Last updated: 2026-05-13

Temperature Scales

Temperature measures the average kinetic energy of particles in a substance. Three scales are in common use: Celsius, the global standard for weather, medicine, and food; Fahrenheit, used primarily in the United States; and Kelvin, the SI base unit that underpins physical science.

All three describe the same physical reality. They differ only in their zero point and the size of a degree.

The Scales at a Glance

Scale	Freezing point of water	Boiling point of water	Absolute zero
Celsius	0 °C	100 °C	−273.15 °C
Fahrenheit	32 °F	212 °F	−459.67 °F
Kelvin	273.15 K	373.15 K	0 K

Conversion Formulas

All three scales are linearly related, so conversion is straightforward:

• °C → °F: multiply by 9/5, then add 32
• °F → °C: subtract 32, then multiply by 5/9
• °C → K: add 273.15
• K → °C: subtract 273.15

There is no direct °F ↔ K formula in common use; the usual path goes through °C.

Why Fahrenheit Survived in the United States

Gabriel Fahrenheit proposed his scale in 1724, calibrating 0 °F to the coldest brine mixture he could create and 96 °F to human body temperature (later refined). By the time the metric system spread globally in the 19th century, Fahrenheit was already embedded in American commerce, weather reporting, and everyday speech. Metrication proposals recur periodically, but the US remains one of only three countries not to have made the official switch.

Kelvin in Practice

The Kelvin scale was defined by Lord Kelvin in 1848 specifically so that 0 K represents absolute zero — the point at which a classical ideal gas would have zero volume. One kelvin is identical in size to one Celsius degree; only the zero point differs. Scientists use Kelvin because it makes thermodynamic equations cleaner: gas laws, blackbody radiation, and entropy calculations all behave more neatly when temperature cannot be negative. In cryogenics, temperatures are routinely expressed in millikelvin (mK) or even microkelvin (μK).

Reference Points Worth Knowing

Reference	Celsius	Fahrenheit	Kelvin
Absolute zero	−273.15 °C	−459.67 °F	0 K
Liquid nitrogen boiling point	−195.79 °C	−320.42 °F	77.36 K
Dry ice (CO₂) sublimation	−78.5 °C	−109.3 °F	194.65 K
Water freezing	0 °C	32 °F	273.15 K
Normal body temperature	37 °C	98.6 °F	310.15 K
Water boiling (at sea level)	100 °C	212 °F	373.15 K
Oven: medium heat	180 °C	356 °F	453.15 K
Surface of the Sun	≈ 5,500 °C	≈ 9,932 °F	≈ 5,773 K

Frequently Asked Questions (FAQ)

What is the difference between Celsius and Fahrenheit?

Celsius (°C) is used in most countries and in science. Water freezes at 0 °C and boils at 100 °C. Fahrenheit (°F) is used primarily in the United States. Water freezes at 32 °F and boils at 212 °F. To convert: °F = °C × 9/5 + 32.

What is absolute zero?

Absolute zero is the lowest possible temperature: 0 K, −273.15 °C, or −459.67 °F. At this point, particles have minimum thermal motion. It is a theoretical limit — no physical object can reach exactly 0 K.

What is normal body temperature in Celsius and Fahrenheit?

Normal human body temperature is approximately 37 °C (98.6 °F). A fever is generally defined as a body temperature above 38 °C (100.4 °F).