A fever is usually considered a body temperature of 100.4°F (38°C) or higher, but the exact cutoff depends on how and where you measure it.

Quick Scoop

Normal vs fever

  • Normal adult body temperature averages around 98.6°F (37°C), but healthy ranges are roughly 97–99°F (36.1–37.2°C). Values above this range may signal a developing fever.
  • Many doctors use 100.4°F (38°C) as the practical point where “this is a fever ” rather than just “a little warm.”

By thermometer type

Most major medical sources give these common fever thresholds:

  • Oral (mouth):
    • Fever: about 100–100.4°F (37.8–38°C) or higher.
  • Rectal / ear / forehead (temporal):
    • Fever: 100.4°F (38°C) or higher.
  • Armpit (axillary):
    • Fever: around 99–99.5°F (37.2–37.5°C) or higher.

In plain terms: if a reliable thermometer reads around 100.4°F (38°C) or more, most doctors would call that a fever.

When it’s more serious

Get urgent medical help (or call emergency services) if:

  • The person is very hard to wake, confused, or has trouble breathing.
  • There’s a stiff neck, severe headache, chest pain, or a seizure.
  • A baby under 3 months has a rectal temperature of 100.4°F (38°C) or higher.
  • Fever lasts more than a few days or keeps climbing despite fluids and over‑the‑counter medicine.

Quick numbered recap

  1. Under ~99°F (37.2°C): Often normal for adults, depending on the person and time of day.
  1. Around 99–100.3°F (37.2–37.9°C): Low‑grade or “slightly elevated,” may be early infection or normal variation.
  1. 100.4°F (38°C) and above: Generally considered a true fever.
  1. 102–104°F (38.9–40°C): Higher fever; watch symptoms closely and consider medical advice, especially in children, older adults, or those with health issues.

Bottom note: This is general information, not a diagnosis. If you or someone else feels very unwell or you’re unsure what to do about a fever, contact a health professional.