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Temperature Converter (°C ↔ °F)

Convert temperature between Celsius and Fahrenheit and see whether a reading meets the clinical fever threshold.

How this calculator works

Enter a temperature in Celsius or Fahrenheit and pick the direction you want to convert. The calculator applies the standard linear Celsius/Fahrenheit formula and also checks the Fahrenheit-equivalent of your reading against the CDC’s clinical fever threshold of 100.4°F (38.0°C), regardless of which unit you entered.

The formula

F = C x 9/5 + 32

Inverse:
C = (F - 32) x 5/9

Fever flag:
isFever = fahrenheitEquivalent >= 100.4

Worked example

Converting 37°C to Fahrenheit:

  • Multiply by 9/5: 37 x 9/5 = 66.6
  • Add 32: 66.6 + 32 = 98.6°F
  • Fever check: 98.6°F is below the 100.4°F threshold, so this is not flagged as a fever.

Frequently asked questions (FAQ)

How do I convert Celsius to Fahrenheit by hand?

Multiply the Celsius value by 9/5, then add 32. For example, 37°C x 9/5 is 66.6, and 66.6 + 32 is 98.6°F, which is the commonly cited normal human body temperature.

Is 100°F a fever?

Not by the CDC's clinical definition. The CDC lists 100.4°F (38.0°C) as the fever threshold used for flu and other respiratory illness, so a 100°F reading is elevated but falls just under that line. This calculator flags 100.4°F and above as a fever.

Why does 38°C count as a fever threshold?

38.0°C converts exactly to 100.4°F, the threshold the CDC uses when describing flu symptoms and complications. This calculator checks the Fahrenheit-equivalent of whatever you enter, in either direction, against that same 100.4°F line.

How do I convert Fahrenheit to Celsius by hand?

Subtract 32 from the Fahrenheit value, then multiply by 5/9. For example, 100°F - 32 is 68, and 68 x 5/9 is about 37.8°C, which is below the 100.4°F fever threshold.

Why is normal body temperature listed as 98.6°F when mine reads differently?

98.6°F (37°C) is a long-used average, but normal body temperature varies by person, time of day, and measurement method by a degree or more in either direction. A single reading a bit above or below 98.6°F is not automatically a fever.

Does this calculator work for weather temperatures too, not just body temperature?

Yes. The Celsius/Fahrenheit formula is the same regardless of what is being measured, so you can use it for weather, cooking, or any other temperature reading. The fever-threshold note is only relevant when the value represents a body temperature.

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