Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin, and Rankine. Unlike most unit conversions, temperature uses offset formulas — not simple multiplication — so each conversion is calculated precisely.
| Unit | Value |
|---|---|
| Celsius (°C) | 100 |
| Fahrenheit (°F) | 100 |
| Kelvin (K) | 100 |
| Rankine (°R) | 100 |
Temperature Conversion Formulas
Temperature scales differ in two ways: where they set their zero point and how large one degree is. Because of the zero-point difference, temperature conversions involve addition or subtraction in addition to multiplication — unlike length or weight conversions, which are pure ratios.
| From | To | Formula |
|---|---|---|
| Celsius (C) | Fahrenheit (F) | F = C × 9/5 + 32 |
| Fahrenheit (F) | Celsius (C) | C = (F − 32) × 5/9 |
| Celsius (C) | Kelvin (K) | K = C + 273.15 |
| Kelvin (K) | Celsius (C) | C = K − 273.15 |
| Celsius (C) | Rankine (°R) | R = (C + 273.15) × 9/5 |
| Fahrenheit (F) | Kelvin (K) | K = (F − 32) × 5/9 + 273.15 |
Common Temperature Reference Points
Memorizing a handful of reference temperatures lets you sanity-check conversions quickly and estimate when you do not have a calculator at hand.
| Description | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Freezing point of water | 0 | 32 | 273.15 |
| Comfortable room temperature | 20–22 | 68–72 | 293–295 |
| Normal human body temperature | 37 | 98.6 | 310.15 |
| Boiling point of water (sea level) | 100 | 212 | 373.15 |
| Celsius = Fahrenheit crossover | −40 | −40 | 233.15 |
Understanding the Temperature Scales
Celsius (°C)
Developed by Swedish astronomer Anders Celsius in 1742. The scale is defined by two fixed points: the freezing point of water (0°C) and the boiling point of water (100°C) at standard atmospheric pressure. Celsius is the standard scale for everyday use in most of the world and in all scientific contexts where Kelvin is not required.
Fahrenheit (°F)
Proposed by German physicist Daniel Gabriel Fahrenheit in 1724. Originally calibrated to the coldest temperature Fahrenheit could reproduce in the lab (0°F) and human body temperature (96°F, later adjusted to 98.6°F). Water freezes at 32°F and boils at 212°F. Used today primarily in the United States for weather forecasts, cooking, and everyday temperature references.
Kelvin (K)
Named after Lord Kelvin (William Thomson), the Kelvin scale is the SI base unit for temperature. It starts at absolute zero (the lowest possible temperature, where molecular motion ceases) and uses the same degree size as Celsius. There are no negative Kelvin values. Kelvin is essential in physics, chemistry, and astronomy where absolute temperature matters (e.g., ideal gas law: PV = nRT requires T in Kelvin).
Rankine (°R)
The Rankine scale is an absolute scale using Fahrenheit degree size. Like Kelvin for Celsius, Rankine starts at absolute zero but advances in Fahrenheit-sized steps. Primarily used in certain US engineering disciplines (thermodynamics, aerospace). 0 Rankine = 0 Kelvin = −273.15°C = −459.67°F.
Frequently Asked Questions
How do I convert Celsius to Fahrenheit?
Use the formula F = (C × 9/5) + 32. Example: 100°C = (100 × 9/5) + 32 = 180 + 32 = 212°F. To go the other way: C = (F − 32) × 5/9.
What is absolute zero in Celsius and Fahrenheit?
Absolute zero — the coldest possible temperature — is 0 Kelvin = −273.15°C = −459.67°F. At this temperature, all molecular motion stops.
What is the formula to convert Celsius to Kelvin?
K = °C + 273.15. The Kelvin scale uses the same degree size as Celsius but starts from absolute zero instead of the freezing point of water. Example: 25°C = 25 + 273.15 = 298.15 K.
At what temperature are Celsius and Fahrenheit equal?
Celsius and Fahrenheit are equal at −40°. That is, −40°C = −40°F. You can verify: F = (−40 × 9/5) + 32 = −72 + 32 = −40°F.
What is the Rankine scale?
Rankine is an absolute temperature scale based on Fahrenheit degrees. 0 Rankine = absolute zero, and 1 degree Rankine = 1 degree Fahrenheit. The freezing point of water is 491.67°R and boiling point is 671.67°R. It is used in some engineering contexts in the United States.
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