linear tube is not: near the tube the falloff is closer to linear, and it only approaches inverse square at distances large compared with the tube length. This calculator uses an exponent of 1.4 for tubes, 1.9 for compacts and 2.0 for mercury vapour, which is a rough way of capturing that difference.
| What changes | Effect on UVB reaching the animal |
|---|---|
| Moving the lamp further away | Falls sharply, faster for point sources than for tubes |
| Glass or acrylic between lamp and animal | Blocks most or all UVB — plain glass is effectively opaque to it |
| Fine mesh screen between lamp and animal | Cuts a substantial fraction, varying with the mesh |
| Lamp age | Output declines steadily while visible light looks unchanged |
| Reflector fitted or not | Changes output at a given distance considerably |
The percentage this tool produces is a relative comparison against a manufacturer chart, useful for deciding whether moving a lamp two inches matters. It is not a UV index and it cannot substitute for the manufacturer distance chart for the exact lamp, or for a UV index meter, which is the only way to know what is actually reaching the animal.
Lamp replacement is a calendar job
UVB lamps stop producing useful UVB long before they stop producing light, and there is no visual cue at all. That is the single most common failure in a setup that otherwise looks correct: an eighteen month old lamp glowing brightly over an animal getting almost nothing from it. Note the install date on the lamp in permanent marker, set a reminder, and replace it on the interval the manufacturer states for that specific lamp rather than on a general rule.
The hours-run figure here is there for the same reason. A lamp on for fourteen hours a day ages faster in calendar terms than one on for eight, and if you run long photoperiods it is worth shortening the replacement interval accordingly.
Related tools
Heat loss through the walls of a box is the same arithmetic as heat loss through the walls of a room, and the heat loss calculator handles the building-scale version if you are heating the room instead of the enclosure. For the electricity a lamp or mat costs to run, the appliance running cost calculator takes wattage and hours and gives a monthly figure. Humidity is the other half of most enclosure specifications and the humidifier sizing calculator covers the room-scale version of that problem. For aquatic setups the aquarium volume and filter calculator is the equivalent tool, and if a small mammal shares the room, the hutch and enclosure size calculator handles its sizing.
Questions people ask
What wattage heat lamp do I need for my terrarium?
The heat loss arithmetic on this page gives a continuous wattage to hold a given ambient lift, and a practical fitting is usually one and a half to two and a half times that because a thermostat cycles it rather than running it flat out. But wattage is the wrong thing to fix on. What matters is the surface temperature at the basking site and the gradient across the enclosure, and both depend on lamp height, reflector, basking surface material and airflow as much as on the rating. Pick a fitting in the right range, put it on a thermostat, then measure and adjust. The correct target temperature itself comes from a species care sheet, not from here.
Do I really need a thermostat with a heat mat?
Yes. An unregulated mat or cable can reach surface temperatures high enough to cause contact burns, and reptiles do not reliably move off a heat source that is injuring them, which is why thermal burns are a common presentation at exotics practices. It is also a fire risk in a cabinet full of substrate. Every heat source goes on a thermostat sized for it, with the probe fixed where the animal can reach and secured so it cannot be pushed aside, plus a separate thermometer as an independent check that the thermostat is reading what you think it is.
How far should the UVB lamp be from the basking spot?
The lamp manufacturer publishes a distance chart for that specific lamp, and that chart is the answer. The right distance depends on the lamp, whether it has a reflector, whether there is mesh in the path, and the UV index range appropriate to the species — which is a species question for a care sheet and an exotics vet, not something a general calculator supplies. What this tool does is compare two distances: it tells you roughly what fraction of the chart figure you get at your actual distance. That is useful for deciding whether a two inch change matters. Confirming the actual UV index at the animal requires a UV index meter.
Does mesh or glass block UVB?
Glass and most acrylic block essentially all of the useful UVB, which is why a lamp mounted outside a glass lid does nothing at all no matter how it is rated. Mesh screen blocks a substantial and variable fraction depending on the wire diameter and opening, commonly a third or more, and the manufacturer chart may or may not account for it. If a lamp sits above a screen top, the effective distance and the effective output are both worse than the raw numbers suggest, which is one of the arguments for mounting inside the enclosure with an appropriate guard.
How often should a UVB lamp be replaced?
On the interval the manufacturer states for that lamp, which is commonly six to twelve months but varies by type and product, and which you should read from the packaging rather than assume. The reason it is a calendar job is that UVB output declines steadily while the lamp still looks perfectly bright, so there is no visual warning at all. Write the install date on the lamp itself and set a reminder. If you run a long daily photoperiod the lamp accumulates hours faster and the interval should shorten. A UV index meter removes the guesswork entirely and pays for itself across a few lamps.