The most reliable way to tell if someone has a fever is with a thermometer: a reading of 100.4°F (38°C) or higher from the mouth, ear, or forehead confirms a fever. But you can also pick up on a fever through physical signs like flushed skin, chills, and unusual warmth to the touch, especially when a thermometer isn’t handy.
What Temperature Counts as a Fever
A fever is defined at slightly different thresholds depending on where you measure:
- Oral, rectal, ear, or forehead: 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
These numbers matter because “normal” body temperature isn’t the 98.6°F most people learned growing up. A large Stanford Medicine analysis found that the average adult body temperature is closer to 97.9°F, with a normal range spanning 97.3°F to 98.2°F. Your temperature also shifts throughout the day, running coolest in the early morning and peaking around 4 p.m. So a reading of 99.5°F at 7 a.m. is more significant than the same number in the late afternoon.
How to Check With a Thermometer
A basic digital contact thermometer works for any age and is the simplest option for most households. Here’s how the different types compare.
Oral thermometers are the most common choice for older children and adults. They’re nearly as accurate as rectal readings and far more comfortable. One thing to keep in mind: wait at least 15 minutes after eating or drinking before taking an oral temperature, since hot coffee or ice water will throw off the reading.
Rectal thermometers give the most accurate measurement and are the standard for infants when a fever is suspected. They’re invasive and uncomfortable, which is why they’re mostly reserved for babies and young children where precision matters most.
Ear (tympanic) thermometers use an infrared sensor to read the temperature inside the ear canal. They’re fast, comfortable, and work well for children older than 7 months and adults. They’re not recommended for infants under 7 months because a small ear canal, earwax, or an ear infection can skew the results. Very hot or cold room temperatures can also affect accuracy.
Forehead and contactless thermometers measure temperature without entering the body, which makes them convenient for sleeping children or quick screenings. They’re less precise than oral or rectal readings but still useful for spotting a fever.
One important note: temperatures from different body sites don’t convert neatly to one another. There’s no reliable formula for adding or subtracting a degree to match them up. The best approach is to use the same method each time so you can compare readings consistently.
How to Tell Without a Thermometer
When you don’t have a thermometer available, several physical signs can help you gauge whether someone is running a fever.
Touch their forehead. Use the back of your hand, which is more sensitive to heat than your palm. If the person’s forehead feels noticeably hot compared to your own skin, a fever is likely. You can also rest your cheek gently against their forehead to feel for excess warmth.
Look for flushed skin. Fever often causes reddish or unusually colored cheeks. In infants and young children, flushed or red-looking skin across the face and body is a common visible sign.
Check for dehydration. Fever pulls water from the body, so signs of dehydration can be a clue. Gently pinch the skin on the back of the hand and release it. Well-hydrated skin snaps back immediately. If it moves slowly, the person may be dehydrated. Dark yellow or orange urine, or urine with a strong odor, is another signal that the body is losing fluids, which often accompanies fever.
Watch for behavioral changes. Someone with a fever often looks and acts unwell in ways that are hard to miss: shivering or chills, body aches, headache, fatigue, and general lethargy. In children, irritability, poor eye contact, or unusual sleepiness can point to a fever even before you touch their skin.
None of these methods are as precise as a thermometer, but taken together, they give a reasonable picture. If the person feels hot, looks flushed, and is shivering or complaining of aches, it’s safe to treat them as though they have a fever until you can confirm with a reading.
Fever Signs in Babies and Young Children
Young children can’t describe how they feel, so you have to rely on what you see. A baby with a fever may have flushed or red skin, feel very warm to the touch, seem unusually fussy or irritable, or become limp and unresponsive. Repeated vomiting or refusal to eat can also accompany a fever in small children.
The age of the child changes how urgently you need to act. For any baby younger than 3 months, any fever at all warrants a call to a healthcare provider, even if the baby otherwise seems fine. For babies 3 to 6 months old, a temperature above 100.4°F or any fever paired with signs of illness (poor feeding, unusual fussiness, lethargy) needs medical attention. For children 6 to 24 months old, a fever above 100.4°F that persists for more than a day is the threshold for calling a provider.
Rectal temperature is the preferred method for infants because it’s the most accurate. Ear thermometers aren’t reliable in babies under 7 months.
When a Fever Needs Urgent Attention
Most fevers in otherwise healthy people are harmless and resolve on their own. But certain symptoms alongside a fever signal something more serious.
In adults, seek immediate care if a fever comes with any of the following: a severe headache, stiff neck (especially pain when bending the head forward), unusual sensitivity to bright light, mental confusion or altered speech, a new rash, persistent vomiting, difficulty breathing, chest pain, abdominal pain, pain during urination, or seizures. A temperature reaching 103°F (39.4°C) or higher also warrants a call to a provider, even without other symptoms.
In children, watch for listlessness, confusion, poor eye contact, repeated vomiting, severe headache or stomachache, or a fever lasting longer than three days. A seizure triggered by fever requires a call to 911 if it lasts more than five minutes or if the child doesn’t recover quickly. Any fever that develops after a child has been left in a hot car is a medical emergency regardless of the temperature reading.