A normal body temperature for adults between ages 11 and 65 falls in the range of 97.6°F to 99.6°F (36.4°C to 37.6°C). The old standard of 98.6°F is a rough midpoint, but your actual baseline can sit anywhere within that range and still be perfectly healthy. A fever starts at 100.4°F (38°C), while anything below 95°F (35°C) is considered hypothermia.
Why 98.6°F Isn’t the Magic Number
The 98.6°F figure dates back to a German study from 1851 and has stuck around ever since. But modern research paints a more nuanced picture. Body temperature varies from person to person, and several factors shift your personal baseline up or down. Women tend to run slightly warmer than men. Younger adults run warmer than older adults. Even the time of day matters: your temperature is typically lowest in the early morning and highest in the late afternoon.
If you’ve ever taken your temperature and seen 97.8°F or 99.1°F and wondered whether something was wrong, it almost certainly wasn’t. Those readings are well within the normal adult range.
How Age Changes Your Baseline
Body temperature rises from childhood into adulthood, then gradually dips in later life. For adults older than 65, the typical range drops to 96.4°F to 98.5°F (35.8°C to 36.9°C). That’s a meaningful difference. An older adult with a reading of 99°F might already have a significant fever relative to their own baseline, even though that number would be unremarkable in a younger person.
How Hormones Affect Temperature
If you menstruate, your body temperature shifts predictably across your cycle. After ovulation, your basal body temperature (the lowest temperature your body reaches during rest) rises by roughly 0.4°F to 1°F. This bump lasts through the luteal phase until your period begins. It’s the principle behind fertility tracking methods that use a thermometer each morning. The shift is small enough that you wouldn’t notice it physically, but a sensitive thermometer picks it up reliably.
Where You Measure Matters
Different spots on your body give slightly different readings, so what counts as “normal” depends on your thermometer type. Oral thermometers read about 0.4°F lower than rectal or ear thermometers, while armpit readings run about 1°F lower than oral. In practical terms, that means a fever threshold of 100.4°F applies to rectal, ear, and temporal artery readings, while an oral temperature of 100°F or an armpit temperature of 99°F signals the same thing.
For the most consistent results at home, use the same method each time. Oral thermometers are the most common choice for adults. If you’re using a forehead (temporal artery) scanner, keep in mind that ambient temperature and sweat can affect the reading.
When a Temperature Becomes a Fever
A fever is generally defined as a body temperature above 100.4°F (38°C). Low-grade fevers, those hovering just above the threshold, are common with mild infections and often resolve on their own. A fever is not an illness itself. It’s your immune system raising your internal thermostat to make the environment less hospitable for viruses and bacteria.
A temperature of 103°F (39.4°C) or higher warrants a call to your doctor. At that level, the fever itself can cause significant discomfort: chills, body aches, fatigue, and dehydration.
Certain symptoms alongside a fever signal something more urgent, regardless of the number on the thermometer:
- Severe headache or stiff neck
- Rash or unusual sensitivity to bright light
- Confusion, altered speech, or strange behavior
- Persistent vomiting
- Difficulty breathing or chest pain
- Seizures
Dangerously High Temperatures
A body temperature above 106.7°F (41.5°C) is a medical emergency called hyperpyrexia. At this level, the heat itself begins damaging organs. Symptoms escalate from dizziness, nausea, and a fast heart rate to confusion, seizures, muscle rigidity, and loss of consciousness. Hyperpyrexia is life-threatening without immediate treatment and most often occurs with heatstroke, severe infections, or certain drug reactions rather than a typical illness.
When Your Temperature Is Too Low
Hypothermia occurs when your core temperature drops below 95°F (35°C). Prolonged cold exposure is the most common cause, but older adults, people with certain chronic conditions, and those on medications that affect circulation can develop hypothermia even in mildly cool environments. Early signs include shivering, clumsiness, and confusion. As temperature continues to fall, shivering may actually stop, which is a dangerous sign that the body is losing its ability to warm itself.
For older adults whose baseline already sits in the mid-96°F range, even a modest dip can push them toward hypothermia more quickly than a younger person.