Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin, and Rankine.
About the Temperature Converter
The temperature converter translates between the four major temperature scales: Celsius, Fahrenheit, Kelvin, and Rankine. Unlike most other unit conversions, temperature conversion involves both a scaling factor and an offset, because the four scales place zero at different points.
Celsius (originally called centigrade) places zero at the freezing point of water and 100 at the boiling point, both at standard atmospheric pressure. Anders Celsius proposed the scale in 1742 with the reverse orientation (100 at freezing, 0 at boiling); the modern orientation was set after his death. The scale was officially renamed Celsius in 1948 by the 9th CGPM.
Fahrenheit places 32 at the freezing point of water and 212 at the boiling point. Daniel Gabriel Fahrenheit defined it in 1724 using a mixture of ice, water, and ammonium chloride as the zero point and human body temperature as 96 (later refined to 98.6). The scale remains in everyday use in the United States for non-scientific purposes.
Kelvin is the SI base unit of thermodynamic temperature. It places zero at absolute zero (the point of zero thermal motion) and uses the same degree size as Celsius, so 0°C = 273.15 K. Per the 2019 SI redefinition, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 * 10^-23 J/K. Rankine uses the same zero as Kelvin but the same degree size as Fahrenheit, so it is to Fahrenheit what Kelvin is to Celsius.
How It Works
The converter uses Celsius as the pivot, with two helper functions:
toCelsius(v, from):
C <- v (if from is C)
C <- (v - 32) * 5/9 (if from is F)
C <- v - 273.15 (if from is K)
C <- (v - 491.67) * 5/9 (if from is R)
fromCelsius(c, to):
v <- c (if to is C)
v <- c * 9/5 + 32 (if to is F)
v <- c + 273.15 (if to is K)
v <- (c + 273.15) * 9/5 (if to is R)The key formulas are the Celsius-to-Fahrenheit and Celsius-to-Kelvin conversions:
F = C * 9/5 + 32 K = C + 273.15
C = (F - 32) * 5/9 C = K - 273.15The Rankine scale is to Fahrenheit what Kelvin is to Celsius: same zero point (absolute zero), same degree size. The conversion is R = (C + 273.15) * 9/5. Absolute zero is -273.15°C, -459.67°F, 0 K, and 0°R.
The converter rejects inputs below absolute zero, since the third law of thermodynamics forbids temperatures below this point. The check uses the Celsius equivalent: if any input converts to a Celsius value below -273.15, the tool reports an error. Note that negative inputs are valid in Celsius and Fahrenheit (winter temperatures below 0°C or 0°F are common), but the equivalent Kelvin value must remain non-negative.
Worked Examples
Converting 25°C to Fahrenheit: the result is 25 * 9/5 + 32 = 45 + 32 = 77°F. This is a comfortable room temperature, often cited as the standard for indoor settings.
Converting 100°C to Kelvin: the result is 100 + 273.15 = 373.15 K. This is the boiling point of water at standard atmospheric pressure, which the Kelvin scale expresses as 373.15 K rather than as a degree value (the kelvin is not a degree).
Converting 98.6°F to Celsius: the result is (98.6 - 32) * 5/9 = 66.6 * 5/9 = 37°C. This is normal human body temperature. Converting 0 K to Fahrenheit gives -459.67°F, the absolute zero of temperature, which the tool accepts as valid input. Converting -500°C to Fahrenheit is rejected, since the equivalent Kelvin value of -226.85 K is below absolute zero.
When to Use This Tool
Use the temperature converter when you need to:
- Convert cooking temperatures in international recipes (a 350°F oven is about 180°C).
- Interpret scientific or engineering data reported in Kelvin.
- Work with thermodynamic calculations that use absolute temperature.
- Convert body temperature readings between Celsius and Fahrenheit.
- Translate HVAC or refrigeration specifications between scales.
- Convert weather forecasts between Celsius and Fahrenheit for travel.
- Teach the relationships between the four temperature scales.
Limitations & Disclaimer
This converter uses the standard linear conversion formulas between Celsius, Fahrenheit, Kelvin, and Rankine. It does not handle the historical Reaumur or Delisle scales. Inputs below absolute zero (-273.15°C, -459.67°F, 0 K, 0°R) are rejected as physically impossible per the third law of thermodynamics. Results are subject to IEEE 754 floating-point precision. See our disclaimer for full details.
Frequently Asked Questions
Why does temperature conversion involve addition and subtraction, not just multiplication?
Because the four temperature scales have different zero points. Celsius places zero at the freezing point of water, Fahrenheit places it at the freezing point of brine, and Kelvin and Rankine place it at absolute zero. Converting between scales requires both a scaling factor (for the degree size) and an offset (for the zero point).
What is absolute zero and why does it matter?
Absolute zero is the theoretical lowest possible temperature, at which all thermal motion ceases. It is -273.15°C, -459.67°F, 0 K, and 0°R. The third law of thermodynamics forbids temperatures below this point. The converter reports any input below absolute zero as physically impossible.
Why is the Kelvin scale the SI base unit of temperature?
Because it places zero at absolute zero, a physically meaningful reference point, and it uses the same degree size as Celsius, which makes it compatible with everyday measurements. Per the 2019 SI redefinition, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 * 10^-23 J/K.
What is the difference between Celsius and centigrade?
They are the same scale. The name 'centigrade' was used because the scale has 100 degrees between the freezing and boiling points of water. The 9th CGPM in 1948 officially renamed it Celsius to honour Anders Celsius and to align with the SI convention of naming units after people.
Why is Celsius not written with a degree symbol but Fahrenheit is?
Actually both are written with a degree symbol (°C and °F). Kelvin is the only temperature scale that does not use the degree symbol, because the kelvin is an SI base unit and SI does not use degree symbols for base units. So we write 25°C or 25°F but 298.15 K.
What is the Rankine scale used for?
Rankine is to Fahrenheit what Kelvin is to Celsius: it uses absolute zero as its zero point but keeps the Fahrenheit degree size. It is used in some engineering contexts in the United States, particularly in thermodynamics and aerospace engineering, where English units are still common. Most other fields use Kelvin.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team