What Body Temperature Is Fever? A Clear Guide to Understanding Fever Thresholds
When someone asks, “What body temperature is fever?” the answer is often quoted as 100.4°F (38°C) or higher. However, the definition can vary slightly depending on the method of measurement, age, and individual differences. This article explains the commonly accepted thresholds, how fever is measured, and why it matters for health and daily life.
1. Normal Body Temperature Ranges
Body temperature is regulated by the hypothalamus, the brain’s thermostat. The average normal temperature for a healthy adult is typically around 98.6°F (37°C), but it can range from 97°F to 99°F (36.1°C to 37.2°C). Children and infants often have slightly higher baseline temperatures.
- Oral (mouth): 97.8°F to 99.5°F (36.5°C to 37.5°C)
- Rectal (anal): 98.6°F to 100.4°F (37°C to 38°C)
- Axillary (armpit): 97.7°F to 99.5°F (36.5°C to 37.5°C)
- Tympanic (ear): 98.6°F to 100.4°F (37°C to 38°C)
Because different sites measure temperature slightly differently, the thresholds for fever also differ. A temperature of 100.4°F (38°C) taken orally is widely accepted as the starting point for a fever.
2. How Fever Is Defined
In clinical practice, a fever is generally considered a body temperature that rises above the normal range for a particular measurement method. The most common definitions are:
- Oral: ≥ 100.4°F (38°C)
- Rectal: ≥ 100.4°F (38°C)
- Axillary: ≥ 99.5°F (37.5°C)
- Tympanic: ≥ 100.4°F (38°C)
These thresholds are based on studies that correlate temperature readings with the presence
