Body Temperature: Fahrenheit to Celsius

A parent sees 37.8°C on a digital ear thermometer and the discharge instructions say 100.4°F is a fever. The conversion is not intuitive, and a 0.5°C difference is clinically significant. Use the normal body temperature Celsius values, the fever thresholds from the CDC and NHS, and the conversion chart from 95°F to 106°F in 0.2°F steps to read one scale and act on the other.

The widely cited normal of 98.6°F (37.0°C) comes from the 1868 work of physician Carl Reinhold August Wunderlich. Modern research, including Protsiv et al. 2020 in eLife, shows that the average body temperature in the 2010s had declined to 97.5°F (36.4°C). The decline is believed to reflect reduced chronic inflammation, better health, and changes in measurement methods. The 98.6°F figure is a historical average, not a universal standard. Normal varies by individual, time of day, age, and measurement site.

Normal Ranges by Measurement Site (CDC)

Oral: 97.0°F to 100.3°F (36.1°C to 37.9°C). Rectal: 98.6°F to 100.6°F (37.0°C to 38.1°C). Axillary (under arm): 96.6°F to 99.7°F (35.9°C to 37.6°C). Tympanic (ear): 97.6°F to 100.0°F (36.4°C to 37.8°C). Temporal artery (forehead): 97.6°F to 100.0°F (36.4°C to 37.8°C).

Daily Variation

Body temperature is lowest in the early morning (around 4:00 to 6:00 a.m.) and highest in the late afternoon or early evening. The daily range can be up to 1.0°F (0.6°C). Exercise, hot environments, hormonal changes, and heavy clothing can raise it temporarily. A reading of 99.5°F (37.5°C) in the evening is not a fever.

The CDC defines a fever as a temperature of 100.4°F (38°C) or higher. The NHS uses the same 100.4°F (38°C) threshold for children but defines a high temperature in adults as 100.0°F (37.8°C) or above. The American Academy of Pediatrics (AAP) states that a rectal temperature of 100.4°F (38°C) or higher is a fever in children. Use the same threshold for tympanic, axillary, and temporal artery readings, per the NHS and AAP.

Infants Under 3 Months

For infants under 3 months, a rectal temperature of 100.4°F (38°C) or higher is a fever per the AAP. Any fever in this age group requires a call to the doctor immediately.

Fever Chart for Quick Reference

100.4°F (38.0°C) is the fever line. 100.0°F (37.8°C) is the NHS adult threshold. 100.6°F (38.1°C) is the top of the normal rectal range. 101.0°F (38.3°C) is a low-grade fever. 102.0°F (38.9°C) requires monitoring. 103.0°F (39.4°C) or higher in an adult warrants a call to the doctor.

The conversion of 98.6°F to Celsius is 37.0°C. This number was based on a small sample of men in 1868. Protsiv et al. 2020, using temperature measurements from the US, found that the mean body temperature in the 2010s was 97.5°F (36.4°C). The decline is about 1.1°F (0.6°C) per century. If a thermometer reads 98.6°F (37.0°C), it is normal, not a fever. If a thermometer reads 98.6°F and the guidance says 100.4°F is a fever, the difference of 1.8°F (1.0°C) is the margin between normal and fever.

The conversion of 100.4°F to Celsius is 38.0°C. This is the CDC and AAP definition of a fever. The NHS uses the same 100.4°F (38.0°C) for children. For adults, the NHS threshold is slightly lower at 100.0°F (37.8°C). A reading of 100.4°F (38.0°C) is a fever regardless of the measurement site, per the CDC. A reading of 99.9°F (37.7°C) is not a fever.

Normal body temperature in Celsius ranges from about 36.1°C (97.0°F) oral to 37.9°C (100.3°F) oral. Rectal readings are higher: 37.0°C (98.6°F) to 38.1°C (100.6°F). Axillary readings are lower: 35.9°C (96.6°F) to 37.6°C (99.7°F). Ear and forehead readings are similar to oral: 36.4°C (97.6°F) to 37.8°C (100.0°F). A temperature of 37.0°C (98.6°F) is within normal range for most people. A temperature of 38.0°C (100.4°F) is a fever.

°F	°C
95.0	35.0
95.2	35.1
95.4	35.2
95.6	35.3
95.8	35.4
96.0	35.6
96.2	35.7
96.4	35.8
96.6	35.9
96.8	36.0
97.0	36.1
97.2	36.2
97.4	36.3
97.6	36.4
97.8	36.6
98.0	36.7
98.2	36.8
98.4	36.9
98.6	37.0
98.8	37.1
99.0	37.2
99.2	37.3
99.4	37.4
99.6	37.6
99.8	37.7
100.0	37.8
100.2	37.9
100.4	38.0
100.6	38.1
100.8	38.2
101.0	38.3
101.2	38.4
101.4	38.6
101.6	38.7
101.8	38.8
102.0	38.9
102.2	39.0
102.4	39.1
102.6	39.2
102.8	39.3
103.0	39.4
103.2	39.6
103.4	39.7
103.6	39.8
103.8	39.9
104.0	40.0
104.2	40.1
104.4	40.2
104.6	40.3
104.8	40.4
105.0	40.6
105.2	40.7
105.4	40.8
105.6	40.9
105.8	41.0
106.0	41.1

Rectal temperature is the gold standard for infants and young children, per the AAP. It is the closest to core body temperature. For older children and adults, an oral reading under the tongue is sufficient. Ear (tympanic) and forehead (temporal artery) thermometers are convenient but less accurate if not positioned correctly. Axillary (under arm) readings are the least accurate and can be up to 1.0°F (0.6°C) lower than oral. Never rely on an axillary reading to rule out a fever.

How to Adjust Between Sites

If you have an oral reading and need the equivalent rectal reading, add 0.5°F (0.3°C). If you have an axillary reading and need the oral equivalent, add 1.0°F (0.6°C). These are approximations; the CDC ranges above are the authoritative guide.

The CDC recommends calling a doctor for a fever of 104°F (40°C) or higher in an adult. For children, the NHS advises calling a doctor if a fever reaches 100.4°F (38°C) in an infant under 3 months, or if the fever lasts more than 5 days in a child of any age. The AAP adds that any fever in an infant under 3 months requires immediate medical evaluation. Signs that warrant a call include: the child is dehydrated, has a seizure, is breathing rapidly, or is unresponsive. For adults, call if the fever is 104°F (40°C) or higher, or if there is confusion, dizziness, or severe pain.

What Not to Do

Do not give aspirin to a child or teenager with a fever; it can cause Reye syndrome. Do not alternate acetaminophen and ibuprofen without a doctor's instruction. Do not use cold baths or alcohol rubs to lower a fever; they can cause shivering and raise the temperature. The body needs fluids, not ice.

Reference conversions and fever thresholds from the CDC, NHS, and American Academy of Pediatrics are provided here.

It does not replace medical advice. If you are unsure whether a temperature reading indicates a fever, or if the person is ill, call a healthcare provider. Temperature readings are affected by the device, site, time of day, and individual variation. A single reading above 100.4°F (38°C) does not always mean a fever; the trend over time matters.

Reading a thermometer in one scale and the guidance in the other is the failure case. A parent sees 37.8°C on a digital ear thermometer, knows 98.6°F (37°C) is normal, and assumes 37.8°C is fine. It is not. The NHS adult fever threshold is 37.8°C (100°F). The CDC threshold is 38°C (100.4°F). At 37.8°C, you are at the border. Always convert to the scale of the guidance before acting.