What are the four main temperature scales?

The four main temperature scales are Celsius, Fahrenheit, Kelvin, and Rankine. Celsius and Fahrenheit are used for everyday measurement, Kelvin is the SI base unit for science, and Rankine is an absolute scale used in some US engineering fields.

What is the difference between Celsius and Kelvin?

Celsius and Kelvin use the same degree size, so a change of 1 degree Celsius equals a change of 1 kelvin. The only difference is the zero point: 0 K is absolute zero, while 0 degrees Celsius is the freezing point of water. To convert, add 273.15 to Celsius to get kelvin.

How do Fahrenheit and Rankine relate?

Rankine uses the same degree size as Fahrenheit but starts at absolute zero. To convert Fahrenheit to Rankine, add 459.67. So 0 degrees Fahrenheit equals 459.67 degrees Rankine, just as 0 degrees Celsius equals 273.15 kelvin.

Which temperature scale do scientists use?

Scientists use Kelvin because it is the SI base unit of temperature and starts at absolute zero, the point where molecular motion stops. This makes thermodynamic and gas-law calculations simpler, since there are no negative temperatures to account for.

Why does the United States still use Fahrenheit?

The United States adopted Fahrenheit through its British colonial heritage and never fully transitioned to the metric system for everyday use. Most of the world standardized on Celsius during the 20th century, but Fahrenheit remains the common scale for US weather, cooking, and body temperature.

Temperature Scales Compared (C, F, K, R)

The Four Scales

Four temperature scales dominate science, engineering, and daily life. Each measures the same physical quantity but anchors its zero point and degree size differently. Understanding how they relate lets you read any thermometer, recipe, or research paper with confidence.

Celsius (°C) is the everyday scale across most of the world. It sets the freezing point of water at 0°C and the boiling point at 100°C, giving a clean 100-degree span between the two. Fahrenheit (°F) is the everyday scale in the United States, placing water's freezing point at 32°F and boiling point at 212°F — a 180-degree span.

Kelvin (K) is the SI base unit of temperature. It uses the same degree size as Celsius but starts at absolute zero, the coldest possible temperature, so it never goes negative. Rankine (°R) is the absolute counterpart to Fahrenheit: it shares Fahrenheit's degree size but, like Kelvin, begins at absolute zero. Rankine appears mainly in US thermodynamics and aerospace engineering.

Reference Points Compared

These five landmark temperatures show how the scales line up. Notice that Celsius and Kelvin always differ by exactly 273.15, while Fahrenheit and Rankine differ by exactly 459.67.

Reference point	Celsius (°C)	Fahrenheit (°F)	Kelvin (K)	Rankine (°R)
Absolute zero	−273.15	−459.67	0	0
Water freezes	0	32	273.15	491.67
Room temperature	21	69.8	294.15	529.47
Body temperature	37	98.6	310.15	558.27
Water boils	100	212	373.15	671.67

Conversion Formulas

Every pair of scales is related by a simple linear equation. The table below covers all the common conversions. Scaling by 9/5 (or 1.8) bridges the Celsius-sized degrees and the Fahrenheit-sized degrees, while the constants shift between the different zero points.

Conversion	Formula
Celsius to Fahrenheit	°F = (°C × 9/5) + 32
Fahrenheit to Celsius	°C = (°F − 32) × 5/9
Celsius to Kelvin	K = °C + 273.15
Kelvin to Celsius	°C = K − 273.15
Fahrenheit to Kelvin	K = (°F − 32) × 5/9 + 273.15
Kelvin to Fahrenheit	°F = (K − 273.15) × 9/5 + 32
Fahrenheit to Rankine	°R = °F + 459.67
Rankine to Fahrenheit	°F = °R − 459.67
Kelvin to Rankine	°R = K × 9/5
Celsius to Rankine	°R = (°C + 273.15) × 9/5

Which Scale Is Used Where

Everyday life. Most countries report weather, oven settings, and body temperature in Celsius. The United States and a handful of territories use Fahrenheit for the same purposes, which is why a forecast of 30°C and one of 86°F describe the very same warm day.

Science. Kelvin rules the laboratory. Because it starts at absolute zero, thermodynamic relationships and gas laws stay clean — doubling the kelvin temperature genuinely doubles the average molecular energy, something no relative scale can claim. Chemists, physicists, and astronomers report temperatures in kelvin worldwide.

US engineering. Rankine survives in American mechanical and aerospace engineering, where calculations are done in imperial units but still require an absolute temperature. Combustion analysis, jet engine cycles, and steam tables in US references often list values in degrees Rankine.

Whichever scale you start from, you can switch to any of the other three in one step with our free temperature converter, which handles Celsius, Fahrenheit, Kelvin, and Rankine with full decimal precision.