Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin and Rankine instantly.
The four temperature scales explained
Unlike length or weight, temperature conversions are not simple multiplications — they involve both scaling and shifting because each scale uses a different zero point. Understanding why each scale was designed the way it was makes the formulas much easier to remember.
Celsius (°C)
Proposed by Swedish astronomer Anders Celsius in 1742, the Celsius scale anchors its two reference points to water: 0°C is the freezing point and 100°C is the boiling point at sea level (standard atmospheric pressure). It is the most widely used temperature scale in the world, adopted by nearly every country for everyday and scientific use.
Because water's phase transitions are universally reproducible, Celsius provides intuitive reference points: below 0 is freezing, body temperature is 37°C, a hot summer day is around 35°C, and a comfortable indoor temperature is 20–22°C.
Fahrenheit (°F)
Developed by German physicist Daniel Gabriel Fahrenheit in 1724, the Fahrenheit scale originally set 0°F to the coldest temperature he could reliably reproduce (a brine ice mixture) and 96°F to human body temperature. This resulted in water freezing at 32°F and boiling at 212°F — exactly 180 degrees apart, a number divisible by many factors.
The US, Belize, and a handful of Caribbean nations still use Fahrenheit for everyday temperature. Scientists and engineers in the US use Celsius or Kelvin. The Fahrenheit scale offers finer granularity between common outdoor temperatures (roughly −20°F to 100°F), which some argue makes it more intuitive for weather reporting.
Kelvin (K)
Kelvin is the SI base unit of temperature and the standard in scientific work. Unlike Celsius and Fahrenheit, Kelvin has no negative values — it starts at absolute zero (0 K = −273.15°C), the theoretical point at which all molecular motion stops. One kelvin is the same size as one degree Celsius; only the zero point differs.
The Kelvin scale is essential in physics, chemistry, and astronomy. Gas laws (Boyle's law, Charles's law) require temperatures in Kelvin. The cosmic microwave background radiation sits at about 2.7 K. The surface of the Sun is approximately 5,778 K.
Rankine (°R)
Rankine is to Fahrenheit what Kelvin is to Celsius — an absolute scale with the same degree size as Fahrenheit, but starting at absolute zero (0°R = −459.67°F). It is used in some US engineering applications, particularly in thermodynamics and aerospace, where absolute temperatures are required but the Fahrenheit degree size is preferred. Outside these specialist fields, Rankine is rarely encountered.
Conversion formulas
| From → To | Formula |
|---|---|
| °C → °F | (C × 9/5) + 32 |
| °F → °C | (F − 32) × 5/9 |
| °C → K | C + 273.15 |
| K → °C | K − 273.15 |
| °F → K | (F − 32) × 5/9 + 273.15 |
| K → °F | (K − 273.15) × 9/5 + 32 |
| °C → °R | (C + 273.15) × 9/5 |
| °R → °C | (R × 5/9) − 273.15 |
Useful temperature reference points
Absolute zero: 0 K = −273.15°C = −459.67°F
Water freezes: 273.15 K = 0°C = 32°F
Room temperature: ~293 K = 20°C = 68°F
Human body: ~310 K = 37°C = 98.6°F
Water boils: 373.15 K = 100°C = 212°F
Sun's surface: ~5778 K = ~5505°C = ~9941°FCommon real-world uses
Tips for quick mental conversions
- °C to °F fast approximation: Double the Celsius value and add 30. This gives a rough result (e.g. 20°C → 20×2+30 = 70°F; exact is 68°F). For accurate work, always use the full formula.
- Body temperature check: Normal body temperature is 37°C = 98.6°F. A fever above 38°C (100.4°F) is medically significant. Knowing these anchors helps you quickly judge whether a reported temperature is normal.
- Oven temperatures: 180°C (conventional oven) = 356°F ≈ 350°F (commonly rounded in US recipes). 200°C = 392°F ≈ 400°F. Always verify: the 20°F rounding in baking can affect results for sensitive recipes.
- Kelvin has no degree symbol. You write 300 K, never 300°K. This is a common typographical mistake in student work and informal writing. In formal scientific documents, the symbol matters.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 and add 32. For example, 20°C × 9/5 + 32 = 68°F.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For example, (98.6°F − 32) × 5/9 = 37°C.
What is Kelvin and how does it relate to Celsius?
Kelvin is the scientific temperature scale starting at absolute zero. To convert, add 273.15 to Celsius — so 0°C equals 273.15 K.
At what temperature are Celsius and Fahrenheit equal?
At −40 degrees. −40°C is exactly −40°F, the only point where the two scales meet.
Converts temperature between Celsius, Fahrenheit, Kelvin and Rankine in real time. All four values update simultaneously as you type.
°F↔°C: (F−32)×5/9 | °C→K: C+273.15 | Absolute zero = −273.15°C = 0K