What is Celsius?
Celsius (symbol: °C) is a temperature scale widely used for everyday measurements worldwide. Named after Swedish astronomer Anders Celsius, who proposed it in 1742, the scale is based on the behavior of water at standard atmospheric pressure [4].
The Celsius scale defines 0°C as the freezing point of water and 100°C as the boiling point, creating a practical 100-degree interval. It's the standard scale for weather forecasts, cooking, medical applications, and most daily temperature measurements outside of scientific contexts.
What is Kelvin?
Kelvin (symbol: K, not °K) is the base unit of temperature in the International System of Units (SI), as defined by the Bureau International des Poids et Mesures [2]. Named after Lord Kelvin (William Thomson), who proposed the absolute temperature scale in 1848, Kelvin starts at absolute zero—the theoretical point where all molecular motion ceases.
Unlike Celsius or Fahrenheit, Kelvin is an absolute temperature scale with no negative values. It's essential in physics, chemistry, astronomy, and thermodynamics because it directly relates to the kinetic energy of particles. One Kelvin increment equals one Celsius degree, making conversions straightforward.
Celsius to Kelvin Conversion Formula
The formula to convert Celsius to Kelvin is simple and direct:
This formula adds 273.15 to the Celsius temperature because absolute zero (0 K) equals -273.15°C. The 273.15 offset represents the difference between the Celsius scale's zero point (water's freezing point) and absolute zero, as established by the NIST Temperature Reference [1].
For most practical purposes, you can round 273.15 to 273, but scientific calculations require the precise value of 273.15 for accuracy.
What is 1°C in Kelvin?
One degree Celsius equals 274.15 Kelvin. Using the formula:
This means that 1°C, just above water's freezing point, corresponds to 274.15 K on the absolute temperature scale. The small increment from 273.15 K (0°C) to 274.15 K (1°C) represents the same temperature change in both scales.
Is 1 Kelvin 273 Celsius?
No, this is a common misconception. 1 Kelvin is not 273°C. Let's clarify the relationship:
1 Kelvin equals -272.15°C. Using the reverse formula:
What people often confuse is that 0°C equals 273.15 K, not the other way around. The number 273.15 is the offset between the scales, not a direct conversion factor.
Why do we add 273 to convert Celsius to Kelvin?
We add 273.15 (often rounded to 273) because of the fundamental difference in how the two scales define their zero points:
Celsius zero (0°C) is based on water's freezing point—a practical reference for everyday life. Kelvin zero (0 K) is absolute zero—the theoretical point where all molecular motion stops and no more heat can be removed from a system.
The difference between these two zero points is exactly 273.15 degrees. Absolute zero occurs at -273.15°C, so to convert from Celsius to Kelvin, we must add this offset to shift from the relative Celsius scale to the absolute Kelvin scale.
How to Convert Celsius to Kelvin (Step-by-Step)
Converting Celsius to Kelvin follows a simple two-step process:
Step-by-Step Conversion Process
- Start with the temperature in Celsius that you want to convert
- Add 273.15 to that value
- The result is the temperature in Kelvin
Worked Examples of Celsius to Kelvin Temperature Conversion
- Start with: 25°C (typical room temperature)
- Add 273.15: 25 + 273.15 = 298.15
- Start with: 100°C (water boils at sea level)
- Add 273.15: 100 + 273.15 = 373.15
- Start with: -273.15°C (absolute zero)
- Add 273.15: -273.15 + 273.15 = 0
- Start with: -196°C (boiling point of liquid nitrogen)
- Add 273.15: -196 + 273.15 = 77.15
- Start with: 5,505°C (surface temperature of the Sun)
- Add 273.15: 5,505 + 273.15 = 5,778.15
For the reverse conversion, see our Kelvin to Celsius converter.
Celsius to Kelvin Conversion Table
Use this comprehensive table for quick reference. All values calculated with the exact formula K = °C + 273.15.
| Celsius (°C) | Kelvin (K) |
|---|---|
| -273.15°C | 0.0000 K |
| -250.00°C | 23.1500 K |
| -200.00°C | 73.1500 K |
| -196.00°C | 77.1500 K |
| -150.00°C | 123.1500 K |
| -100.00°C | 173.1500 K |
| -50.00°C | 223.1500 K |
| -40.00°C | 233.1500 K |
| -20.00°C | 253.1500 K |
| 0.00°C | 273.1500 K |
| 4.00°C | 277.1500 K |
| 10.00°C | 283.1500 K |
| 20.00°C | 293.1500 K |
| 25.00°C | 298.1500 K |
| 37.00°C | 310.1500 K |
| 50.00°C | 323.1500 K |
| 100.00°C | 373.1500 K |
| 200.00°C | 473.1500 K |
| 500.00°C | 773.1500 K |
| 1,000.00°C | 1,273.1500 K |
| 2,000.00°C | 2,273.1500 K |
| 5,505.00°C | 5,778.1500 K |
Why Do I Need to Convert Celsius to Kelvin?
Converting Celsius to Kelvin is essential in various scientific and technical contexts:
Scientific Research
Physics, chemistry, and astronomy require Kelvin for thermodynamic calculations, gas laws (PV=nRT), and energy calculations. Kelvin's absolute nature makes mathematical relationships more straightforward.
Engineering Applications
Thermodynamics, heat transfer, and HVAC systems use Kelvin for efficiency calculations, Carnot cycle analysis, and thermal engineering design, as detailed in the Engineering Toolbox [6].
Cryogenics
Working with extremely low temperatures (liquid nitrogen, liquid helium) requires Kelvin to accurately describe conditions near absolute zero.
Astronomy
Stellar temperatures, cosmic microwave background radiation, and planetary atmospheres are measured in Kelvin because they span enormous temperature ranges.
International Standards
Scientific publications, international standards (ISO, SI), and technical documentation require Kelvin as the official temperature unit.
Common Temperature Reference Points
Understanding key temperature reference points across all three scales helps develop intuition for scientific applications:
| Reference Point | Celsius (°C) | Kelvin (K) | Fahrenheit (°F) |
|---|---|---|---|
| Absolute zero | -273.15°C | 0 K | -459.67°F |
| Liquid nitrogen boiling point | -196.00°C | 77.15 K | -320.80°F |
| Antarctic record low | -89.20°C | 183.95 K | -128.56°F |
| Water freezes | 0.00°C | 273.15 K | 32.00°F |
| Human body temperature | 37.00°C | 310.15 K | 98.60°F |
| Water boils (sea level) | 100.00°C | 373.15 K | 212.00°F |
| Oven baking temperature | 180.00°C | 453.15 K | 356.00°F |
| Iron melting point | 1,538.00°C | 1,811.15 K | 2,800.40°F |
| Sun's surface | 5,505.00°C | 5,778.15 K | 9,941.00°F |
How to Read a Thermometer in C and K
Most consumer thermometers display Celsius or Fahrenheit, but scientific thermometers often show Kelvin:
Digital Scientific Thermometers
Laboratory thermometers typically display Kelvin with high precision (often to 0.01 K). They may have a mode switch between K, °C, and °F. Look for the unit indicator on the display.
Analog Laboratory Thermometers
Some analog thermometers show dual scales (°C and K). Read the value where the liquid column aligns with the appropriate scale. Kelvin scales start at 273.15 for water's freezing point.
Infrared Thermometers
Industrial infrared thermometers often display Kelvin for high-temperature applications (furnaces, kilns). Check the settings menu to switch between units if needed.
Celsius vs Fahrenheit vs Kelvin
Understanding the differences between these three temperature scales helps you choose the right one for your application:
| Characteristic | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|---|
| Primary use | Everyday worldwide | USA, some territories | Scientific worldwide |
| Zero point | Water freezes (0°C) | Brine solution (0°F) | Absolute zero (0 K) |
| 100° point | Water boils (100°C) | Not a standard reference | Not a standard reference |
| Water freezes | 0°C | 32°F | 273.15 K |
| Water boils | 100°C | 212°F | 373.15 K |
| Absolute zero | -273.15°C | -459.67°F | 0 K |
| Degree size | 1°C = 1 K | 1°F = 0.56°C | 1 K = 1°C |
| Negative values? | Yes | Yes | No (absolute scale) |
| Degree symbol? | Yes (°C) | Yes (°F) | No (just K) |
| Named after | Anders Celsius (1742) | Daniel Fahrenheit (1724) | Lord Kelvin (1848) |
When to Use Each Scale
Use Celsius for everyday temperature measurements, weather, cooking, and medical applications in most countries. Use Fahrenheit for daily life in the USA, reading American weather forecasts, and following American recipes. Use Kelvin for scientific research, thermodynamics, physics calculations, astronomy, and any context requiring absolute temperature measurements.
History of Celsius
The Celsius scale was proposed in 1742 by Swedish astronomer Anders Celsius (1701-1744). Interestingly, his original scale was inverted: 0° was the boiling point of water, and 100° was the freezing point. He chose this arrangement to avoid negative numbers during Swedish winters, as documented in his original paper [4].
Shortly after Celsius's death in 1744, the scale was reversed to its current form by Carl Linnaeus (the famous taxonomist) and Martin Strömer, Celsius's successor at Uppsala University. This arrangement was more intuitive because temperature numbers increase as things heat up. In 1948, an international conference officially renamed the scale "Celsius" to honor its creator and avoid confusion with "centigrade," which was also used for angular measurement.
History of Kelvin (Lord Kelvin, 1848)
The Kelvin scale was proposed in 1848 by British physicist William Thomson, later ennobled as Lord Kelvin (1824-1907). He recognized the need for an absolute temperature scale that started at the theoretical point where all molecular motion ceases—absolute zero, as outlined in his landmark paper [3].
Lord Kelvin based his scale on the Carnot cycle and thermodynamic principles, establishing that absolute zero was -273.15°C. The scale was originally called "absolute temperature" but was renamed "Kelvin" in his honor in 1968 by the International Committee for Weights and Measures. Lord Kelvin's contributions to thermodynamics, electrical engineering, and the transatlantic telegraph cable made him one of the most influential scientists of the 19th century. For a comprehensive overview, see the Kelvin Wikipedia article [7].
Frequently Asked Questions
Can Kelvin be negative?
No, Kelvin cannot be negative. The Kelvin scale starts at absolute zero (0 K), the theoretical point where all molecular motion stops. You cannot have a temperature lower than absolute zero, so Kelvin values are always zero or positive. This is why Kelvin is called an "absolute" temperature scale.
Why is it called Kelvin and not degrees Kelvin?
According to the International System of Units (SI), the unit is called "Kelvin" (symbol: K), not "degrees Kelvin." The correct usage is "273 Kelvin" or "273 K," never "273 degrees Kelvin" or "273°K." This naming convention distinguishes it from relative scales like Celsius and Fahrenheit, which use the degree symbol.
Is 0 K the same as -273.15°C?
Yes, exactly. Absolute zero (0 K) equals -273.15°C. This is the theoretical temperature where all molecular motion ceases and no more heat can be removed from a system. Scientists have approached within billionths of a Kelvin above absolute zero in laboratory settings, but absolute zero itself cannot be reached.
Why do scientists use Kelvin instead of Celsius?
Scientists use Kelvin because it's an absolute scale that directly relates to the kinetic energy of particles. Many physical laws and equations (like the ideal gas law PV=nRT) work more naturally with Kelvin. Kelvin also avoids negative values, making mathematical calculations simpler and more intuitive in thermodynamics.
Can I use 273 instead of 273.15 for conversions?
For most everyday purposes, rounding 273.15 to 273 is acceptable. However, for scientific work, cryogenics, or high-precision applications, you must use the exact value of 273.15. The 0.15 difference may seem small, but it can significantly affect results in precise thermodynamic calculations.
What is room temperature in Kelvin?
Standard room temperature is typically defined as 25°C, which equals 298.15 K. Some scientific contexts use 20°C (293.15 K) as room temperature. The exact definition varies by field, but 298.15 K is the most common reference in chemistry and physics.
References and Sources
- NIST Temperature Reference — National Institute of Standards and Technology. Official conversion formulas and historical information about temperature scales.
- BIPM (International Bureau of Weights and Measures) — Global organization maintaining international measurement standards, including the SI definition of the Kelvin.
- Lord Kelvin (1848). "On an Absolute Thermometric Scale." Philosophical Magazine. Original paper establishing the absolute temperature scale based on thermodynamic principles.
- Anders Celsius (1742). "Observations of two persistent degrees on a thermometer." Proceedings of the Royal Swedish Academy of Sciences. Original document proposing the temperature scale that became Celsius.
- WHO (World Health Organization) — International guidelines on body temperature and medical standards.
- Engineering Toolbox — Comprehensive technical reference for engineering calculations, including thermodynamic formulas and temperature conversions.
- Kelvin — Wikipedia — Comprehensive overview of the Kelvin scale and Lord Kelvin's contributions to science.
- CRC Handbook of Chemistry and Physics — Authoritative reference for chemical and physical properties, including precise temperature constants.
All conversions in this article use the standard formula K = °C + 273.15, as established by international agreement and used in scientific contexts worldwide. Historical information is drawn from primary sources and peer-reviewed research.