Guide
Fever temperature: Celsius vs Fahrenheit, explained
A US thermometer says 100.4°F, a European one says 38°C. Same fever, different number — here's the map between them.
By Buğra SözeriPublished
“You have a fever” means a different number depending on which thermometer you're holding. In the United States that number is usually 100.4°F; almost everywhere else it’s 38.0°C. They're the same temperature — the confusion is purely about scale, not biology — but it trips people up constantly, especially parents reading an imported thermometer or travelers checking symptoms abroad. This guide lays out the actual thresholds on both scales, why “normal” isn't one fixed number, and how to read a reading correctly regardless of which unit your thermometer speaks.
The fever threshold, side by side
Clinical sources generally define an adult fever as an oral temperature at or above the following, which is the same cutoff expressed on both scales:
| Reading | Celsius | Fahrenheit |
|---|---|---|
| Typical “normal” average | 36.4–37.0°C | 97.5–98.6°F |
| Low-grade fever | 37.2–38.0°C | 99.0–100.4°F |
| Fever (standard threshold) | 38.0°C | 100.4°F |
| High fever — seek care | 39.4°C+ | 103°F+ |
For anything in between, the conversion is exact and mechanical: °F = °C × 9/5 + 32, and the reverse is °C = (°F − 32) × 5/9. Our Celsius to Fahrenheit converter does this instantly if you'd rather not do the arithmetic while someone in the house is sick.
Why “98.6°F” isn't a hard target
98.6°F (37.0°C) is the number most people were taught as “normal,” and it comes from measurements a German physician published in the 1850s. Later, larger studies have found the real average running closer to 97.5°F (36.4°C), and that individual normal temperature varies by roughly a degree Fahrenheit from person to person, plus a daily rhythm — lower in early morning, higher in late afternoon. That means a reading of 99.0°F isn't automatically a fever for everyone; what matters clinically is less the single number and more whether it's meaningfully above your own usual baseline, plus how you actually feel.
Measurement site changes the number too
Where you take the temperature shifts the reading independent of the Celsius/Fahrenheit question. Rectal and ear (tympanic) readings tend to run about 0.5–1°F (0.3–0.6°C) higher than oral readings, and armpit (axillary) readings tend to run about 0.5–1°F lower. A thermometer's instructions usually state which site its displayed thresholds assume — mixing sites without adjusting is a more common source of false alarm than the unit conversion itself.
When to seek care
General guidance from health authorities such as the CDC treats a persistent fever, a very high reading (103°F / 39.4°C or above in adults), or a fever accompanied by symptoms like stiff neck, difficulty breathing, or confusion as reasons to contact a healthcare provider rather than waiting it out. For infants and young children the thresholds are stricter and age-dependent — a pediatrician's specific guidance, not this table, is the right source there. This guide is educational only, not medical advice.
If you're comparing this to how the two scales behave outside the body — weather, ovens, freezers — see our Celsius vs Fahrenheit comparison for the broader picture of where each scale is used and why.
Frequently asked questions
- What temperature counts as a fever in Celsius?
- Most clinical guidance treats a temperature at or above 38.0°C (100.4°F) taken orally as a fever in adults. Normal body temperature isn't a single fixed number — it varies by person, time of day, and measurement site — but 38.0°C is the commonly used threshold that separates normal variation from a true fever.
- What temperature counts as a fever in Fahrenheit?
- 100.4°F is the standard adult threshold used by most US clinical sources, equivalent to 38.0°C. A reading of 103°F (39.4°C) or higher, or a fever lasting more than a couple of days, is generally when guidance recommends contacting a healthcare provider.
- Is 98.6°F still considered normal body temperature?
- 98.6°F (37.0°C) is the traditional average, based on 19th-century measurements, but modern studies find the real population average runs slightly lower, around 97.5°F (36.4°C), and normal varies by roughly a full degree Fahrenheit between individuals and across the day. A single reading a bit above or below 98.6°F isn't automatically abnormal — what matters more is a change from your own baseline.
- Why do fever thermometers show different scales in different countries?
- It's the same split as everyday weather reporting: the US and a handful of other countries use Fahrenheit, while most of the world, including most medical literature outside the US, uses Celsius. A thermometer bought in the US typically defaults to °F; one bought almost anywhere else defaults to °C. Many digital thermometers can toggle between the two in their settings.
Sources & references
Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.
- CDC — Flu Symptoms & Complications (fever thresholds) — CDC's plain-language guidance on fever as a flu symptom(as of )
- MedlinePlus (NIH) — Fever — NIH consumer-health reference defining fever thresholds and when to seek care(as of )
- WHO — Thermometers and body temperature measurement guidance — WHO publications index for clinical measurement standards(as of )
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Published September 25, 2026