A fever in adults starts at 100.4°F (38.0°C) when measured with a thermometer. Anything between 99.1°F and 100.3°F is considered a low-grade fever, and readings above 102.4°F (39.1°C) are high-grade. But before you even grab a thermometer, your body usually gives you clear signals that your temperature is climbing.
What a Fever Feels Like
Most people suspect a fever before they confirm one. The earliest sign is often chills, which feel paradoxical because your body is actually heating up. Your brain has reset its internal thermostat higher, so your current normal temperature feels cold by comparison. That triggers shivering, goosebumps, and the urge to pile on blankets.
As your temperature rises, you may notice a flushed face, warm skin (especially on your forehead and chest), body aches, and a general sense of fatigue or weakness. Some people get a headache or lose their appetite. Once the fever peaks or starts to break, you’ll often sweat heavily as your body works to cool itself back down. If you’re experiencing several of these symptoms together, there’s a good chance your temperature is elevated.
How to Get an Accurate Reading
The only way to confirm a fever is with a thermometer, but where you measure matters more than most people realize. Rectal readings are the closest to your true core temperature. Oral readings average about 1.1°F lower than rectal, and in some cases can be off by nearly 3°F in either direction. Ear (tympanic) thermometers are similarly unreliable for individual readings. While the average difference from rectal is small (around 0.2°F), any single ear reading can be off by up to 2°F higher or 1.6°F lower than rectal.
Forehead thermometers are convenient but tend to be the least consistent. For most adults at home, an oral digital thermometer is the most practical choice. Place the tip under your tongue, keep your mouth closed, and wait for the beep. Avoid eating, drinking, or exercising for at least 15 minutes beforehand, as all of these can skew the result.
For infants and very young children, a rectal thermometer is the standard. Pediatricians define fever in babies as a rectal temperature of 100.4°F (38.0°C) or higher.
Your Temperature Changes Throughout the Day
Normal body temperature isn’t a fixed 98.6°F. It fluctuates by as much as 0.9°F over the course of a day, reaching its lowest point around 4 a.m. and peaking near 6 p.m. This means a reading of 99.3°F in the evening could be perfectly normal, while that same number at 6 a.m. might actually reflect a low-grade fever. If you’re borderline, take your temperature at the same time of day when rechecking so you’re comparing apples to apples.
Fever Ranges in Adults
Not all fevers carry the same weight. The categories break down like this:
- Low-grade: 99.1°F to 100.4°F (37.3°C to 38.0°C). Common with mild infections, allergies, or even after vigorous exercise. Often manageable at home.
- Moderate: 100.6°F to 102.2°F (38.1°C to 39.0°C). Typical of many viral illnesses like the flu or COVID. Your body is actively fighting an infection.
- High-grade: 102.4°F to 105.8°F (39.1°C to 41.0°C). This level warrants close attention. A temperature of 103°F (39.4°C) or higher is the threshold where medical guidance becomes important.
Fever Thresholds for Babies and Children
The rules are stricter for young children, especially newborns. Any rectal temperature of 100.4°F or higher in a baby under 3 months old needs prompt medical evaluation, even if the baby looks fine otherwise. Infants in this age range can have serious infections without showing obvious signs of illness.
For older children, a fever alone isn’t necessarily alarming. What matters more is how the child is acting. A child with a 102°F fever who is still drinking fluids and making eye contact is in a very different situation than a child with the same temperature who is limp, confused, or inconsolable.
Signs a Fever Needs Medical Attention
A fever is your immune system doing its job, and most fevers resolve on their own within a few days. But certain symptoms alongside a fever point to something more serious. In adults, seek immediate care if a fever comes with any of the following: a stiff neck with pain when bending your head forward, mental confusion or altered speech, unusual sensitivity to bright light, persistent vomiting, difficulty breathing or chest pain, a rash, seizures, or pain when urinating.
In children, the red flags include listlessness, poor eye contact, repeated vomiting, or a severe headache. A seizure lasting more than five minutes is a 911 situation for any age. And any fever in a child who was recently left in a hot car requires emergency care regardless of how high the reading is.
Checking Without a Thermometer
If you don’t have a thermometer handy, you can get a rough sense by pressing the back of your hand (not your palm, which is less sensitive) against your forehead or neck. Warm, flushed skin combined with chills and body aches strongly suggests a fever. But this method can’t tell you whether you’re at 100°F or 103°F, and that distinction matters. A basic digital thermometer costs a few dollars and is worth keeping in your medicine cabinet for moments exactly like this.