A leopard gecko is cold-blooded, which just means its body takes its temperature from the room around it. It can't shiver or sweat to fix things. Instead it walks between warm and cool spots. So the job of an owner is to build a gradient: one warm end, one cool end, and a cosy middle where the gecko can choose.
Too cold, and a gecko struggles to digest food and may stop eating. Too hot, and it has nowhere to escape and can get stressed or burned. The good news is that a couple of cheap digital thermometers tell you almost everything. Type in your readings below and see where you land.
Care sheets from vets and keepers don't match exactly, and that's worth knowing. For the warm side, one vet sheet says 85–90°F (about 29–32°C), another says around 90°F, and a UK keeper guide gives 30–32°C. For the cool side they range from about 22°C to 26–27°C. Rather than pretend one number is magic, the tool above uses the overlap and flags the edges as "close" rather than "wrong".
At night the picture is simpler. Leopard geckos cope with a drop into the high teens, and sheets only call for night heat when the room falls toward 15°C (59°F). The light should be off for about 12 hours, because geckos are most active at dusk and in the dark.
Measure at gecko height, on both ends, with a digital probe. A sticker thermometer on the glass can be off by several degrees.
A vet sheet notes that the best heat source is a heat lamp or ceramic heat emitter above the enclosure, and that under-tank heaters and hot rocks are less safe and less effective. Hot rocks in particular are a known cause of burns. Whatever you use should be plugged into a thermostat, so a stuck heater can't cook your gecko.
Add a UVB light if your vet or the species guide recommends one for your setup. Sheets differ on strength and distance, so follow the maker's instructions for the distance from your gecko's highest perch. Provide at least three hides: warm, cool, and a humid hide filled with damp moss to help shedding.
Temperature is the first thing to check, but it isn't the whole story. If your gecko stops eating for more than a short spell, loses a lot of weight in its tail, has stuck shed on its toes, breathes with its mouth open or is wobbly, book a vet who knows reptiles. We can't diagnose from a thermometer reading and neither can a website. Routine check-ups every 6 to 12 months are suggested by one vet sheet.