A fever in adults is generally defined as a body temperature of 100.4°F (38°C) or higher. That said, the exact number depends on where you take your temperature, and certain groups, especially older adults, may run fevers at lower readings than the standard cutoff.
Fever Thresholds by Measurement Site
Your thermometer reading shifts depending on where you measure. Rectal and ear readings run higher than oral ones, and armpit readings run lower. These are the standard thresholds:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral (under the tongue): 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
There is no exact formula for converting between these sites. You may have heard rules like “add a degree for armpit readings,” but the Mayo Clinic notes there’s no reliable way to adjust temperatures taken from different parts of the body. The simplest approach is to compare your reading to the threshold for the method you used.
Why 98.6°F Isn’t a Magic Number
Normal body temperature isn’t a fixed point. Studies show it ranges from about 97°F (36.1°C) to 99°F (37.2°C) across healthy adults. Your temperature is lowest in the early morning and naturally rises through the afternoon and evening. Physical activity, hormonal cycles, heavy clothing, and even a hot meal can nudge it higher. A reading of 99.2°F after a workout doesn’t necessarily mean you’re sick.
This natural variation is why the fever threshold sits above 100°F for oral readings rather than at some precise margin above 98.6°F. If your temperature is in the 99 to 100°F range and you feel fine, you’re likely just seeing normal fluctuation.
Fever Thresholds in Older Adults
Adults over 65, particularly those who are frail, tend to have lower baseline body temperatures than the commonly cited 98.6°F. That makes it harder for their bodies to hit the standard fever cutoff, even when a serious infection is present. CDC guidelines for older adults in long-term care use a broader definition: a single oral reading above 100°F, repeated oral readings above 99°F, or any rise of more than 2°F above that person’s usual baseline.
This is important because infections in older adults can become dangerous quickly. A temperature that looks borderline in a younger person may represent a significant fever in someone over 65. Knowing your normal baseline temperature makes it much easier to spot a meaningful change.
Low-Grade vs. High-Grade Fevers
Not all fevers carry the same urgency. A low-grade fever, roughly 100 to 102.2°F (37.8 to 39°C), is the body’s standard immune response to infections like a cold or flu. It’s uncomfortable but rarely dangerous on its own.
High-grade fevers range from about 102.4 to 105.8°F (39.1 to 41°C). Adults with temperatures of 103°F or higher typically look and feel noticeably ill. Harvard Health recommends calling a doctor if your fever reaches 104°F (40°C), as sustained temperatures at that level can signal a more serious infection or complication that needs medical evaluation.
Which Thermometer Is Most Accurate
Rectal thermometers give the most accurate core body temperature reading, but for obvious reasons most adults prefer an oral digital thermometer, which provides similar accuracy. If you use one, wait at least 15 minutes after eating or drinking before taking a reading, since hot coffee or ice water will temporarily skew the number.
Forehead (temporal artery) thermometers are convenient but less reliable. Direct sunlight, cold air, and sweat on the forehead can all throw off the reading. Ear thermometers face their own issues: earwax buildup, ear infections, and even very hot or cold room temperatures can affect accuracy. If you get a surprising reading from a forehead or ear thermometer, it’s worth double-checking with an oral reading.
Armpit thermometers are the least accurate option. They’re better than nothing, but if the number seems off, switch to oral measurement to confirm.
When a Fever Needs Treatment
A fever itself isn’t an illness. It’s your immune system raising the thermostat to make the body less hospitable to viruses and bacteria. Low-grade fevers often don’t need medication at all, especially if you’re tolerating the discomfort reasonably well. Staying hydrated, resting, and wearing light clothing can be enough.
Over-the-counter options like acetaminophen and ibuprofen can bring a fever down when the aches, chills, or general misery are keeping you from resting. These work by different mechanisms, so they can be alternated if one alone isn’t providing enough relief. Follow the dosing instructions on the package, and avoid exceeding the daily maximum for either one.
A fever that climbs above 104°F, lasts more than three days, or comes with a stiff neck, severe headache, confusion, persistent vomiting, or a rash warrants prompt medical attention. These combinations can point to infections that need more than rest and fluids to resolve.