Normal temperature by age group
Body temperature is not identical across a lifespan. Newborns and infants tend to run slightly warmer and regulate temperature less efficiently, while older adults often run a little cooler and may not mount as strong a fever when ill. Cleveland Clinic and MedlinePlus both highlight these age-related differences. The ranges below are typical normal values, and the exact figure still depends on the measurement method described later in this article.
| Age group | Normal range (Celsius) | Normal range (Fahrenheit) |
|---|
| Newborn (0 to 3 months) | 36.4C to 37.6C | 97.5F to 99.7F |
| Child (3 months to 12 years) | 36.1C to 37.2C | 97.0F to 99.0F |
| Adult (12 to 65 years) | 36.1C to 37.2C | 97.0F to 99.0F |
| Older adult (65 and over) | 35.8C to 36.9C | 96.4F to 98.5F |
The most important practical point about age is the fever threshold for the very young. Because a fever in a newborn can be the first sign of a serious infection, medical guidance is stricter for them. The CDC and Mayo Clinic advise that any infant under three months old with a rectal temperature of 100.4F (38C) or higher should be evaluated by a doctor promptly, even if the baby otherwise seems well.
Normal temperature by measurement method
Where you place the thermometer changes the number it shows. A reading taken in the armpit is naturally lower than one taken inside the body, because skin is cooler than core tissue. This is the single most common reason two thermometers seem to disagree. The table below gives the typical normal ranges for the four most common methods, drawn from Mayo Clinic and Cleveland Clinic guidance.
| Method | Normal range (Celsius) | Normal range (Fahrenheit) |
|---|
| Oral (under the tongue) | 35.5C to 37.5C | 95.9F to 99.5F |
| Rectal (most accurate) | 36.6C to 38.0C | 97.9F to 100.4F |
| Ear (tympanic) | 35.8C to 38.0C | 96.4F to 100.4F |
| Armpit (axillary) | 34.7C to 37.3C | 94.5F to 99.1F |
As a rule of thumb, rectal and ear readings run about 0.5C to 1C (1F to 2F) higher than oral readings, and armpit readings run a similar amount lower. When you record a temperature for a doctor, always note which method you used, because the same core temperature looks different depending on the site.
What raises or lowers body temperature
A normal temperature is not a fixed point. It moves throughout the day and responds to what you are doing. According to MedlinePlus, body temperature follows a daily rhythm that is lowest in the early morning, often around 4 a.m., and highest in the late afternoon or early evening. That natural swing is about 0.5C, or close to 1F, so a reading of 98.4F at 9 a.m. and 99.0F at 6 p.m. can both be normal for the same healthy person.
Several everyday factors push a reading up or down without any illness being involved:
- Exercise and physical activity raise core temperature, sometimes by a full degree or more, and it can take 30 minutes or longer to return to baseline.
- Hot or cold environments shift skin and oral readings, which is why a thermometer used right after coming in from the cold can read low.
- Eating, drinking, or smoking just before an oral reading can warm or cool the mouth. Wait about 15 to 20 minutes for an accurate measurement.
- The menstrual cycle raises baseline temperature slightly after ovulation, an effect long used in fertility tracking.
- Age matters at both ends of life, with infants running warmer and older adults often running cooler, as noted above.
- Stress and strong emotion can nudge a reading up by a small amount through their effect on the nervous system.
When a reading is too high: fever
A fever is the body deliberately raising its own temperature, usually to help fight an infection. The CDC and Mayo Clinic generally define a fever as a temperature of 100.4F (38C) or higher. Readings between roughly 99F and 100.3F are sometimes called low grade and may simply reflect the time of day, recent activity, or a warm room rather than illness.
A fever on its own is not usually dangerous in an otherwise healthy adult, and it often does not require treatment beyond rest and fluids. Mayo Clinic advises that adults seek medical care for a temperature of 103F (39.4C) or higher, or for a fever that lasts more than three days. The thresholds are stricter for children, and any fever in an infant under three months old should be checked by a doctor right away. Seek urgent help if a fever comes with a stiff neck, confusion, severe headache, difficulty breathing, a rash that does not fade under pressure, or a seizure.
When a reading is too low: hypothermia
At the other end of the scale, a body temperature below 95F (35C) is defined as hypothermia, according to the CDC, and it is a medical emergency. Hypothermia means the body is losing heat faster than it can produce it, and it most often happens after exposure to cold air or water. Warning signs include intense shivering, slurred speech, drowsiness, clumsiness, and confusion. In infants, cold skin and unusual quietness or low energy can be signs.
It is worth separating a true low core temperature from a misleading reading. A single low number from an armpit thermometer, a cold mouth, or a device used right after coming inside is usually a measurement issue, not hypothermia. A genuinely low core temperature with symptoms, on the other hand, calls for immediate medical attention and active rewarming.
How to measure body temperature accurately
A thermometer is only as reliable as the way it is used. A few simple habits keep your readings trustworthy:
- Pick the right method for the age. Use a rectal thermometer for newborns and infants, since it is the most accurate. Oral, ear, or forehead devices are fine for older children and adults.
- Wait after eating, drinking, or exercising. Leave 15 to 20 minutes before an oral reading and around 30 minutes after activity.
- Follow the device instructions. Ear and forehead thermometers need correct positioning to be accurate, and a digital oral thermometer should stay in place until it beeps.
- Record the method and time. Note whether the reading was oral, rectal, ear, or armpit, and roughly what time of day it was taken.
- Compare to your own baseline. Knowing your usual morning and evening temperatures makes it far easier to spot a meaningful change.
If you take readings in one scale but your thermometer or your doctor uses the other, the converter above turns any Celsius value into Fahrenheit and back in a single step. The fixed formula is straightforward: to go from Celsius to Fahrenheit, multiply by 9, divide by 5, and add 32. To go the other way, subtract 32, multiply by 5, and divide by 9. For body-temperature ranges specifically, 36.6C equals 97.9F and 38C equals 100.4F.
The bottom line on normal body temperature
Normal body temperature is a range, not a single magic number. For most healthy adults that range runs from about 97F to 99F (36.1C to 37.2C), with a modern average near 97.9F (36.6C) rather than the historic 98.6F. Your reading shifts with the time of day, your age, your activity, and the thermometer method you use, and all of those shifts can be completely normal. Pay attention to clear thresholds instead: a reading of 100.4F (38C) or higher signals a fever, and one below 95F (35C) signals hypothermia. For everything in between, the most useful number is your own personal baseline. As always, this guide is general information, so consult a healthcare professional for advice about a specific reading or symptom.