For most adults, a fever starts at 100.4 F, or 38 C. That is the common threshold doctors and health agencies use when they are trying to separate a normal warm day from a real rise in body temperature.
What matters just as much as the number is how someone feels. A low fever with dehydration, trouble breathing, chest pain, confusion, or symptoms that are getting worse deserves attention even if the reading is not very high.
So the quick answer is 100.4 F or 38 C and up. If the person is very young, very old, immunocompromised, or clearly unwell, the thermometer is only part of the story.
Sources
This is general information, not professional medical advice. For decisions about your situation, talk to a qualified professional.
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