Yes, 38°C (100.4°F) is the standard threshold for fever in adults. If your thermometer reads 38°C or higher, you have a fever. The CDC defines fever as a measured temperature of 100.4°F (38°C) or greater, and this is the cutoff used by most hospitals and clinics worldwide.
Why 38°C Is the Cutoff
The average normal oral temperature for adults is 37°C (98.6°F), so 38°C sits a full degree above that baseline. That one-degree jump signals your immune system is actively fighting something, whether it’s a viral infection, bacterial illness, or another inflammatory process. Your body deliberately raises its temperature because many pathogens reproduce less efficiently in warmer conditions.
That said, body temperature isn’t fixed throughout the day. It tends to run a little lower in the morning and a bit higher in the afternoon and evening. So a reading of 37.8°C after dinner might be less concerning than 37.8°C first thing in the morning. A reading of exactly 38°C is right at the fever line, meaning you could be fighting off an infection even if you feel mostly fine.
How Your Thermometer Affects the Number
Where you take your temperature matters, because different methods give slightly different results. Oral readings (under the tongue) are considered the standard reference point. If you’re using a different type of thermometer, here’s how the numbers shift:
- Ear (tympanic) thermometers read 0.3 to 0.6°C higher than oral. So an ear reading of 38°C may actually correspond to an oral temperature of about 37.4 to 37.7°C, which isn’t quite a fever.
- Armpit (axillary) thermometers read 0.3 to 0.6°C lower than oral. An armpit reading of 38°C likely means your true core temperature is closer to 38.3 to 38.6°C, which is a clear fever.
- Forehead (temporal) thermometers vary by brand and technique, so they’re generally less precise than oral or ear readings.
If your armpit thermometer says 38°C, take it seriously. If your ear thermometer says 38°C, you’re probably running warm but may not have crossed the clinical fever threshold.
Fever Ranges and What They Mean
Not all fevers carry the same weight. A temperature of 38°C is a low-grade fever. Your body is responding to something, but in most healthy adults this level doesn’t require aggressive treatment. Here’s how the range breaks down:
- 38°C to 39°C (100.4°F to 102.2°F): Low-grade fever. Common with colds, flu, and minor infections. Usually manageable at home with rest and fluids.
- 39°C to 39.4°C (102.2°F to 103°F): Moderate fever. You’ll likely feel noticeably unwell. Worth monitoring closely.
- 39.4°C to 40°C (103°F to 104°F): High fever. The Mayo Clinic recommends calling your healthcare provider at 39.4°C (103°F) or above.
- Above 40°C (104°F): Harvard Health advises calling a doctor at this point regardless of other symptoms.
- Above 41.5°C (106.7°F): This is hyperpyrexia, a medical emergency requiring immediate treatment.
Managing a Low-Grade Fever at Home
At 38°C, you don’t necessarily need medication. A low-grade fever is your immune system working, and letting it run its course can actually help your body clear infections faster. Focus on staying hydrated, resting, and wearing light clothing.
If the fever is making you miserable with chills, headache, or body aches, over-the-counter pain relievers can bring your temperature down and ease discomfort. Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) both work well for fever. For acetaminophen, the typical adult dose is 650 to 1,000 milligrams every four to six hours as needed, with a maximum of 3,000 to 4,000 milligrams in 24 hours depending on the product. Don’t exceed the limit on the label, as too much acetaminophen can cause serious liver damage.
When a Fever Needs Medical Attention
The temperature number alone doesn’t always tell the full story. A fever of 38°C in an otherwise healthy adult who has cold symptoms is rarely an emergency. But a fever at any level paired with certain symptoms needs prompt evaluation. These red flags include:
- Stiff neck, especially with pain when bending your head forward
- Confusion, unusual behavior, or altered speech
- Difficulty breathing or chest pain
- Persistent vomiting
- Severe headache or unusual sensitivity to bright light
- Pain when urinating or foul-smelling urine
- Seizures or loss of consciousness
- New rash
- Severe pain anywhere in the body
Also pay attention to duration. A low-grade fever lasting more than three days without improvement, or one that keeps coming back, is worth a call to your doctor even if none of the above symptoms are present.
Older Adults and Lower Baselines
Adults over 65 tend to run cooler baseline temperatures, which means a reading of 38°C can represent a more significant jump for them than for a younger adult. An older person with a temperature of 38°C and new confusion, fatigue, or loss of appetite may have a serious infection even though the number looks “barely a fever.” In this age group, a lower threshold for seeking medical advice is appropriate.