In adults, a fever is generally any body temperature of 100.4°F (38°C) or higher when measured orally.

Quick Scoop: Key Numbers

  • Normal adult temperature: about 97°F–99°F (36.1°C–37.2°C).
  • Fever (adult, oral): 100.4°F (38°C) or above.
  • Low‑grade fever: roughly 99°F–100.4°F (37.2°C–38°C), especially if above your usual baseline.
  • High fever: around 102–103°F (38.9°C–39.4°C) or higher in adults.
  • Very high/dangerous (hyperpyrexia): 104°F (40°C) or higher – this is a medical emergency.

Why the Method Matters

Different thermometer sites have slightly different “fever” cutoffs.

  • Oral: 100.4°F (38°C) or higher counts as a fever.
  • Rectal/ear/temporal: 100.4°F (38°C) or higher is also considered a fever.
  • Armpit (axillary): about 99°F (37.2°C) or higher may indicate fever.

When to Worry and Call a Doctor

For an adult, get urgent medical care if:

  • Temperature is 104°F (40°C) or higher.
  • Fever lasts more than 3 days, or keeps coming back.
  • You have serious symptoms: trouble breathing, chest pain, confusion, stiff neck, severe headache, rash, persistent vomiting, or signs of dehydration (very little urine, dizziness, extreme weakness).

An example: if an adult has 101.5°F, feels very ill, and it persists more than a couple of days, that should prompt a call to a healthcare provider even though it is not extreme.

Information gathered from public forums or data available on the internet and portrayed here.