A fever is any body temperature at or above 100.4°F (38°C), whether measured orally, rectally, or in the ear. That’s the threshold used by the CDC and most medical guidelines. But even without a thermometer, your body gives you several reliable clues that your temperature is elevated.
What Counts as a Fever
The exact number depends on where you measure. An oral or rectal reading of 100.4°F (38°C) or higher is a fever. An armpit reading of 99°F (37.2°C) or higher also qualifies, though armpit measurements tend to be less precise. These thresholds apply to both adults and children.
Fevers also fall into a rough spectrum. Harvard Health breaks it down this way:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
A low-grade fever often resolves on its own within a day or two. High-grade fevers deserve closer attention, especially if they persist or come with other concerning symptoms.
Signs You Have a Fever Without a Thermometer
Not everyone has a thermometer handy, and the CDC acknowledges that. Feeling warm to the touch, having a flushed face, or experiencing chills are all recognized signs of a possible fever even when you can’t get a number. If someone places the back of their hand on your forehead or neck and you feel noticeably warm, that’s a reasonable signal.
Beyond warmth, fevers typically come bundled with other symptoms that are hard to miss: chills, body aches, headache, and a general feeling of exhaustion. Chills happen because your brain has temporarily raised its internal thermostat. Your body thinks it’s too cold relative to this new, higher set point, so it constricts blood vessels and triggers shivering to generate heat. That’s why you can feel freezing even though you’re actually running hot. Once the fever breaks, the process reverses: blood vessels open up, you start sweating, and the chills fade.
Why Your Body Raises Its Temperature
Fever isn’t a malfunction. It’s a deliberate defense. When your immune system detects an infection, it releases signaling molecules that act on a temperature-control center deep in the brain. These signals raise your body’s target temperature, much like turning up a thermostat. Your body then works to reach that new set point by conserving heat (pulling blood away from the skin, which is why your hands and feet can feel cold) and generating heat through shivering.
This elevated temperature makes it harder for many bacteria and viruses to replicate, and it also speeds up certain immune responses. The process works the same way whether the trigger is a cold, the flu, a urinary tract infection, or something else entirely.
Getting an Accurate Thermometer Reading
If you do have a thermometer, how you use it matters. Rectal thermometers are the most accurate, followed closely by oral thermometers. Ear and forehead thermometers are convenient but less reliable. Earwax, a curved ear canal, or an ear infection can all throw off an ear reading. Forehead thermometers lose accuracy in direct sunlight, cold environments, or when the skin is sweaty.
For an oral reading, place the thermometer under your tongue and keep your mouth closed until it beeps. Avoid eating, drinking, or smoking for at least 15 minutes beforehand, since a hot cup of coffee or a cold glass of water will temporarily skew the number. For armpit readings, tuck the thermometer snugly against dry skin and hold your arm tight against your body. Keep in mind that armpit temperatures run lower than oral ones, so a reading of 99°F in the armpit is roughly equivalent to 100.4°F orally.
Pacifier thermometers and fever strips are not recommended because they’re too imprecise to be useful. Clean the tip of your thermometer before and after every use.
Measuring Temperature in Babies and Children
For babies from birth to 3 months, a rectal thermometer is the standard. This age group needs the most accurate reading possible because fevers in very young infants can signal serious infections. From 3 months to 4 years, you can use a rectal, armpit, or forehead thermometer. By age 4, most children can hold a digital thermometer under their tongue long enough for an oral reading.
If you use a thermometer for rectal readings, label it and keep a separate one for oral use. Never use the same thermometer in both places. And never leave a child unattended with a thermometer in place.
When a Fever Is Concerning
Most fevers in otherwise healthy adults and older children are uncomfortable but not dangerous. They typically mean your immune system is doing its job. But certain combinations of symptoms alongside a fever signal something more serious.
In adults, a fever paired with a stiff neck, confusion, sensitivity to light, or a severe headache can indicate meningitis, which requires immediate medical attention. A fever with rapid breathing, severe muscle or joint pain, cold hands and feet, or a dark purple rash may point to a serious bloodstream infection. Persistent vomiting, chest pain, or difficulty breathing alongside a fever are also red flags.
In babies, the threshold for concern is lower. Any fever of 100.4°F or higher in an infant under 3 months old warrants a call to a healthcare provider, even if the baby seems otherwise fine. In older infants, watch for poor feeding, unusual irritability, sluggishness, vomiting, or a bulging soft spot on the skull.
A fever that lasts more than three days in adults, or more than a day in young children, is also worth getting checked out, particularly if it keeps climbing or doesn’t respond to rest and fluids.