What Counts as a Normal Body Temperature, and When It Is a Fever
What is a normal body temperature?
The textbook figure of 98.6°F (37°C) is a 19th-century average. Modern studies put the typical adult range at roughly 97°F to 99°F (36.1–37.2°C), with readings naturally lowest in the early morning and highest in the late afternoon. Body temperature also declines slightly with age and varies by about half a degree across the menstrual cycle.
Get an accurate reading
- Wait 15 minutes after eating, drinking, or exercise before an oral reading.
- Measure at the same time of day when tracking a trend - afternoon readings run about 1°F higher than early morning.
- Stick to one method. Ear, forehead and oral readings differ by up to a degree, so comparisons only work within one method.
Normal ranges by age
| Age group | Normal range (°F) | Normal range (°C) |
|---|---|---|
| Babies (0–2 years) | 97.9–100.4°F | 36.6–38.0°C |
| Children (3–10) | 97.0–100.0°F | 36.1–37.8°C |
| Adults (11–65) | 97.0–99.0°F | 36.1–37.2°C |
| Older adults (65+) | 96.4–98.5°F | 35.8–36.9°C |
Measurement method matters
| Method | Compared with oral | Notes |
|---|---|---|
| Oral | Baseline | Wait 15 min after hot/cold drinks |
| Rectal | +0.5 to +1°F higher | Most accurate for infants |
| Ear (tympanic) | +0.5 to +1°F higher | Position affects accuracy |
| Forehead (temporal) | ≈0 to -0.5°F lower | Fast, slightly less precise |
| Armpit (axillary) | -0.5 to -1°F lower | Least accurate common method |
When is it a fever?
The widely used medical threshold for fever is 100.4°F (38°C). Between about 99.1°F and 100.3°F is often described as a low-grade elevation. See our fever temperature chart for severity levels and when to seek care. A single slightly-high reading in a comfortable, well-appearing person usually just reflects the daily rhythm, exercise, or a warm room - recheck after 30 quiet minutes.
When a temperature is too low: hypothermia
A body temperature below 95°F (35°C) is hypothermia, and it is a medical emergency. It usually follows exposure to cold air or cold water, and it creeps up on people: as the body cools, the ability to think clearly goes too, so someone with hypothermia may not realise they need help.
Warning signs include confusion, drowsiness, pale and cold skin, slowed breathing or heart rate, shivering that cannot be controlled (it can stop altogether once the body is very cold), and weakness or loss of coordination. Call 911 or your local emergency number whenever hypothermia is suspected, and do not warm the person with direct heat such as hot water or a heating pad.
A single low reading on a thermometer, such as 96°F (35.6°C), is far more often a measuring problem than hypothermia: a reading taken right after a cold drink, an under-arm reading, or a forehead scan in a cold room all run low. Re-check with an oral or rectal reading before worrying.
When to call a doctor about a temperature
Most fevers come from minor infections and settle with rest and fluids. These are the points at which MedlinePlus (the US National Library of Medicine) advises contacting a medical provider right away:
- Babies 3 months or younger: a rectal temperature of 100.4°F (38°C) or higher.
- Babies 3 to 12 months: a fever of 102.2°F (39°C) or higher.
- Children 2 years or younger: a fever lasting longer than 24 to 48 hours; older children, longer than 48 to 72 hours.
- Adults: a fever that stays at or keeps rising above 103°F (39.4°C), a fever lasting longer than 48 to 72 hours, or any fever of 105°F (40.6°C) or higher that does not come down readily with treatment.
- Anyone with a fever plus a serious long-term illness, a weakened immune system, a new rash or bruises, pain when urinating, or recent travel abroad.
Call 911 or your local emergency number if a fever comes with confusion, difficulty waking, trouble breathing, blue lips or nails, a very bad headache, a stiff neck or a seizure. The fever chart linked above breaks readings down by age and measuring method. This guide is general information, not a diagnosis.
Why the textbook 98.6°F is outdated
The famous 98.6°F comes from a German study of the 1860s. Modern datasets tell a cooler story: average adult body temperature has drifted down over the past 150 years, and large recent studies put today's typical oral average near 97.9°F (36.6°C). Better thermometers, lower rates of chronic infection and changed metabolism all likely contribute.
The practical takeaway: judge readings against a range (97-99°F for most adults) and against your own baseline, not against a single 19th-century number.
What moves your temperature day to day
- Time of day: lowest around 4-6 AM, highest in late afternoon - a swing of about 1°F.
- Exercise: hard workouts can raise core temperature 2-3°F for a while afterward.
- The menstrual cycle: a sustained rise of roughly 0.5-1°F follows ovulation (the basis of basal body temperature charting).
- Age: older adults run cooler and may mount smaller fevers, so a “mild” reading can understate illness.
- Hot drinks, meals and weather: transient effects worth waiting out before you measure.