How do I convert Celsius to Fahrenheit?

Multiply the Celsius temperature by 9/5 (1.8) and then add 32. For example, 20°C × 1.8 + 32 = 68°F. To go the other way, subtract 32 and multiply by 5/9: (68 − 32) × 5/9 = 20°C.

What is absolute zero in Celsius and Fahrenheit?

Absolute zero - the lowest theoretically possible temperature - is −273.15°C or −459.67°F, which is 0 K (Kelvin) exactly. At absolute zero, the thermal motion of particles theoretically reaches its minimum. It has never been achieved in practice, though scientists have come within billionths of a degree.

At what temperature are Celsius and Fahrenheit the same?

Celsius and Fahrenheit are equal at exactly −40°. That is, −40°C = −40°F. This is the only temperature where the two scales intersect, which makes it a useful sanity check when verifying the conversion formula.

What is the difference between Kelvin and Celsius?

Kelvin and Celsius use the same degree size - a change of 1°C is exactly equal to a change of 1 K. The only difference is the zero point: Celsius is based on water's freezing point (0°C), while Kelvin is based on absolute zero (0 K = −273.15°C). To convert Celsius to Kelvin, add 273.15.

What is Rankine and when is it used?

Rankine (°R) is an absolute temperature scale that uses the Fahrenheit degree size rather than the Celsius degree size. Like Kelvin, 0°R is absolute zero. The Rankine scale is used in some branches of engineering in the United States, particularly in thermodynamics and fluid mechanics, because it allows absolute-temperature calculations using Fahrenheit-sized degrees.

What is normal body temperature in Celsius and Fahrenheit?

The traditionally quoted normal human body temperature is 37°C / 98.6°F. Modern research shows that actual average body temperature is closer to 36.6°C / 97.9°F and varies by individual, time of day, and measurement site. A fever is generally defined as a core temperature above 38°C / 100.4°F.

Temperature Converter

Celsius to Fahrenheit and Fahrenheit to Celsius

Celsius (°C) and Fahrenheit (°F) are the two scales most people encounter in daily life. The conversion formula is: °F = °C × 9/5 + 32. To reverse it: °C = (°F − 32) × 5/9. The two scales cross at exactly −40°, the one temperature that is the same in both Celsius and Fahrenheit.

Common temperature references

Reference	°C	°F	K
Absolute zero	−273.15	−459.67	0
Dry ice (CO₂ sublimation)	−78.5	−109.3	194.65
Water freezes	0	32	273.15
Cold winter day	−10	14	263.15
Comfortable room temperature	20	68	293.15
Human body temperature	37	98.6	310.15
Hot summer day	40	104	313.15
Oven: medium heat	180	356	453.15
Water boils (sea level)	100	212	373.15
C and F are equal	−40	−40	233.15

The four temperature scales

Celsius (°C)

The Celsius scale was defined by the Swedish astronomer Anders Celsius in 1742. Its two anchors are the freezing point (0°C) and boiling point (100°C) of water at standard atmospheric pressure. The Celsius scale is used by most of the world for everyday temperature measurement and is the scientific standard alongside Kelvin.

Fahrenheit (°F)

Proposed by Daniel Gabriel Fahrenheit in 1724, the Fahrenheit scale was historically anchored to the freezing point of a brine solution (0°F) and human body temperature (96°F in the original, later adjusted). Water freezes at 32°F and boils at 212°F. The Fahrenheit scale is used primarily in the United States for everyday temperatures. Its 180-degree span between freezing and boiling (vs. 100° in Celsius) gives slightly finer resolution for typical weather and body temperatures.

Kelvin (K)

The Kelvin scale (the SI base unit of thermodynamic temperature) begins at absolute zero (the lowest theoretically possible temperature) and uses the same degree size as Celsius. 0 K = −273.15°C. Kelvin values are never negative by definition. Kelvin is used throughout physics, chemistry, and engineering where absolute temperatures are required. For example, in the ideal gas law (PV = nRT), where T must be in Kelvin.

The 2019 SI redefinition fixed the Kelvin by tying it to the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K), making it more precise than the previous triple-point-of-water definition.

Rankine (°R)

The Rankine scale uses the same size degree as Fahrenheit but starts at absolute zero, analogous to Kelvin's relationship with Celsius. 0°R = absolute zero = −459.67°F. Water freezes at 491.67°R and boils at 671.67°R. Rankine is used in some US engineering contexts, particularly in thermodynamics and aerodynamics, where absolute temperatures are needed but Fahrenheit-sized degrees are preferred.

Why temperature conversion is different from other unit conversions

Most unit conversions are purely multiplicative: multiply by a fixed factor. Temperature is different because Celsius and Fahrenheit have different zero points as well as different degree sizes. Converting between them requires both a multiplication and an addition (or subtraction). Kelvin and Rankine, being absolute scales that start at the same physical zero, only require a multiplication by 9/5 (or 5/9) to convert between themselves.

Practical cooking temperatures

Reference	°C	°F
Rare steak (internal)	52	125
Medium steak (internal)	60	140
Well-done steak (internal)	71	160
Poultry - safe minimum (USDA)	74	165
Bread - internal done	88–93	190–200
Soft ball candy stage	115	240
Hard crack candy stage	149	300
Deep frying oil	175–190	347–375

Weather context anchors

A quick reference for travelers or anyone switching between Celsius and Fahrenheit for everyday weather:

Condition	°C	°F
Freezing point of water	0	32
Cold day (wear a heavy coat)	5	41
Cool day (jacket needed)	10	50
Mild / comfortable	20	68
Warm / t-shirt weather	25	77
Hot day	30	86
Very hot / dangerous heat	40	104
Heat stroke risk	43+	110+