Fever: Fahrenheit to Celsius, and What Counts

Temperature is the one conversion on this site with an offset, because the two scales start from different zeros: °C = (°F − 32) ÷ 1.8. So 98.6 °F is 37.0 °C, and the American fever line of 100.4 °F is exactly 38.0 °C.

The chart, and the number that counts as fever

MedlinePlus gives fever as a temperature above 100.4 °F (38.0 °C), and puts normal body temperature at an average of 98.6 °F (37 °C) inside a range from 97 °F (36.1 °C) to 99 °F (37.2 °C).

Fahrenheit	Celsius	Where it sits
97.0 °F	36.1 °C	Bottom of the normal range
98.6 °F	37.0 °C	The traditional average
99.0 °F	37.2 °C	Top of the normal range
100.0 °F	37.8 °C	Warm, below the fever line
100.4 °F	38.0 °C	Fever
101.0 °F	38.3 °C	Fever
102.0 °F	38.9 °C	Fever
103.0 °F	39.4 °C	Fever
104.0 °F	40.0 °C	Call a clinician at any age

Note that 100.4 and 38.0 are the same temperature to a tenth of a degree — one of the very few places where a threshold is a round number on both scales. That is not luck. The clinical cutoff was set at 38.0 °C and 100.4 °F is what that converts to.

Where you take it changes the answer

A thermometer measures the temperature of the place you put it, and the places are not the same temperature. MedlinePlus gives different fever thresholds for children depending on site:

Site	Fever begins at
Rectal	100.4 °F (38.0 °C)
Oral	99.5 °F (37.5 °C)
Axillary, under the arm	99.0 °F (37.2 °C)

The rectal reading runs highest and the underarm reading lowest, with roughly a degree Fahrenheit between them. An underarm reading of 99.5 °F and an oral reading of 99.5 °F carry different meanings, and comparing today’s underarm number with yesterday’s ear number tells you very little.

Two practical consequences. Use the same site and the same thermometer when you are tracking a fever over a day. And when you report a temperature to a clinician, say where you took it — the number alone is incomplete.

The reading is not the illness

A fever is the body’s response to infection, not the infection, and its height is a poor guide to how serious the cause is. A mild virus can produce 103 °F; something serious can run at 100.5 °F. What clinicians weigh alongside the number is how the person looks and behaves, how long the temperature has been up, and what else is happening.

Age is the exception that overrides all of that. The American Academy of Pediatrics advises calling the pediatrician for an infant younger than three months who reaches 100.4 °F (38.0 °C), because at that age a fever is investigated rather than watched. The AAP also lists a temperature repeatedly above 104 °F (40.0 °C) at any age, a fever lasting more than 24 hours in a child under two, and more than three days in an older child.

Converting a foreign thermometer

If you have a thermometer bought abroad, or a device that switched to Celsius after a battery change, most models toggle units by holding the power button. Until you find it, the two anchors worth memorising are the ones above: 37 is normal, 38 is fever. On the Fahrenheit side those are 98.6 and 100.4.

For the other unit conversions that show up on a lab report rather than a thermometer, see blood sugar mg/dL to mmol/L. For the weight that fever-reducing doses in children are calculated from, pounds to kg.

This content is informational and does not replace medical evaluation. Seek care for a fever with warning signs, and immediately for any fever in a newborn.