Temperature: Celsius, Fahrenheit & Kelvin

Enter a temperature in any scale and read it instantly in all the others, including the scales still used in science and older engineering documents.

Everyday reference points

To convert Celsius to Fahrenheit in your head, double the number, subtract a tenth of that result, then add 32. It is accurate to within a degree across normal weather temperatures.

Temperature is one of the most vital thermodynamic physical properties in human existence. It dictates weather forecasts, cooking chemistry, medical diagnostics, refrigeration standards, chemical manufacturing, and aerospace engineering. Yet, depending on whether you are reading an American recipe, analyzing a European meteorological chart, or running an astrophysics thermodynamic simulation, temperature values are expressed in three radically different scales: Celsius, Fahrenheit, and Kelvin

Because temperature scales do not share a common zero point, mental conversions are notoriously tricky: simply multiplying by a decimal will yield wildly inaccurate numbers. Our client-side Temperature Converter provides instant cross-conversion across Celsius, Fahrenheit, Kelvin, and Rankine with precision floating-point formulas and complete client-side privacy.

Table of Contents

The Physics and History of the Three Major Scales

Understanding the origins of each temperature scale illuminates why their conversion formulas require both scaling ratios and coordinate offset adjustments:

Fahrenheit (°F – 1724): Formulated by German physicist Daniel Gabriel Fahrenheit, this was the first standardized scale utilizing mercury thermometers. Fahrenheit calibrated his zero point (0°F) using a stabilized freezing mixture of ice water and ammonium chloride salt. He set 32°F at the freezing point of pure water and 96°F at human body temperature. On this scale, pure water boils at 212°F, creating an interval of exactly 180 degrees between freezing and boiling.
Celsius (°C – 1742): Developed by Swedish astronomer Anders Celsius, this decimal scale is based purely on the physical phase transitions of water at standard atmospheric pressure (1 atm): pure water freezes at exactly 0°C and boils at exactly 100°C Adopted by the metric system, Celsius is the daily standard for 96% of the global population.
Kelvin (K – 1848): Proposed by Scottish physicist William Thomson (Lord Kelvin), the Kelvin scale is the international thermodynamic baseline (SI unit). Unlike Celsius and Fahrenheit, Kelvin does not use “degrees”—it is an absolute scale starting at Absolute Zero (0 K), the theoretical temperature where all classical atomic kinetic motion ceases. Absolute zero corresponds to -273.15°C or -459.67°F

Physical Thermodynamic Event	Celsius (°C)	Fahrenheit (°F)	Kelvin (K)
Absolute Zero (Zero Atomic Motion)	`-273.15 °C`	`-459.67 °F`	`0 K`
Freezing Point of Pure Water (1 atm)	`0.00 °C`	`32.00 °F`	`273.15 K`
Comfortable Room Temperature	`21.00 °C`	`69.80 °F`	`294.15 K`
Normal Human Body Temperature	`37.00 °C`	`98.60 °F`	`310.15 K`
Boiling Point of Pure Water (1 atm)	`100.00 °C`	`212.00 °F`	`373.15 K`

The Mathematical Conversion Formulas

Because the Fahrenheit scale has 180 divisions between water’s phase changes while Celsius has 100, the ratio between the two is 180 / 100 = 9/5 = 1.8 To convert accurately:

Celsius to Fahrenheit: Multiply by 1.8 and add 32: °F = (°C × 1.8) + 32
Fahrenheit to Celsius: Subtract 32 and divide by 1.8: °C = (°F - 32) / 1.8
Celsius to Kelvin: Add 273.15: K = °C + 273.15
Kelvin to Celsius: Subtract 273.15: °C = K - 273.15

Key Inquiries Regarding Temperature

At what temperature are Celsius and Fahrenheit equal?

Celsius and Fahrenheit intersect at exactly -40 degrees (-40°C = -40°F). At this extreme sub-zero threshold, both scales report identical values.

Why is Kelvin written without a degree symbol (°)?

Degrees indicate arbitrary divisions referenced against chosen zero marks (like freezing water). Because Kelvin is an absolute measurement based directly on thermal energy density from absolute zero, it is an absolute unit of measure (like meters or kilograms) and does not take a degree symbol.

Is this temperature converter completely private?

Yes. All conversions run 100% locally within your browser’s JavaScript engine. No data is transmitted to cloud servers.