Convert between Celsius, Fahrenheit, Kelvin, and Rankine instantly. Shows all four scales at once with common reference points.
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Which Scale Should You Use?
Cooking and weather (US)
Fahrenheit is standard for US kitchens and weather reports — most US oven dials and recipes use °F.
Cooking and weather (most other countries)
Celsius is the global standard elsewhere — most ovens, weather reports, and body-temperature readings outside the US use °C.
Science and engineering
Kelvin is the SI base unit, used whenever a calculation involves absolute temperature (gas laws, thermodynamics) — it has no negative values, since 0K is absolute zero.
How to Use the Temperature Converter
Enter your temperature value — type any number including decimals and negative values.
Select the input unit — choose Celsius (°C), Fahrenheit (°F), or Kelvin (K).
Read all conversions — the result shows your temperature in all three scales simultaneously.
Use reference points — the converter shows common temperature benchmarks (water freezing/boiling, body temperature) for context.
Convert negative values — works for sub-zero temperatures, including temperatures below absolute zero detection (Kelvin cannot go below 0).
🌡️ Quick mental conversions: °C to °F: multiply by 1.8, add 32. °F to °C: subtract 32, divide by 1.8. A useful shortcut: double the Celsius temperature and add 30 gives an approximate Fahrenheit value (accurate within a few degrees for everyday temperatures).
Understanding Temperature Scales
🌡️ Celsius (°C)
Created by Anders Celsius in 1742. Based on water: 0°C = freezing point, 100°C = boiling point (at sea level). Used by virtually every country in the world for everyday temperature. Also called Centigrade (100 gradations between freezing and boiling). The standard scale for scientific work alongside Kelvin.
🌡️ Fahrenheit (°F)
Created by Daniel Fahrenheit in 1724. Originally based on brine (salt water) freezing (0°F) and human body temperature (96°F, later recalibrated to 98.6°F). Water freezes at 32°F, boils at 212°F. Used primarily in the United States, its territories, and a few other countries for everyday temperature.
🌡️ Kelvin (K)
The SI base unit of temperature. Absolute zero (0 K) = −273.15°C — the theoretical point where all molecular motion ceases. No degree symbol used (just K, not °K). Used exclusively in science and engineering. Conversion: K = °C + 273.15. Kelvin and Celsius degrees are the same size; Kelvin just starts at absolute zero.
📊 Conversion Formulas
°F = (°C × 9/5) + 32. °C = (°F − 32) × 5/9. K = °C + 273.15. °C = K − 273.15. °F = (K − 273.15) × 9/5 + 32. K = (°F − 32) × 5/9 + 273.15. The Celsius-Fahrenheit relationship is linear but has different starting points and scale factors.
🌡️ Reference Temperatures
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. Body temperature: 310 K / 37°C / 98.6°F. Water boils: 373.15 K / 100°C / 212°F. Sun surface: ~5778 K / ~5505°C / ~9941°F.
⚗️ Rankine Scale
A fourth temperature scale: Rankine (°R or °Ra). Like Kelvin, starts at absolute zero, but uses Fahrenheit-sized degrees. 0°R = absolute zero. Water freezes at 491.67°R, boils at 671.67°R. Used in some US engineering fields (thermodynamics, aerospace). °R = °F + 459.67. Rarely encountered outside specialised US engineering contexts.
Temperature in Science and Daily Life
Why the US uses Fahrenheit
The United States adopted the Fahrenheit scale before the metric system was established and never made the switch that most other countries completed in the 20th century. Fahrenheit has the psychological advantage of providing more granularity in the common human comfort range: 0–100°F (−18–38°C) roughly spans 'very cold' to 'very hot' in human experience — easy to intuitively grasp. The Celsius range for the same human experience is 0–40°C — requiring more decimal precision to describe small changes. Despite this intuitive advantage, Fahrenheit is isolated internationally, creating conversion needs constantly.
Temperature in cooking
Ovens in the US use Fahrenheit; ovens in most of the world use Celsius. Common conversions: 325°F = 163°C (slow oven). 350°F = 177°C (moderate oven — most cake recipes). 375°F = 191°C. 400°F = 204°C (hot oven — roasting). 425°F = 218°C. 450°F = 232°C (very hot — pizza). A simple rule: subtract 32 from °F and divide by 2 gives approximate °C (slightly high but useful for cooking where precision is less critical than in science).
Absolute zero and quantum physics
Absolute zero (0 K, −273.15°C) is the theoretical minimum temperature where all classical molecular motion stops. It is theoretically unreachable — as you approach it, the energy required to cool further approaches infinity. The coldest temperatures achieved by humans are within billionths of a degree above absolute zero, in physics laboratories studying quantum phenomena. At these temperatures, matter behaves in extraordinary ways: superconductivity (zero electrical resistance) and superfluidity (zero viscosity) emerge, enabling applications in MRI machines, quantum computers, and particle accelerators.
🌡️ Temperature conversion quick reference: −40°C = −40°F (the only point where both scales are equal). 0°C = 32°F. 20°C = 68°F. 37°C = 98.6°F. 100°C = 212°F. For Kelvin: add 273 to °C. Common mistake: confusing a temperature difference (ΔT) with an absolute temperature — a 1°C difference = 1.8°F difference, but 1°C absolute ≠ 1.8°F absolute.
Conversion Formulas
°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15 · °R = (°C + 273.15) × 9/5
Worked example
Converting 100°C (boiling water at sea level): °F = 100 × 9/5 + 32 = 212°F. K = 100 + 273.15 = 373.15 K. °R = 373.15 × 9/5 = 671.67°R.
Avoid These Mistakes
Confusing a difference with an absolute value
A change of 1°C equals a change of 1.8°F — but that's not the same as saying 1°C absolute equals 1.8°F absolute. Convert absolute readings with the full formula, not just a scaling factor.
Forgetting Kelvin has no negative values
0 K is absolute zero — there is no such thing as a negative Kelvin reading. If a calculation produces one, the input or formula has an error.
Mixing up Rankine with Kelvin
Both start at absolute zero, but Rankine uses Fahrenheit-sized degrees while Kelvin uses Celsius-sized degrees — they are not interchangeable without their own conversion.
Kelvin is the SI base unit for temperature, used in science and engineering. 0K is absolute zero — the coldest possible temperature. Kelvin = Celsius + 273.15. There are no negative Kelvin values.
What is normal body temperature?
37°C = 98.6°F = 310.15K. A temperature above 38°C (100.4°F) is generally considered a fever. Normal body temperature can vary between 36.1°C and 37.2°C depending on the person and time of day.
What temperature does water boil and freeze?
At sea level (1 atm pressure): water freezes at 0°C (32°F, 273.15K) and boils at 100°C (212°F, 373.15K). At higher altitudes, boiling point decreases because atmospheric pressure is lower.
References
🏛️ International System of Units (SI)
Kelvin is one of the seven SI base units, formally defined via the Boltzmann constant since the 2019 SI redefinition.
📄 NIST
The US National Institute of Standards and Technology publishes the official Celsius-Fahrenheit-Kelvin-Rankine conversion formulas and reference points used here.