Temperature Converter

About Temperature Converter

How Temperature Conversion Works

Every temperature conversion between Celsius and Fahrenheit involves two steps: scaling the degree size and shifting the zero point. A single Celsius degree covers 1/100 of the range between water's freezing and boiling points. A Fahrenheit degree covers 1/180 of that same range, making each Fahrenheit degree smaller than each Celsius degree.

To convert Celsius to Fahrenheit, multiply by 9/5 (or 1.8) to adjust for the different degree sizes, then add 32 to shift the zero point. For example, 25°C becomes (25 × 1.8) + 32 = 77°F.

Going the other way, subtract 32 first, then multiply by 5/9. So 98.6°F becomes (98.6 - 32) × 5/9 = 66.6 × 0.5556 = 37°C.

Kelvin is simpler because each kelvin is exactly the same size as one degree Celsius. The only difference is the zero point: 0 K sits at absolute zero (-273.15°C) instead of at water's freezing point. Converting between Celsius and Kelvin means adding or subtracting 273.15. To go from Fahrenheit to Kelvin, convert to Celsius first, then add 273.15.

Worked example - Kelvin to Fahrenheit: convert 310.15 K. First subtract 273.15 to get Celsius: 310.15 - 273.15 = 37°C. Then apply the Celsius-to-Fahrenheit formula: (37 × 1.8) + 32 = 66.6 + 32 = 98.6°F. That is normal human body temperature.

There is also a fourth scale called Rankine, used mainly in US engineering. It works like Kelvin but uses Fahrenheit-sized degrees: 0 °R equals absolute zero, and water freezes at 491.67 °R. It sees limited use outside specialised thermodynamics work.

Conversion Formulas

From	To	Formula	Example
Celsius	Fahrenheit	(C × 9/5) + 32	100°C = 212°F
Fahrenheit	Celsius	(F - 32) × 5/9	72°F ≈ 22.22°C
Celsius	Kelvin	C + 273.15	0°C = 273.15 K
Kelvin	Celsius	K - 273.15	373.15 K = 100°C
Fahrenheit	Kelvin	(F - 32) × 5/9 + 273.15	32°F = 273.15 K
Kelvin	Fahrenheit	(K - 273.15) × 9/5 + 32	0 K = -459.67°F

The Celsius and Kelvin scales use the same increment size (a 1-degree change is the same in both). They differ only in their zero point: Kelvin starts at absolute zero (-273.15°C), while Celsius starts at the freezing point of water.

A Brief History of Temperature Scales

Daniel Gabriel Fahrenheit, a German-Dutch physicist, created the Fahrenheit scale in 1724. He set 0°F at the temperature of a brine solution (a mix of ice, water, and ammonium chloride) and originally calibrated 96°F as human body temperature. The scale became the default in English-speaking countries for over two centuries.

Anders Celsius, a Swedish astronomer, introduced his scale in 1742. His original version was actually inverted: 0 was the boiling point and 100 was freezing. Carl Linnaeus reversed it to the modern orientation, with 0 at freezing and 100 at boiling, creating a clean 100-degree range between water's two phase changes.

William Thomson, later Lord Kelvin, proposed an absolute temperature scale in 1848. He published "On an Absolute Thermometric Scale" arguing for a scale starting where molecular thermal energy reaches its minimum. The General Conference on Weights and Measures adopted the kelvin as the SI base unit for thermodynamic temperature in 1954, and it remains the standard for scientific work worldwide.

The Three Scales Compared

Property	Celsius	Fahrenheit	Kelvin
Zero point	Freezing point of water	Brine solution (Daniel Fahrenheit, 1724)	Absolute zero (no molecular motion)
100 mark	Boiling point of water	Roughly human body temp (original intention)	N/A (arbitrary in this scale)
Degree size	1/100 of water's freeze-boil range	1/180 of water's freeze-boil range	Same as Celsius
Used by	Most countries, science	United States, a few Caribbean nations	Physics, chemistry, engineering
Negative values?	Yes (below water's freezing point)	Yes (below brine's freezing point)	No (starts at absolute zero)

Key Temperature Reference Points

Point	Celsius	Fahrenheit	Kelvin
Absolute zero	-273.15	-459.67	0
Liquid nitrogen	-196	-321	77
Dry ice (CO2)	-78.5	-109.3	194.65
Where F and C are equal	-40	-40	233.15
Water freezes	0	32	273.15
Room temperature	20-22	68-72	293-295
Human body	37	98.6	310.15
Water boils (sea level)	100	212	373.15
Oven (moderate)	180	350	453.15
Paper burns (Fahrenheit 451)	233	451	506

What Is Absolute Zero?

Absolute zero is the lowest physically possible temperature: 0 K, equal to -273.15°C or -459.67°F. At this point, atoms have minimal thermal energy, though quantum mechanics says they still retain a small residual vibration called zero-point energy. They never truly stop moving entirely.

No experiment has ever reached absolute zero, and thermodynamic theory says it is impossible to do so in a finite number of steps. The closest recorded laboratory temperature is about 50 picokelvin (0.00000000005 K), achieved with ultracold atomic gases. Near absolute zero, matter behaves in unusual ways: metals can become superconductors with zero electrical resistance, and certain fluids become superfluids that flow without friction or can climb up the walls of their container. William Thomson proposed the Kelvin scale specifically because an absolute starting point makes physics equations simpler. In the ideal gas law (PV = nRT), temperature must be in Kelvin - using Celsius would produce negative volumes, which makes no physical sense.

Is Normal Body Temperature Really 37°C?

The widely cited 37°C (98.6°F) standard comes from German physician Carl Wunderlich, who averaged measurements from about 25,000 people in the 1860s. But a 2020 study from Stanford University School of Medicine found that average human body temperature has been declining since the 19th century. Men born in the 1990s run about 0.59°C (1.06°F) cooler than men born in the early 1800s.

Current research puts the modern average closer to 36.4-36.6°C (97.5-97.9°F). Reduced chronic inflammation and lower metabolic rates from improved living conditions likely explain the drop. The normal range for any individual is about 36.1-37.2°C (97.0-99.0°F), and it varies by time of day, physical activity, and age. A fever is generally considered to start at 38°C (100.4°F), though this threshold has not been officially revised to reflect the lower modern average.

Quick Mental Conversions

For rough estimates without a calculator, a few shortcuts work well. The "double and add 30" method for Celsius to Fahrenheit works best between 0°C and 35°C, which covers most weather temperatures. Outside that range the approximation drifts further from the real answer, so use the exact formula or this converter for precision.

Method	How	Accuracy
C to F (approximate)	Double the C value and add 30	Close enough for weather (20°C to 70°F, actual: 68°F)
F to C (approximate)	Subtract 30, then halve	Close enough for weather (80°F to 25°C, actual: 26.7°C)
Exact at -40	-40°C = -40°F exactly	The only point where both scales agree

Which Countries Use Fahrenheit?

Celsius is the official temperature scale in nearly every country. The United States is the largest nation still using Fahrenheit for everyday purposes, including weather forecasts, cooking, and medical thermometers. Other countries using Fahrenheit include Belize, the Bahamas, the Cayman Islands, Palau, the Marshall Islands, and several other small Caribbean and Pacific island nations. Around 14 countries use Fahrenheit in total, according to World Population Review.

Some countries use both scales in practice. In Canada, weather reports use Celsius but many recipes list oven temperatures in Fahrenheit. The UK switched to Celsius for weather forecasts as part of its 1960s-70s metrication process, though older generations still sometimes reference Fahrenheit for hot summer days. Australia made the change in 1972, and most former British colonies followed between 1960 and 1980. The United States has resisted full metrication despite several government efforts, most recently the Omnibus Trade and Competitiveness Act of 1988. That act designated the metric system as the preferred system for trade and commerce but left everyday use voluntary, so Fahrenheit remains the standard for weather, cooking, and medicine in the US.

Cooking Temperature Reference

Converting oven temperatures is a common task when following recipes from other countries. American recipes use Fahrenheit, most European recipes use Celsius, and older UK recipes sometimes use Gas Mark. Gas Mark is a scale specific to gas ovens where each mark equals a 25°F (14°C) increment, starting at Gas Mark 1 = 275°F (135°C). Fan-assisted ovens typically run about 20°C hotter than conventional ovens, so reduce the stated temperature by 20°C when using a fan oven.

Oven Setting	Celsius	Fahrenheit	Gas Mark (UK)
Very cool	110°C	225°F	1/4
Cool	140°C	275°F	1
Moderate	180°C	350°F	4
Moderately hot	190°C	375°F	5
Hot	200°C	400°F	6
Very hot	220°C	425°F	7
Extremely hot	240°C	475°F	9

Gas Mark 4 (180°C / 350°F) is the most common baking temperature for cakes and biscuits in UK recipes.

Weather Temperature Ranges

Temperature perception varies significantly by region and personal acclimatisation. What counts as "warm" in the Scottish Highlands would feel "cool" in southern Spain. The ranges below are general descriptions for temperate-climate residents, though comfort also depends on humidity, wind, sun exposure, and individual tolerance.

Description	Celsius	Fahrenheit
Extreme cold	Below -20°C	Below -4°F
Freezing	-10 to 0°C	14 to 32°F
Cold	0 to 10°C	32 to 50°F
Cool	10 to 15°C	50 to 59°F
Mild	15 to 20°C	59 to 68°F
Warm	20 to 25°C	68 to 77°F
Hot	25 to 35°C	77 to 95°F
Extreme heat	Above 40°C	Above 104°F

For converting other unit types, the length converter handles metric and imperial distances, the weight converter covers mass units, and the cooking unit converter helps with volumes, weights, and other recipe measurements. All tools run entirely in your browser.

Sources

• BIPM - SI Brochure (kelvin as SI base unit)
• NIST - SI Units including Kelvin
• Stanford Medicine - Human body temperature has decreased
• Encyclopedia Britannica - Celsius temperature scale
• Encyclopedia Britannica - Fahrenheit temperature scale
• World Population Review - Countries that use Fahrenheit