Article — Celsius Converter
Celsius Converter: From °C to °F, K, and °R
A Celsius converter turns a temperature in °C into Fahrenheit, Kelvin, or Rankine using the exact 9/5 factor and the 273.15 offset. Celsius is the everyday metric scale for temperature, anchored at 0 °C (water freezing) and 100 °C (water boiling) at one atmosphere.
For everyday use it carries you through weather, cooking, body temperature, and refrigeration. For physics and chemistry the Kelvin column matters; for US recipes and HVAC equipment, Fahrenheit and Rankine matter.
What is a Celsius converter?
A Celsius converter takes a temperature in °C and returns the same physical state in another scale. The calculator above produces three outputs at once: Fahrenheit for US weather and cooking, Kelvin for physics and chemistry, and Rankine for US thermodynamics. None of the conversions involves measurement error — the relationships are exact by definition.
Quick-pick buttons cover the values most people search for: −40 °C (the only point where C and F agree), 0 °C (water freezing), 20 °C (room), 37 °C (body), 100 °C (water boiling), and 180 °C (oven baking). The calculator also flags values below absolute zero (−273.15 °C), which are physically impossible.
Anders Celsius's original 1742 scale ran backwards: 0 was the boiling point of water and 100 was the freezing point. The scale was inverted the next year by Carl Linnaeus, who saw the rising-number-equals-warming-temperature convention as more intuitive.
Celsius to Fahrenheit formula
Multiply Celsius by 1.8 (or 9/5) and add 32. So 20 °C times 1.8 is 36, plus 32 gives 68 °F. The reverse direction subtracts 32 first, then multiplies by 5/9.
For mental math at typical weather temperatures, double the Celsius value and add 30. 25 °C doubled is 50, plus 30 is 80 °F (the true value is 77 °F — about 3 degrees high). Good enough to decide on a jacket; not good enough for an oven dial.
Celsius to Kelvin conversion
Add 273.15. The kelvin uses the same interval size as Celsius but starts at absolute zero, so the conversion is a pure offset. 25 °C becomes 298.15 K. 0 °C becomes 273.15 K. The reverse direction subtracts the same constant.
Scientists prefer Kelvin because absolute temperature appears in every gas law, every radiation formula, and every statistical-mechanics equation. Using Celsius in the ideal gas law PV = nRT would let you "double the temperature" by going from 1 to 2 °C and predict a doubling of pressure — obviously wrong.
Celsius converter cheat sheet
°F = °C × 1.8 + 32 °C = (°F − 32) / 1.8K = °C + 273.15 °C = K − 273.15°R = (°C + 273.15) × 1.8 −40 °C = −40 °FFor temperature differences rather than absolute values, only the scale factor applies. A 10 °C rise is an 18 °F rise; the 32 offset cancels out when you subtract two readings. This matters for engineering specifications written in temperature deltas.
The Celsius scale: origin and modern definition
Anders Celsius proposed the 100-degree scale in 1742 while working at Uppsala University. The original calibration used water's freezing and boiling points at one atmosphere as the two anchors. Linnaeus reversed the direction in 1743 to give the modern orientation.
The modern definition is tied to the Kelvin via the SI. Since the 2019 revision, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10−23 J/K. Celsius is then defined as kelvin minus 273.15 exactly. The water-based calibration no longer defines the unit; it is now a measured property.
Celsius in cooking and medicine
Most published recipes outside the US use Celsius. Standard oven settings run 120 °C (very low) to 240 °C (extremely hot), with most baking happening at 180 °C. Convection ovens typically run 15 to 25 °C cooler than conventional ovens for the same browning result.
Medicine uses Celsius in most of the world. Normal body temperature is 36.1 to 37.2 °C (oral), with 37.0 °C as the textbook average. Hypothermia begins below 35 °C; medical-emergency fever crosses 40 °C. Some clinical labs use older Fahrenheit reference values; the numbers convert directly with the 9/5 + 32 formula.
Always convert the temperature, not just the number. A US recipe that calls for "350 degrees" means 350 °F = 177 °C, not 350 °C (which would be 662 °F and would carbonize most foods within minutes).
Common Celsius conversion mistakes
- Wrong order with Fahrenheit — multiplying first then subtracting 32 instead of subtracting then multiplying when going F to C.
- Using 1.8 in the wrong direction — 1.8 is correct for C to F; the reverse uses 5/9 (about 0.556).
- Forgetting the offset for Kelvin — "25 °C in Kelvin is 25" is wrong by 273. The right answer is 298.15 K.
- Below absolute zero — any input below −273.15 °C is physically impossible. The converter rejects it.
- Mixing differences with absolute values — a 10 °C difference is an 18 °F difference, not 50 °F.
Celsius reference temperatures
Standard reference points anchor the rest of the scale. Liquid nitrogen boils at −195.79 °C and is the standard cryogen for biology and superconductor work. Dry ice sublimates at −78.5 °C and is used to ship temperature-sensitive medical samples. Domestic freezers run at −18 °C, the FDA-recommended setpoint for long-term frozen storage. Water freezes at 0 °C and boils at 100 °C at sea level; at 1500 metres elevation it boils closer to 95 °C because atmospheric pressure is lower.
Comfortable indoor air sits between 18 and 24 °C for most adults. Body temperature is 37 °C. Hot summer weather climbs to 35 °C in temperate climates, with heat-wave warnings starting at 40 °C. Pizza ovens reach 450 °C, blast furnaces over 1200 °C, iron melts at 1538 °C, and the Sun's photosphere glows at about 5505 °C.
Two values matter most for everyday work: 37 °C (body temperature) and 100 °C (water boiling at sea level). Once those are anchored, the rest of the scale falls into place. The −40 crossover with Fahrenheit is also worth memorizing — it is the one point where the two scales agree exactly. Below that, Fahrenheit reads lower; above it, Fahrenheit reads higher.