Free Temperature Converter Online

The Temperature Converter instantly translates values between the three major temperature scales: Celsius (°C), Fahrenheit (°F), and Kelvin (K). Type a temperature in any field and the other two scales update in real time with your selected decimal precision (0-4 places). The tool handles the full range from absolute zero upward.

Because the two scales have both different unit sizes and different zero points. The 9/5 factor converts the Celsius degree size to the smaller Fahrenheit degree size: 1 degree Celsius spans the same temperature interval as 1.8 degrees Fahrenheit. The +32 accounts for the offset between the two zero points: 0°C = 32°F. Mathematically, it is a linear transformation y = mx + b where m = 9/5 and b = 32.

Kelvin enables proportional reasoning that Celsius and Fahrenheit prevent. Doubling a temperature from 10°C to 20°C does NOT double the thermal energy. But doubling 283.15 K to 566.3 K (which is 293.15°C) does roughly double the average kinetic energy, because Kelvin starts at true zero. This is why the ideal gas law (PV = nRT) and Stefan-Boltzmann law use Kelvin.

At -40 degrees. This is the one temperature where both scales read the same numeric value: -40°C = -40°F. You can solve for it by setting the conversion formula equal to itself: x = (9/5)x + 32. This is useful as a quick mental check for any temperature converter — enter -40 and verify both Celsius and Fahrenheit show the same number.

Kelvin is an absolute scale where 0 K (absolute zero) is the theoretical minimum temperature in the universe — the point at which all atomic motion ceases. It is physically impossible to reach temperatures below 0 K according to the laws of thermodynamics. The converter enforces this constraint by displaying an error if you attempt to enter a negative Kelvin value.

The Rankine scale (°R) is to Fahrenheit what Kelvin is to Celsius: an absolute scale where 0°R = absolute zero, but each degree is the same size as one Fahrenheit degree. The conversion is °R = °F + 459.67. Rankine is used in some engineering thermodynamics texts but is increasingly rare. This converter focuses on the three most widely used scales: Celsius, Fahrenheit, and Kelvin.

In Python: f = c * 9/5 + 32 converts Celsius to Fahrenheit. In JavaScript: (c * 9/5 + 32).toFixed(2). In Go, use c * 9.0 / 5.0 + 32 with float64 types. For Celsius to Kelvin, all languages use c + 273.15. For Fahrenheit to Celsius: (f - 32) * 5/9. Most standard libraries do not include built-in temperature conversion functions, so you typically write the formulas directly. Be careful with integer division in languages like C and Java — always use floating-point types to avoid truncation.

Kelvin is an absolute thermodynamic scale, not a relative scale like Celsius or Fahrenheit. The kelvin (unit symbol K) is one of the seven SI base units, and by international convention (BIPM) it is written without the degree sign. The correct notation is "373.15 K", not "373.15°K". The degree symbol was officially dropped in 1967 by the 13th General Conference on Weights and Measures (CGPM). This distinction emphasizes that Kelvin measures absolute temperature starting from absolute zero, not a relative interval.

Isaac Newton proposed a temperature scale around 1700 with 0° as freezing and 12° as body heat (approximately 33°C). Each Newton degree was roughly 2.8°C. While the Newton scale is obsolete today, it is historically significant as one of the first attempts to create a quantitative temperature scale with fixed reference points. Newton's work influenced later scientists including Anders Celsius and Daniel Fahrenheit. The scale occasionally appears in historical physics contexts and trivia.

The Celsius scale was originally called the "centigrade" scale (from Latin centum = 100 and gradus = steps) because it divided the interval between freezing and boiling water into 100 equal degrees. In 1948, the name was officially changed to "Celsius" by the International Conference on Weights and Measures to avoid confusion with the French unit of angle measurement (also called centigrade). The two terms refer to the same scale, but "Celsius" is the correct modern name. Anders Celsius originally defined 0° as boiling and 100° as freezing — the scale was inverted after his death.