Body Temperature in Celsius and Fahrenheit: What Counts as a Fever?

98.6 F and 37 C are the same temperature, yet one reading can look calm and the other alarming. A thermometer set to the wrong scale can turn a normal night into a scare.

This guide shows how to convert a reading in four quick moves. It also covers where the normal range sits and what counts as a fever. You will see why mouth, ear and armpit readings disagree, and when health sources say to call for help.

How Do You Convert a Temperature Reading From F to C?

Subtract 32 from the Fahrenheit number, multiply by 5, and divide by 9. The formula is C = (F – 32) x 5/9. The fever line of 100.4 F makes a clean test case, because it lands exactly on 38.0 C.

1. Note the reading and the site. Write down 100.4 F and where you took it, such as the mouth.
2. Subtract 32. 100.4 minus 32 leaves 68.4. This removes the offset between the two scales.
3. Multiply by 5. 68.4 times 5 gives 342.
4. Divide by 9. 342 divided by 9 is exactly 38.0 C.
5. Round to one decimal. Home thermometers show tenths, so 38.0 C is the number to compare with a source.

Going the other way, multiply Celsius by 9, divide by 5, and add 32. For 39 C, that is 351 divided by 5, or 70.2, plus 32. The answer is 102.2 F.

Remember the step size too. One degree Celsius equals 1.8 degrees Fahrenheit. So a rise from 37.0 C to 37.5 C is a rise of 0.9 F, from 98.6 F to 99.5 F.

Normal Body Temperature Is a Range, Not One Number

MedlinePlus calls 98.6 F (37 C) the generally accepted average. It also notes studies showing a normal range from 97 F (36.1 C) to 99 F (37.2 C). So a reading of 97.5 F on a quiet morning is not a warning sign by itself.

Your own number moves through the day. MedlinePlus says body temperature is usually highest in the evening. In adults, it is lowest in the early morning.

Daily life nudges it up as well. MedlinePlus lists physical activity, strong emotion, eating, heavy clothing, medicines, a hot room and high humidity. It also notes a rise of 1 F or more in the second half of the menstrual cycle. That 1 F rise is only about 0.56 C.

This is why one reading means less than a pattern. A reading taken after a jog in a warm room tells you little about fever. Sit quietly for a while in a comfortable room, then measure again.

Our converter shows three scales at once. For 98.6 F it returns 37.0 C and 310.15 K, the Kelvin value used in science classes.

What Temperature Counts as a Fever?

The NHS treats 38 C (100.4 F) or above as a high temperature, which it also calls a fever. MedlinePlus says a reading over 100.4 F most often means a fever caused by an infection or illness.

MedlinePlus goes further for children and splits the line by site. A child has a fever at or above 100.4 F (38 C) in the bottom. The line is 99.5 F (37.5 C) in the mouth and 99 F (37.2 C) under the arm.

For adults, MedlinePlus says a fever is probable when a mouth reading is above 99 F to 99.5 F. The exact line depends on the time of day. The table lines up the key numbers from both sources.

MedlinePlus adds that a simple cold can cause a high fever of 102 F to 104 F (38.9 C to 40 C). That does not mean the illness is serious.

Why Mouth, Ear and Armpit Readings Disagree

Each site gives a slightly different number, so the fever line shifts with it. MedlinePlus sets the child line at 38.0 C in the bottom but 37.2 C under the arm. That is a gap of 0.8 C, or about 1.4 F.

The NHS explains how to use a digital thermometer under the tongue or in the armpit. For an ear thermometer, it says to follow the maker’s instructions. It warns against forehead strips because they are not accurate.

When Should a Fever Get Medical Attention?

Health sources set clear lines, and they are stricter for babies. This section repeats what they say. It is general education, not a diagnosis.

Babies and Children

MedlinePlus sets the first line for babies 3 months or younger. A rectal reading of 100.4 F (38 C) or higher means calling the provider right away. The line for ages 3 to 12 months is 102.2 F (39 C). The NHS says to call 111 for a baby under 3 months at 38 C or higher. It gives 39 C or higher for ages 3 to 6 months.

Time matters too. MedlinePlus flags a fever lasting longer than 24 to 48 hours in a child 2 years or younger. The NHS flags a high temperature that has lasted 5 days or more.

Adults

MedlinePlus says to call a provider right away for a fever of 105 F (40.6 C) or more that does not come down readily. The same applies to a fever that stays at or keeps rising above 103 F (39.4 C). A fever lasting longer than 48 to 72 hours also qualifies.

MedlinePlus says to call 911 for a fever with confusion, trouble breathing, a stiff neck, a very bad headache or a seizure. The NHS also advises plenty of fluids during a fever. Our guide to the signs of dehydration and how to prevent it explains what to watch for.

Try it with a real reading. Enter the number and the site in the body temperature converter to see it in both scales.

Body Temperature: Frequently Asked Questions

Sources and Further Reading

This article explains temperature conversion and published fever thresholds for general education only. It does not diagnose illness or replace advice from a health professional. Reviewed for accuracy by Dr. Abdullah Khalil, MBBS. Last updated September 27, 2026.