What Temperature Is a Fever? A Celsius and Fahrenheit Guide

A fever is generally a body temperature of 100.4F (38C) or higher, measured by mouth or rectally. That is the threshold used by the Mayo Clinic and Cleveland Clinic, and it is the number most people are looking for. A fever is not an illness on its own. It is a sign that your body is fighting something, most often an infection, and it can also follow vaccination, heat exposure, or certain medications.

The trouble is that the answer depends on how you take the temperature and whether you are reading the result in Celsius or Fahrenheit. A thermometer that says 38 means a fever, but so does one that says 100.4, because those are the same temperature. This guide lays out the thresholds in both scales, shows how the cutoff shifts between oral, rectal, ear, armpit, and forehead readings, and explains when a fever is a reason to call a doctor.

This article is general health information and is not a substitute for professional medical advice. If you are worried about a fever in yourself or someone in your care, contact a doctor or healthcare provider.

Fever thresholds in Celsius and Fahrenheit

Body temperature exists on a spectrum, and clinicians group it into broad bands rather than a single on or off point. The table below shows the standard ranges for an oral or rectal reading in an adult, in both Fahrenheit and Celsius. These bands are consistent with guidance from the Mayo Clinic, Cleveland Clinic, and the United States National Library of Medicine resource MedlinePlus.

Treat these bands as a practical guide rather than rigid rules. The exact cutoffs vary slightly between health organizations, and a few tenths of a degree on either side of a line does not change much. What matters is the trend, how you feel, and whether other symptoms are present. The single most important figure to remember is 100.4F (38C): below it you do not have a fever, and at or above it you do.

Fever thresholds by measurement method

Where you place the thermometer changes the reading. Core temperature is the body temperature deep inside, and different sites approximate it with different amounts of error. A rectal reading is closest to core and runs slightly higher, while an armpit reading sits furthest from core and runs lower. The CDC and Cleveland Clinic both note that rectal temperature is the most accurate, particularly for infants and very young children. Forehead (temporal artery) and ear (tympanic) thermometers are fast and convenient but can be thrown off by technique, earwax, or sweat.

The table below shows the approximate temperature at which each method indicates a fever. Because methods do not read identically, always note which one you used when you record or report a temperature.

As a rule of thumb, an armpit reading runs roughly 0.5F to 1F (about 0.3C to 0.6C) lower than an oral reading, and a rectal reading runs about 0.5F to 1F higher than oral. That is why an armpit temperature of 99F can signal the same underlying state as an oral 100F. Rectal, ear, and forehead methods are designed to approximate core temperature directly, so the familiar 100.4F (38C) cutoff applies to them.

What counts as normal body temperature?

The number most people learned is 98.6F (37C), set by a 19th century study. Modern sources treat that as an average rather than a fixed normal. MedlinePlus describes a healthy temperature as a range, roughly 97F to 99F (36.1C to 37.2C), that varies from person to person. Your temperature is naturally lower in the early morning and higher in the late afternoon and evening, and it rises with exercise, heavy clothing, hot weather, and, in many people, the menstrual cycle. Because your personal baseline may sit a little above or below 98.6F, a fever is best understood as a meaningful rise above your own normal, not just a crossing of one fixed line.

How the Celsius and Fahrenheit conversion works

The two scales meet through one formula. To turn Celsius into Fahrenheit, multiply by 1.8 and add 32. To go the other way, subtract 32 from the Fahrenheit value and divide by 1.8. Working the fever threshold through it confirms the pairing: 38C multiplied by 1.8 is 68.4, and adding 32 gives 100.4F. The same math links every row in the tables on this page, and the converter above does it for any reading so you never have to do the arithmetic while caring for someone who is unwell.

Fever in adults versus children

The 100.4F (38C) threshold applies to both adults and children, but the level of concern differs by age. In a healthy adult, a fever itself is rarely dangerous and often resolves on its own. The Mayo Clinic notes that how you feel matters as much as the number: a moderate fever with mild symptoms usually calls for rest and fluids, while a fever of 103F (39.4C) or higher is a reason to contact a doctor.

Children run fevers often, and the temperature alone is a poor measure of how sick a child is. A child who is still drinking, alert, and playing between temperature spikes is usually less worrying than a listless child with a lower reading. The big exception is the youngest infants. The Mayo Clinic and Cleveland Clinic advise that any rectal temperature of 100.4F (38C) or higher in a baby under three months old should be treated as a medical concern and checked by a doctor the same day, because young infants can become seriously ill quickly and show few outward signs.

When to see a doctor

For most healthy adults, a fever can be managed at home with rest, fluids, and, if appropriate, an over the counter fever reducer. Drawing on Mayo Clinic and Cleveland Clinic guidance, contact a doctor or healthcare provider in these situations:

• An adult temperature of 103F (39.4C) or higher.
• A fever that lasts longer than three days, or keeps returning.
• Any fever in an infant under three months old.
• A fever in a child of any age that is paired with unusual drowsiness, irritability, poor feeding, or signs of dehydration.
• A fever in someone with a weakened immune system or a serious ongoing health condition, where you should follow the advice of your care team.

Emergency warning signs

Some symptoms alongside a fever need urgent care rather than a routine appointment. Seek emergency medical help right away if a fever appears with any of the following:

• A severe headache or a stiff, painful neck.
• Confusion, unusual drowsiness, or difficulty waking the person.
• Trouble breathing, chest pain, or persistent vomiting.
• A skin rash that spreads quickly or does not fade when pressed.
• A seizure, or sensitivity to bright light.
• A very high temperature above 104F (40C) that does not come down.

These warning signs are not a complete list. Trust your judgment: if a fever is accompanied by anything that frightens you or feels wrong, seek care without waiting.

How to take an accurate temperature

A reliable reading starts with good technique. Use a digital thermometer rather than an old mercury one, and avoid measuring within 15 to 30 minutes of eating or drinking something hot or cold, exercising, or a hot bath, since all of those can skew an oral reading. Always record which method you used. When you describe a fever to a doctor or nurse, the most useful details are the highest temperature, the measurement method, how long the fever has lasted, and what other symptoms are present. For the most accurate reading in a young child, follow your pediatrician's advice on using a rectal thermometer.