Most people know that 37 °C (98.6 °F) is "normal" body temperature. But that figure comes from a 19th-century average — modern research shows healthy adults range from 36.1 to 37.2 °C (97–99 °F) depending on age, time of day, and measurement site.
In this guide you'll learn how to interpret temperature readings, when a high temperature counts as fever, how to convert between Celsius and Fahrenheit, and when to seek medical help.
What is a normal body temperature?
The widely cited 37 °C (98.6 °F) comes from a study by German physician Carl Wunderlich published in 1851. A large 2020 study in eLife found the average oral temperature in healthy US adults is closer to 36.6 °C (97.9 °F) — and has been gradually decreasing over the past century.
Normal temperature is not a single number but a range. The accepted oral range for healthy adults is 36.1–37.2 °C (97.0–99.0 °F). Anything above 38 °C (100.4 °F) orally is generally defined as fever.
Normal oral temperature: 36.1–37.2 °C (97.0–99.0 °F)
Fever threshold: ≥ 38.0 °C (100.4 °F)
Celsius to Fahrenheit — the formula
The exact conversion formulas are:
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
| Category | °C | °F |
|---|---|---|
| Hypothermia | < 35.0 | < 95.0 |
| Below normal | 35.0–36.0 | 95.0–96.8 |
| Normal | 36.1–37.2 | 97.0–99.0 |
| Elevated | 37.3–38.0 | 99.1–100.4 |
| Fever | 38.1–39.0 | 100.6–102.2 |
| High fever | 39.1–40.0 | 102.4–104.0 |
| Very high fever | > 40.0 | > 104.0 |
Measurement sites — which is most accurate?
Temperature varies by where you measure it. Rectal measurement most closely reflects core body temperature and is the gold standard in clinical settings. Oral, axillary (armpit), and tympanic (ear) readings are practical but slightly less accurate.
Rectal (most accurate)
36.6–38.0 °C (97.9–100.4 °F). Gold standard. Used for infants and ICU patients.
Oral
35.5–37.5 °C (95.9–99.5 °F). Convenient and accurate in adults. Wait 15 min after eating or drinking.
Axillary (armpit)
34.7–37.3 °C (94.5–99.1 °F). Least accurate. Add ~0.5 °C to estimate core temperature.
Tympanic (ear)
35.8–38.0 °C (96.4–100.4 °F). Fast but requires proper technique for accuracy.
When to see a doctor
Seek emergency care immediately
- Temperature above 40 °C (104 °F) at any age
- Any fever in infants under 3 months
- Temperature below 35 °C (95 °F) — hypothermia
- Fever with stiff neck, severe headache, confusion, or rash
See a doctor within 24 hours
- Fever above 39 °C (102.2 °F) lasting more than 1 day
- Any fever in children under 2 years lasting more than 1 day
- Fever lasting more than 3 days in adults
Natural temperature variation
Body temperature is not fixed — it follows a circadian rhythm, rising and falling by up to 0.5 °C throughout the day:
- Lowest: early morning (around 4–6 am), typically 36.0–36.4 °C
- Highest: late afternoon (around 4–6 pm), typically 36.8–37.2 °C
Other factors that raise temperature slightly include physical exercise, ovulation (by ~0.3 °C), hot drinks, and warm clothing. Factors that lower it include sleep, fasting, and cold exposure.