Is it a fever?
Fever starts at 100.4°F, or 38°C. Enter the reading and where you measured; the checker corrects for method and tells you plainly what to do.
Armpit readings run about half a degree low; the checker corrects for where you measured.
Keep checking through an illness? Log it in the symptom diary.
Your number, answered
Measured by mouth. Armpit readings run about half a degree lower, so add 0.5 before reading this table.
| Celsius | Fahrenheit | What it means |
|---|---|---|
| 35.1 to 36.4 | 95.2 to 97.5 | Low side of normal. Common in the morning and in older adults. |
| 36.5 to 37.2 | 97.7 to 99.0 | Normal. |
| 37.3 to 37.9 | 99.1 to 100.3 | Low-grade. Not officially a fever. Recheck in an hour. |
| 38.0 to 39.9 | 100.4 to 103.9 | Fever. Rest, drink steadily, and watch the red flags below. |
| 40.0 and up | 104 and up | High fever. An adult needs care today. |
Look up an exact reading, 35.5 to 40.5°C
| °C | °F | Is it a fever? |
|---|---|---|
| 35.5 | 95.9 | Not a fever |
| 35.6 | 96.1 | Not a fever |
| 35.7 | 96.3 | Not a fever |
| 35.8 | 96.4 | Not a fever |
| 35.9 | 96.6 | Not a fever |
| 36.0 | 96.8 | Not a fever |
| 36.1 | 97.0 | Not a fever |
| 36.2 | 97.2 | Not a fever |
| 36.3 | 97.3 | Not a fever |
| 36.4 | 97.5 | Not a fever |
| 36.5 | 97.7 | Not a fever |
| 36.6 | 97.9 | Not a fever |
| 36.7 | 98.1 | Not a fever |
| 36.8 | 98.2 | Not a fever |
| 36.9 | 98.4 | Not a fever |
| 37.0 | 98.6 | Not a fever |
| 37.1 | 98.8 | Not a fever |
| 37.2 | 99.0 | Not a fever |
| 37.3 | 99.1 | Not a fever, low-grade |
| 37.4 | 99.3 | Not a fever, low-grade |
| 37.5 | 99.5 | Not a fever, low-grade |
| 37.6 | 99.7 | Not a fever, low-grade |
| 37.7 | 99.9 | Not a fever, low-grade |
| 37.8 | 100.0 | Not a fever, low-grade |
| 37.9 | 100.2 | Not a fever, low-grade |
| 38.0 | 100.4 | Fever |
| 38.1 | 100.6 | Fever |
| 38.2 | 100.8 | Fever |
| 38.3 | 100.9 | Fever |
| 38.4 | 101.1 | Fever |
| 38.5 | 101.3 | Fever |
| 38.6 | 101.5 | Fever |
| 38.7 | 101.7 | Fever |
| 38.8 | 101.8 | Fever |
| 38.9 | 102.0 | Fever |
| 39.0 | 102.2 | Fever, moderate to high |
| 39.1 | 102.4 | Fever, moderate to high |
| 39.2 | 102.6 | Fever, moderate to high |
| 39.3 | 102.7 | Fever, moderate to high |
| 39.4 | 102.9 | Fever, moderate to high |
| 39.5 | 103.1 | Fever, moderate to high |
| 39.6 | 103.3 | Fever, moderate to high |
| 39.7 | 103.5 | Fever, moderate to high |
| 39.8 | 103.6 | Fever, moderate to high |
| 39.9 | 103.8 | Fever, moderate to high |
| 40.0 | 104.0 | High fever |
| 40.1 | 104.2 | High fever |
| 40.2 | 104.4 | High fever |
| 40.3 | 104.5 | High fever |
| 40.4 | 104.7 | High fever |
| 40.5 | 104.9 | High fever |
- Below normal
- Under 35.0°C (95°F). If it stays there, that is hypothermia territory and needs care.
- Not a fever
- 36.5 to 37.2°C (97.7 to 99.0°F) is ordinary. Below 36.5 is the low side of normal, not a problem on its own.
- Not a fever, low-grade
- 37.3 to 37.9°C (99.1 to 100.3°F). Above your usual but not a fever. Recheck in an hour.
- Fever
- 38.0 to 38.9°C (100.4 to 102.0°F). Rest and fluids, and watch the red flags above.
- Fever, moderate to high
- 39.0 to 39.9°C (102.2 to 103.8°F). Rest and fluids, and a same-day call if it is not settling.
- High fever
- 40.0°C (104°F) and above. An adult needs care today.
One exception outranks the whole table: a baby under 3 months at 38°C (100.4°F) needs a doctor today.
Where 100.4 comes from
The line itself is an agreement, not a discovery. 38.0°C, which is 100.4°F, is the number medicine settled on so that every clinic, study and guideline means the same thing by the word fever.
The most consequential place it appears is the rule for young babies. The American Academy of Pediatrics defines fever in an infant 8 to 60 days old as a rectal reading at or above 38.0°C, and builds the whole workup around it. That is why the under-3-months line outranks everything else on this page. AAP clinical practice guideline, Pediatrics, 2021.
The half nobody quotes: 98.6 is out of date. Across more than 670,000 readings from three US groups spanning 1862 to 2017, body temperature has been falling by about 0.05°F every birth decade. Men born in the 2000s ran roughly 1°F cooler than men born in the early 1800s, and the modern average sits nearer 97.9°F.
So 100.4 is further above your normal than the old textbook number suggests, and your own usual reading tells you more than the line does. Protsiv et al, eLife, 2020.
A threshold is a convention, not a cliff. Nothing in the body changes between 100.3 and 100.4. How someone looks, how long it has run and which way it is heading all matter more than one reading. The one exception is a baby under 3 months, where the number alone is enough to go in.
Every figure on this site names where it came from. How we check.
Common questions
What temperature is considered a fever?
100.4 F (38 C) and above is a fever, measured orally. Readings from 99.5 to 100.3 F are low-grade and often just the body warming through the day, after exercise, or early in an infection. Normal is a range: most healthy adults run 97 to 99 F, lowest in the morning.
Is 99.5 a fever?
Not officially. It is low-grade territory, worth rechecking after a few quiet minutes, since a hot drink, a warm room, exercise or simply the afternoon can put a healthy person there. If it keeps climbing past 100.4 F, it is a fever.
Is 37.6 a fever?
Not yet. 37.6 C (99.7 F) is low-grade: above the usual range but under the 38 C line where fever officially starts. Recheck in an hour, sitting quietly. If it climbs to 38 C or you feel properly unwell, treat it as the start of one.
Is 37.2 a fever in adults?
No. 37.2 C (99 F) sits at the top of the normal range, and plenty of healthy adults reach it in the late afternoon, after a meal, or after walking. It only deserves a recheck, not concern.
What is a normal body temperature?
A range, not one number: most healthy adults run 36.1 to 37.2 C (97 to 99 F) by mouth, lowest in the early morning and highest in the late afternoon. Older adults often run a little cooler, which is why a modest rise in a grandparent can matter more than the same number in a teenager.
Is 36.2 a normal temperature?
Yes. 36.2 C (97.2 F) sits comfortably inside the normal range, especially in the morning, in older adults, and in people who simply run cool. It is nowhere near a fever, and it is not worryingly low either; doctors only start thinking about low body temperature below about 35 C. 36.5 is equally unremarkable.
Is 37.8 or 37.9 a fever?
Almost. Both sit at the very top of low-grade, a whisker under the official 38 C (100.4 F) line. Practically, a reading of 37.8 or 37.9 in someone who feels unwell is treated as the start of a fever: rest, fluids, and a recheck in an hour. In someone who feels fine after exercise or a hot afternoon, recheck first.
When is a fever dangerous?
For adults: 104 F (40 C) and above, a fever lasting beyond 3 days, or any fever with a stiff neck, confusion, a rash that does not fade under a glass, trouble breathing, or severe headache; those need same-day care. For a baby under 3 months, 100.4 F is itself the emergency line. Anyone on chemotherapy with a fever needs urgent care regardless of the number.
Should I treat a fever or let it run?
Fever is the immune system working, and treating it is for comfort, not cure. If you feel miserable, acetaminophen or ibuprofen at package doses helps. If you feel tolerable, resting and drinking steadily is a fine choice. What matters is the pattern and the red flags, not forcing the number down.
Do forehead and armpit thermometers read differently?
Yes. Armpit readings run about half a degree F lower than oral; rectal readings run about half a degree higher and are the standard for babies. Forehead scanners are convenient but drift with sweat and technique. Whichever you use, use the same one through an illness so the trend is honest.