Convert temperatures between Celsius, Fahrenheit, Kelvin, Rankine and
historical scales. Alongside conversion results, learn how each scale
works, why temperature formulas include offsets and where different
units are used in weather, cooking, science, medicine and engineering.
For the most frequently used calculations, open the
Celsius to Fahrenheit converter
or use the
Fahrenheit to Celsius converter
for the reverse conversion.
When Is Temperature Conversion Useful?
Temperature conversion is useful when checking an overseas weather
forecast, following an international recipe, setting an oven or
comparing thermostat readings. It is also important in medicine,
laboratory work, refrigeration, HVAC, manufacturing, aviation,
materials testing and scientific research.
An online temperature converter reduces manual calculation errors,
especially because Celsius and Fahrenheit differ in both their zero
points and degree sizes. Scientific conversions may also require users
to distinguish between an actual temperature and a temperature change.
Why Temperature Scales Give Different Numbers
A room can simultaneously be described as
20 °C, 68 °F or
293.15 K. Its physical temperature has not changed;
only the numerical scale and reference point are different.
Temperature conversion is more complicated than many length or weight
conversions. Celsius and Fahrenheit differ in both zero position and
degree size. Kelvin and Rankine begin at absolute zero but use
different-sized intervals.
When 100 Meant Freezing and Zero Meant Boiling
In 1742, Swedish astronomer Anders Celsius published a
hundred-division temperature scale based on two water reference
points. His numbering ran in the opposite direction from the
Celsius scale used today:
-
0 degrees represented the boiling point of
water.
-
100 degrees represented the freezing point of
water.
An ascending version soon appeared. French physicist Jean-Pierre
Christin described a scale with zero at freezing and 100 at boiling
in 1743. Swedish botanist Carl Linnaeus also used the ascending
arrangement during the following years.
It is therefore safer to describe the modern ordering as a
development adopted by several eighteenth-century scientists and
instrument makers rather than crediting the reversal exclusively
to one person.
Celsius died in 1744, but his name eventually became associated
with the ascending scale. The international name
degree Celsius replaced degree centigrade in
1948.
The Modern Relationship Between Celsius and Kelvin
Water’s freezing and boiling points remain useful everyday
references, but they are not the modern formal definition of
Celsius temperature. Celsius is related exactly to thermodynamic
temperature in kelvins:
Temperature in °C = temperature in K − 273.15
One Celsius degree has the same interval size as one kelvin. The
two scales differ by their numerical starting points, while the
kelvin itself is defined using the fixed value of the Boltzmann
constant.
The
NIST guide to temperature units
explains the relationship between Celsius and Kelvin and provides
their conversion equations.
How 37 °C Became the Precise-Looking 98.6 °F
During the nineteenth century, German physician Carl Reinhold
August Wunderlich collected a large number of patient-temperature
observations. His work helped establish approximately 37 °C as a
familiar reference for normal body temperature.
Converting that whole-number Celsius value produces:
(37 × 9 ÷ 5) + 32 = 98.6 °F
The Fahrenheit result contains a decimal place, but that does not
make the original estimate accurate to one-tenth of a Fahrenheit
degree. Its apparent precision is partly a consequence of
converting a rounded Celsius value.
Body temperature is not one exact number shared by everyone. It can
vary with the individual, age, time of day, activity, measurement
site, thermometer and measurement method.
A later clinical study reported a mean oral temperature of
approximately 36.8 °C and argued against treating 98.6 °F as a
universal normal value. The study is available through the
National Library of Medicine
.
This example demonstrates an important rule:
extra digits in a converted result do not add accuracy that
was absent from the original measurement
.
Four Numbers Can Describe the Same Temperature
The freezing point of pure water under familiar reference
conditions provides a useful comparison among four scales:
Approximate freezing point of pure water in four temperature
scales
| Scale |
Temperature |
Reference for zero |
| Celsius |
0 °C |
273.15 K above absolute zero |
| Fahrenheit |
32 °F |
459.67 Fahrenheit degrees above absolute zero |
| Kelvin |
273.15 K |
Absolute zero |
| Rankine |
491.67 °R |
Absolute zero |
These values describe approximately the same physical condition.
The water reference depends on experimental circumstances:
pressure, purity, dissolved substances and supercooling can affect
the point at which water changes phase.
Kelvin and Rankine Are Absolute Temperature Scales
Kelvin begins at absolute zero and uses intervals equal in size to
Celsius degrees. Rankine also begins at absolute zero, but its
intervals are equal in size to Fahrenheit degrees.
- 0 K = −273.15 °C
- 0 °R = −459.67 °F
- A 1 K interval equals a 1 °C interval
- A 1 °R interval equals a 1 °F interval
NIST describes these relationships in its
temperature conversion tables
.
Temperature Values and Temperature Changes Use Different Formulas
Converting a temperature reading requires accounting for the
different zero points of Celsius and Fahrenheit:
°F = (°C × 9 ÷ 5) + 32
°C = (°F − 32) × 5 ÷ 9
You can apply the first equation using our
Celsius to Fahrenheit conversion tool
.
When the starting value is in Fahrenheit, use the
Fahrenheit to Celsius conversion tool
.
A temperature difference has no zero-point offset. A rise of 10 °C
is therefore equivalent to a rise of 18 °F:
10 × 9 ÷ 5 = 18
Adding 32 in that calculation would be incorrect because the value
describes a change, not a position on the Fahrenheit scale. This
distinction matters in engineering, laboratory work, manufacturing
and climate-data analysis.
Where Temperature Converters Are Used Today
Travellers compare Celsius and Fahrenheit weather forecasts, while
cooks convert oven settings when following recipes written in
another country. Medical staff may encounter instruments,
references or records that use different regional scales.
Scientists generally use kelvins for thermodynamic temperature.
Engineers working with some US customary systems may encounter
Fahrenheit and Rankine, particularly in calculations involving
temperature ratios and thermodynamic cycles.
Temperature conversion also appears in HVAC, refrigeration,
aviation, manufacturing, materials testing, climate research and
industrial safety. In each case, users should identify whether the
value represents a temperature point, a temperature interval or an
approximate measurement with limited precision.