Cool end, warm end, basking spot and night range for 13 pet reptiles from the Merck veterinary table, a check of your thermometer readings and the electricity your heat lamp uses.
Reptile Temperature Gradient Calculator - Basking & Night
Cool end, warm end, basking spot and night range for 13 pet reptiles from the Merck veterinary table, a check of your thermometer readings and the electricity your heat lamp uses.
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Reptile temperature gradient by species
A bearded dragon needs air from 77 °F at the cool end to 90 °F at the warm end, a basking spot near 99 °F and nights no lower than 68 °F. A ball python sits lower and narrower: 77-86 °F, basking around 95 °F. This reptile temperature gradient calculator gives those targets for 13 common pet reptiles, straight from the husbandry table in the Merck Veterinary Manual, then checks the readings from your own thermometers spot by spot. Add the wattage of the heater and it also tells you how much electricity the heat lamp uses in a month.
Quick start: choose Bearded dragon and type the readings 78 (cool end), 88 (warm end), 99 (basking) and 70 (night). All four come back in range or on target. Humidity 35% is flagged as above the 20-30% range for this desert lizard.
How the targets are worked out
Merck lists a preferred optimal temperature zone (POTZ) for each species, measured in the air. The calculator places its lower limit at the cool end and its upper limit at the warm end. The table's footnote adds two rules of thumb: the basking spot runs about 5 °C (9 °F) warmer than the zone, and at night the whole zone drops by about 5 °C. Fahrenheit values are rounded to the whole degree, as Merck prints them.
- Species - the 13 snakes, lizards, tortoises and turtles in the Merck table.
- Unit - Fahrenheit or Celsius; readings are typed in the same unit.
- Readings - cool end, warm end, basking spot, night and humidity, all optional. Measure air temperature at animal height, not the glass or the substrate surface.
- Heater - wattage, hours switched on, the share of that time it actually runs on a thermostat, and your price per kWh from the electric bill.
Temperature chart for 13 pet reptiles
Day zone, basking spot and night range in Fahrenheit with Celsius in brackets, plus humidity and lighting. "UVB essential" means Merck marks broad-spectrum lighting (UVB 290-300 nm) as essential; for the others it is recommended but not required.
| Species | Day zone | Basking | Night | Humidity | UVB |
|---|---|---|---|---|---|
| Corn or rat snake | 77-86 (25-30) | 95 (35) | 68-77 (20-25) | 30-70% | recommended |
| Boa constrictor | 82-88 (28-31) | 97 (36) | 73-79 (23-26) | 70-95% | recommended |
| Ball python | 77-86 (25-30) | 95 (35) | 68-77 (20-25) | 50-80% | recommended |
| Leopard gecko | 77-86 (25-30) | 95 (35) | 68-77 (20-25) | 20-30% | recommended |
| Green iguana | 84-91 (29-33) | 100 (38) | 75-82 (24-28) | 60-85% | essential |
| Bearded dragon | 77-90 (25-32) | 99 (37) | 68-81 (20-27) | 20-30% | essential |
| Water dragon | 75-86 (24-30) | 95 (35) | 66-77 (19-25) | 80-90% | essential |
| Savannah monitor | 77-90 (25-32) | 99 (37) | 68-81 (20-27) | 20-40% | essential |
| Greek tortoise | 68-82 (20-28) | 91 (33) | 59-73 (15-23) | 30-50% | essential |
| Box turtle | 72-82 (22-28) | 91 (33) | 63-73 (17-23) | 50-80% | essential |
| Leopard tortoise | 77-86 (25-30) | 95 (35) | 68-77 (20-25) | 30-50% | essential |
| Red-footed tortoise | 77-86 (25-30) | 95 (35) | 68-77 (20-25) | 50-90% | essential |
| Red-eared slider | 72-81 (22-27) | 90 (32) | 63-72 (17-22) | aquatic | essential |
Humidity for the dry-land species rises well above these figures while the animal sheds, so a humid hide is part of the setup rather than a sign the enclosure is off. Merck marks the corn snake, the Greek tortoise and the box turtle as species that hibernate, and the slider as one that may.
Eight setups checked
Basking reading 100 °F against a target of about 95 °F: "hotter than the guide". A dimmer thermostat or a lower wattage brings it back.
Night reading 65 °F, range 68-77 °F: below. A light bulb is the wrong fix at night; a ceramic emitter or a mat on a thermostat gives heat without light.
Cool end 74 °F against a floor of 77 °F: below the zone, even with the warm end fine at 85 °F. The gradient is 11 °F wide when the zone needs only 9 °F.
Basking 88 °F: warmer than the 82 °F top of the zone but short of about 91 °F, so "a little cool".
Both thermometers read 80 °F: the result warns that there is no gradient. Merck's advice is a heater toward one end of the enclosure.
Readings 28 and 31 °C, humidity 65%: both ends in range, humidity below the 70-95% a rainforest boa needs.
100 W, 12 h a day, on 60% of the time: 0.72 kWh a day, 21.6 kWh a month. At $0.17 per kWh, $3.67 a month and $44.68 a year.
150 W, 24 h, thermostat on half the time: 1.80 kWh a day, 54.0 kWh a month, 657 kWh a year.
Heating rules from the same chapter
The Merck chapter lists incandescent bulbs, infrared ceramic bulbs, heat pads and mats, warming cables, tubular heaters, radiators and convector heaters as usable sources. Whatever the source, it should be the right size, run through a thermostat, sit behind a guard and stand toward one end, because that end-to-end placement is what creates the gradient. Two warnings stand out: hot rocks often burn larger animals, and light bulbs cannot heat the enclosure at night. Some mercury-halide bulbs give heat and UVB at once.
Why does the share of time on matter so much for the bill? Because a thermostat switches the heater off once the probe reaches the set point. A lamp that runs flat out all day costs the full watts times hours; the same lamp cycling 60% of the time costs 40% less. If you do not know the share, leave it empty and the result shows the worst case, on the whole time.
The chart below shows the worst case, a heater that never switches off, for a month of 30 days at $0.17 per kWh. Multiply by the share of time your thermostat keeps it on.
| Heater | 12 h a day | Cost a month | 24 h a day | Cost a month |
|---|---|---|---|---|
| 25 W | 9.0 kWh | $1.53 | 18.0 kWh | $3.06 |
| 50 W | 18.0 kWh | $3.06 | 36.0 kWh | $6.12 |
| 75 W | 27.0 kWh | $4.59 | 54.0 kWh | $9.18 |
| 100 W | 36.0 kWh | $6.12 | 72.0 kWh | $12.24 |
| 150 W | 54.0 kWh | $9.18 | 108.0 kWh | $18.36 |
Common questions
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