Fever (pyrexia) is a regulated elevation of body temperature above the normal range, typically driven by the immune system's response to infection, inflammation, or other triggers. Unlike hyperthermia (unregulated temperature rise, as in heat stroke), fever involves a deliberate upward resetting of the hypothalamic thermostat by inflammatory signalling molecules called pyrogens.

Normal body temperature in adults is commonly quoted as 37°C (98.6°F), but this figure is an average. Research shows that 'normal' actually spans a range of approximately 36.1–37.5°C (97.0–99.5°F) and varies with time of day (lowest in early morning, highest in late afternoon), measurement site, age, and individual physiology.

Most clinical definitions place the fever threshold at ≥38.0°C (100.4°F) oral equivalent, though some guidelines use 37.8°C or 38.3°C depending on the clinical context and patient population. In infants under three months, any temperature ≥38.0°C warrants urgent medical review.

Celsius (°C) and Fahrenheit (°F) are the two temperature scales encountered in everyday medical practice. Celsius is the standard in most countries and in all scientific contexts; Fahrenheit remains in common use in the United States and a small number of other nations.

The conversion formulas are straightforward and exact:

Oral (sublingual) temperature is the standard reference in clinical practice for cooperative adults. It is accurate when the thermometer is placed under the tongue in the posterior sublingual pocket and the mouth is kept closed for the full measurement duration. Oral readings may be affected by recent hot or cold drinks.

Rectal temperature most closely reflects core body temperature and reads approximately 0.4°C higher than oral. It is the preferred method for infants and young children when accuracy is critical, though it is more invasive.

Axillary (underarm) temperature runs approximately 0.5°C lower than oral and has the highest variability. It is the least accurate common method and is best reserved for screening rather than precise assessment.

Tympanic (ear) thermometers measure infrared radiation from the tympanic membrane, providing a quick result that reads approximately 0.2°C higher than oral. Accuracy can be affected by ear canal anatomy, cerumen (earwax), and improper probe positioning.

Temporal artery thermometers scan the forehead and also read approximately 0.2°C higher than oral on average. They are convenient for children and sleeping patients but require correct sweeping technique and can be affected by sweating or recent exertion.

This tool automatically applies the appropriate site correction offset when classifying your reading, so you can enter the temperature exactly as measured — regardless of site.

Hypothermia is defined as core body temperature below 35.0°C (95.0°F) in adults. It is a medical emergency, with severe hypothermia (<28°C) carrying high mortality without prompt warming and supportive care.

Low-grade fever (37.5–37.9°C / 99.5–100.2°F) is a mild, often transient elevation that may accompany early infection, post-vaccination response, or mild inflammation. Close monitoring without immediate intervention is usually appropriate in otherwise healthy adults.

High fever (39–40.9°C / 102.2–105.6°F) warrants close monitoring and usually antipyretic therapy for comfort. In children, rapidly rising temperatures in this range can trigger febrile convulsions in susceptible individuals.

Hyperpyrexia (≥41°C / 105.8°F) is a medical emergency. Temperatures above this threshold risk protein denaturation, cerebral oedema, multi-organ failure, and death without urgent intervention.