Temperature Conversion Guide: Celsius, Fahrenheit & Kelvin Explained (2026)

The Three Temperature Scales

Three scales dominate temperature measurement worldwide: Celsius, Fahrenheit, and Kelvin. Each was created for different purposes, by different people, at different times in history — and each remains in active use today for distinct reasons.

Celsius (°C)

Anders Celsius, a Swedish astronomer, developed the centigrade scale in 1742. His original scale ran 0° at boiling and 100° at freezing — the reverse of what we use today. Fellow scientist Carl Linnaeus (yes, the taxonomy guy) flipped it. The Celsius scale is now the global standard for everyday temperature: weather reports, cooking, and body temperature in every country except the United States, Myanmar, and Liberia.

Fahrenheit (°F)

Daniel Gabriel Fahrenheit, a Polish-German physicist, introduced his scale in 1724. He set 0°F as the temperature of a brine solution (ice, water, and ammonium chloride) and 96°F as approximate human body temperature. The calibration has shifted slightly since then — modern body temperature lands closer to 98.6°F on his scale. Fahrenheit remains deeply embedded in American life despite efforts over the decades to switch the US to metric.

Kelvin (K)

William Thomson — later Lord Kelvin — proposed the thermodynamic temperature scale in 1848 based on the concept of absolute zero. The Kelvin scale uses the same degree size as Celsius but starts at the coldest possible temperature (0 K), where all molecular motion theoretically ceases. Scientists and engineers worldwide use Kelvin for thermodynamics, astrophysics, chemistry, and any calculation involving absolute temperatures. Note that Kelvin values are written without a degree symbol: 300 K, not 300°K.

Conversion Formulas

Every temperature conversion between the three scales follows a fixed formula. Memorize the two main ones and the rest follow logically.

Celsius to Fahrenheit

°F = (°C × 9/5) + 32

Worked example: Convert 25°C (a warm room) to Fahrenheit.
25 × 9/5 = 25 × 1.8 = 45
45 + 32 = 77°F

Fahrenheit to Celsius

°C = (°F − 32) × 5/9

Worked example: Convert 350°F (oven temperature) to Celsius.
350 − 32 = 318
318 × 5/9 = 318 × 0.5556 = 176.7°C

Celsius to Kelvin

K = °C + 273.15

Worked example: Convert 0°C (water’s freezing point) to Kelvin.
0 + 273.15 = 273.15 K

Kelvin to Celsius

°C = K − 273.15

Worked example: Convert 310 K (approximate body temperature in Kelvin) to Celsius.
310 − 273.15 = 36.85°C

Fahrenheit to Kelvin

K = (°F − 32) × 5/9 + 273.15

Worked example: Convert 212°F (boiling water) to Kelvin.
(212 − 32) × 5/9 + 273.15 = 100 + 273.15 = 373.15 K

Common Reference Temperature Table

This table covers the temperatures you’ll look up most often — from absolute zero to the surface of the sun.

Source for extreme Earth temperatures: World Meteorological Organization (WMO). The coldest temperature ever recorded on Earth was −89.2°C (−128.6°F) at Vostok Station, Antarctica on July 21, 1983. The hottest surface temperature was 56.7°C (134°F) at Furnace Creek, Death Valley, California on July 10, 1913.

Quick Estimation Tricks

You won’t always have a calculator. These mental math shortcuts get you within a few degrees, which is usually close enough.

Celsius to Fahrenheit (rough estimate)

Double the Celsius temperature and add 30. This is about 2° high, but works well for everyday ranges. 20°C → (20 × 2) + 30 = 70°F. Actual answer: 68°F. Close enough for a weather check.

Fahrenheit to Celsius (rough estimate)

Subtract 30 and divide by 2. 80°F → (80 − 30) / 2 = 25°C. Actual answer: 26.7°C. Again, a 2° margin, but fast and practical.

Key anchor temperatures to memorize

• 0°C = 32°F (freezing)
• 10°C = 50°F (cold day)
• 20°C = 68°F (room temperature)
• 30°C = 86°F (hot day)
• 37°C = 98.6°F (body temperature)
• 100°C = 212°F (boiling)
• −40°C = −40°F (equal point)

Temperature in Everyday Life

Cooking and Baking

Oven temperatures are the most common conversion people need. Most recipes from outside the US use Celsius. A standard baking temperature of 180°C equals 356°F — typically rounded to 350°F in American recipes. For roasting meat, 200°C (392°F) is a standard roasting temperature, while 230°C (446°F) creates a crispy high-heat finish.

Weather

Travelers moving between countries quickly learn a few anchor points: 0°C is freezing, 10°C is a coat day, 20°C is comfortable, and 30°C is beach weather. For Americans visiting Europe, the quick rule “30 is hot, 20 is nice, 10 is cool, 0 is ice” covers 90% of daily weather reports.

Medicine

Body temperature is reported in Fahrenheit in the US and Celsius everywhere else. The classic threshold for fever is 38°C (100.4°F). A 2020 Stanford University study analyzing 677,000 temperature measurements found that average human body temperature has declined to approximately 97.5°F (36.4°C) — down from the 98.6°F (37°C) standard Carl Wunderlich established in 1851 using early mercury thermometers. Researchers attributed the decline to reduced metabolic rates and lower rates of chronic infection in modern populations.

Science and Engineering

Kelvin is the SI (International System of Units) base unit for temperature and is used wherever absolute temperature matters. Gas laws like the ideal gas law (PV = nRT) require Kelvin because Celsius and Fahrenheit can produce nonsensical negative values in calculations. Cryogenics, astrophysics, semiconductor manufacturing, and thermodynamics all run on Kelvin.

Why the US Still Uses Fahrenheit

The United States is one of only three countries still using Fahrenheit for everyday temperature — the others being Myanmar and Liberia, according to the National Institute of Standards and Technology (NIST). Every other country uses Celsius as the standard.

The US came close to switching. In the 1970s, Congress passed the Metric Conversion Act (1975) and the Omnibus Trade and Competitiveness Act (1988), both calling for voluntary metrication. Highway signs, speed limits, and consumer packaging never fully converted. The voluntary nature of the legislation meant industries simply didn’t change what didn’t need changing.

The deeper reason is inertia compounded by infrastructure. Weather services, home thermostats, medical equipment, school curricula, and decades of human habit all embed Fahrenheit into American daily life. Changing a temperature scale isn’t just a unit swap — it requires re-educating 330 million people on what “hot” feels like as a number.

There’s also an argument that Fahrenheit is more intuitive for human comfort ranges. Its 0–100 scale spans approximately the coldest survivable outdoor conditions to the hottest uncomfortable day, giving it finer granularity in the range humans actually experience. In contrast, 0–100 on the Celsius scale covers freezing to boiling water — ranges that matter more to science than to daily life.

Interesting Temperature Facts

The crossover point: −40

The only temperature where Celsius and Fahrenheit agree is −40 degrees. Set °F = °C in the conversion formula and solve algebraically: C = (C × 1.8) + 32, which gives C = −40. This temperature comes up occasionally in extreme cold weather reporting — it’s cold enough to be dangerous in either unit.

Absolute zero has never been reached

Scientists have cooled atoms to within a billionth of a degree of absolute zero (0 K = −273.15°C), but reaching exactly 0 K is theoretically impossible due to the third law of thermodynamics. The current record for lowest human-made temperature is about 38 picokelvin (0.000000000038 K), achieved at MIT.

The body temperature myth

98.6°F (37°C) as “normal” body temperature was established by Carl Wunderlich in 1851, based on measurements from a population in Leipzig, Germany. His thermometers were long (took 20 minutes to read) and likely measured armpit temperature, which runs slightly higher than oral readings. A 2020 Stanford study published in eLifefound the true modern average is closer to 97.5°F (36.4°C), with normal range from about 97°F to 99°F depending on time of day, age, and how temperature is measured.

Record temperatures on Earth

The hottest surface temperature ever recorded on Earth was 56.7°C (134°F) at Furnace Creek in Death Valley, California on July 10, 1913, according to the World Meteorological Organization (WMO). The coldest was −89.2°C (−128.6°F) at Vostok Station, Antarctica on July 21, 1983 — a Soviet research station at nearly 3,500 meters above sea level.

Frequently Asked Questions

How do you convert Celsius to Fahrenheit?

Multiply the Celsius value by 9/5 and add 32. Formula: °F = (°C × 9/5) + 32. For example, 20°C × 1.8 = 36, plus 32 = 68°F. For a rough mental estimate, double the Celsius value and add 30 — accurate to within about 2 degrees.

What is the formula to convert Fahrenheit to Celsius?

Subtract 32 from the Fahrenheit value, then multiply by 5/9. Formula: °C = (°F − 32) × 5/9. For example, 72°F − 32 = 40, times 5/9 = 22.2°C. A quick mental shortcut: subtract 30 and divide by 2 for a close estimate.

What is absolute zero in Celsius and Fahrenheit?

Absolute zero is 0 Kelvin, equal to −273.15°C or −459.67°F. It’s the theoretical coldest possible temperature — the point at which all molecular motion stops. It has never been fully achieved, though laboratory conditions have come within a fraction of a billionth of a degree.

What is normal human body temperature in Celsius?

Modern research puts average human body temperature at approximately 36.4°C (97.5°F), not the widely cited 37°C (98.6°F). A 2020 Stanford University study published in eLifeanalyzed 677,000 temperature measurements and found body temperature has declined since Carl Wunderlich’s 1851 measurements — likely due to lower chronic infection rates and improved health in modern populations. Fever is generally defined as 38°C (100.4°F) or above.

Why does the US still use Fahrenheit?

The US uses Fahrenheit due to historical adoption and the massive infrastructure cost of switching. Fahrenheit was in use before the global shift to metric, and the 1975 Metric Conversion Act made adoption voluntary — so industries largely didn’t bother. The US is now one of only three countries still using Fahrenheit, alongside Myanmar and Liberia, per the National Institute of Standards and Technology (NIST). A full switch would require updating weather services, building systems, medical equipment, and re-educating the entire population.

At what temperature are Celsius and Fahrenheit equal?

The two scales intersect at exactly −40 degrees. At −40°C = −40°F, both scales give the same reading. This can be verified by plugging into the formula: °F = (−40 × 9/5) + 32 = −72 + 32 = −40. It’s the only point where the scales agree, and it happens to be dangerously cold in any unit system.