Workshop converter
Temperature Converter
Convert Celsius, Fahrenheit, Kelvin, Rankine, Réaumur, Rømer, Newton, and Delisle temperatures, then learn why temperature scales have different zeros, different degree sizes, and more history than a thermometer has room to print.
Common conversions
Sage’s guided lesson
Temperature is simple, right up until it is not
A thermometer gives you a number, but that number only makes sense after you know the scale, the reference points, where the reading was taken, and what the thermometer was actually sensing. I’ll show you how to convert the number, how to check the math yourself, and why a perfectly converted answer can still be a poor measurement.
First, compare the scales
Temperature conversion needs a shift and a resize
Length conversion is usually multiplication or division because the units share the same zero. Temperature is trickier. Celsius and Fahrenheit use different zero points and different degree sizes, so you normally have to shift the starting point, then resize the interval.
This converter uses Celsius as the middle step. It first turns the starting temperature into Celsius, then converts that Celsius value into the target scale. Celsius is the translator in the middle, even when you are converting between two historical scales.
The most useful formulas
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
K = °C + 273.15
°R = °F + 459.67
20 °C = 68 °F = 293.15 K = 527.67 °R. Nothing became hotter or colder. We only changed the coordinate system used to describe it.
Let’s do one by hand
Converting 68 °F into Celsius and kelvin
Start by subtracting 32. That lines the Fahrenheit zero point up with the Celsius scale. Then multiply by 5/9 because one Fahrenheit degree is smaller than one Celsius degree.
Worked example
(68 − 32) × 5/9 = 20 °C
20 + 273.15 = 293.15 K
68 + 459.67 = 527.67 °R
To go back to Fahrenheit, use (20 × 9/5) + 32 = 68 °F.
For a fast mental estimate, subtract 30 and divide by 2 when going from Fahrenheit to Celsius. For 68 °F, that gives about 19 °C, close to the exact 20 °C. Going the other way, double Celsius and add 30. These are handy guesses, not precision formulas. Random thought: why did the calculator apologize? It made a mistake and felt divided. Anyway, back to things that are hot, cold, or dramatically lukewarm.
These words are not twins
Temperature and heat are related, but they are not the same thing
Temperature describes the thermal state of a material. Heat is energy moving from a warmer place to a cooler one because a temperature difference exists. Once the temperatures become equal and the net transfer stops, the objects are in thermal equilibrium.
That difference explains why a tiny spark can have a very high temperature while carrying little total energy, and a bathtub can have a much lower temperature while containing far more thermal energy. One can sting you; the other can keep an entire person warm.
A metal tool and a wooden bench can sit in the same room long enough to reach the same temperature, yet the metal feels colder. Metal carries heat away from your hand faster. Your nerves notice the rate of heat transfer, not a secret colder temperature hiding in the metal.
Water is useful, not perfectly obedient
Freezing and boiling points depend on conditions
The familiar values, water freezing at 0 °C and boiling at about 100 °C, are reference values tied to specified conditions. Boiling is especially sensitive to pressure. At high altitude, lower air pressure lets water boil at a lower temperature. Inside a pressure cooker, higher pressure raises the boiling point.
Purity, dissolved material, pressure, and the condition of the container can also matter. Very pure, undisturbed water can remain liquid below 0 °C; that is called supercooling. Give it the right disturbance or crystal to start from, and it may freeze very quickly.
At the triple point of water, solid ice, liquid water, and water vapor can exist together in equilibrium. That point is 273.16 K, which is 0.01 °C, under a very specific low pressure. It was once central to the definition of the kelvin and is still useful for calibrating thermometers.
So, no, 100 °C is not a universal kitchen promise. Boiling is a negotiation between temperature and pressure, and water has apparently read the fine print.
Tiny history detour
Thermometers existed before everyone agreed on the numbers
Early devices called thermoscopes could show that something was getting warmer or cooler, but they did not provide a standardized temperature. Once experimenters began adding repeatable reference points and numbered scales, the thermoscope became a thermometer.
Isaac Newton and Ole Rømer proposed early scales around the beginning of the eighteenth century. Daniel Gabriel Fahrenheit visited Rømer and later introduced his own scale in 1724. Fahrenheit’s early reference points included a cold brine mixture and a human body temperature. Later standardization placed water’s freezing point at 32 °F and its boiling point at 212 °F under standard pressure.
Anders Celsius proposed his hundred degree scale in 1742, but he arranged it in the opposite direction from the modern scale: water boiled at 0 and froze at 100. The direction was reversed soon afterward, giving us the familiar Celsius scale.
Between water’s ordinary reference points
Celsius: 0 °C to 100 °C, a span of 100 degrees
Fahrenheit: 32 °F to 212 °F, a span of 180 degrees
1 °C interval = 1.8 °F intervals
Now we remove the arbitrary zero
Kelvin and Rankine begin at absolute zero
The Celsius and Fahrenheit zeros are convenient reference points, not the physical bottom of temperature. Absolute scales begin at absolute zero, the lowest possible thermodynamic temperature. That is 0 K, −273.15 °C, −459.67 °F, and 0 °R.
Kelvin uses the same interval size as Celsius. A rise of 1 K is the same temperature change as a rise of 1 °C. Rankine does the same job with Fahrenheit sized intervals, so a rise of 1 °R equals a rise of 1 °F.
The kelvin used to be defined through the triple point of water. Since 2019, it has been defined by fixing the value of the Boltzmann constant, which links thermodynamic temperature to energy. That gives temperature measurement a foundation in a constant of nature instead of one carefully prepared sample of water.
Kelvin does not use a degree sign; write 293.15 K, not 293.15 °K. Also, absolute zero does not mean every kind of motion vanishes. Quantum mechanical zero point motion remains, even though thermal motion is at its minimum.
The scale drawer gets wonderfully strange
Réaumur, Rømer, Newton, and Delisle did things differently
Before Celsius and Fahrenheit became dominant, scientists proposed many competing scales. The converter includes four of the better known historical examples. Their conventional water reference points make the personalities of the scales fairly obvious.
| Scale | Water freezes | Water boils | What makes it unusual |
|---|---|---|---|
| Réaumur | 0 °Ré | 80 °Ré | Eighty equal intervals between the water points. |
| Rømer | 7.5 °Rø | 60 °Rø | An early scale that helped influence Fahrenheit’s work. |
| Newton | 0 °N | about 33 °N | Newton used body temperature as an early reference and extended the scale for hotter materials. |
| Delisle | 150 °De | 0 °De | The numbers run backward, so larger values mean colder temperatures. |
The formulas here use modern conventional relationships between the scales. A surviving historical thermometer or written record may not match perfectly because the instrument, liquid, reference pressure, calibration method, and local version of the scale could differ.
The converter is exact; your thermometer may not be
Thermometers infer temperature from something else
A thermometer does not reach into matter and pull out a temperature number directly. It watches some property that changes predictably with temperature. A liquid thermometer watches expansion. A resistance thermometer watches electrical resistance. A thermocouple watches voltage. An infrared thermometer watches emitted thermal radiation.
That means the measuring method matters. An infrared thermometer normally reports a surface temperature, not the internal temperature. Shiny metal can give misleading infrared readings because emissivity is low and reflected radiation can influence the result. A probe needs enough contact and enough time to settle. Air temperature changes depending on sunlight, airflow, height, and distance from nearby surfaces.
Sage’s accuracy checklist
Measure the right thing: air, surface, liquid, or internal temperature. Use the right sensor and placement for the job. Allow enough response time for the reading to stabilize. Know the instrument’s accuracy, not only its display resolution. Do not report more converted decimals than the original measurement can support.A lamp labeled 5000 K is describing its color appearance, not necessarily the physical temperature of the bulb. Color temperature compares the light to the color a hot ideal radiator would produce. An LED can look like 5000 K daylight without the lamp itself being anywhere near 5000 K.
Sage keeps a drawer of odd facts
A few temperature facts worth keeping
−40 is the meeting point. At −40, Celsius and Fahrenheit show the same number. It is the one place where those two scales stop arguing.
A temperature interval is not the same as a temperature. A change of 10 °C equals a change of 18 °F, but 10 °C does not equal 18 °F because the zero points are different.
Absolute zero can be approached, not simply reached. Laboratories have cooled matter astonishingly close to 0 K, but the third law of thermodynamics means exactly 0 K cannot be reached through a finite sequence of physical processes.
The last digit may be decorative. Converting 72 °F produces 22.222222... °C mathematically, but a household thermometer that was only accurate to the nearest degree did not suddenly become a precision laboratory instrument.
Feeling hot or cold is not a thermometer. Humidity, airflow, sunlight, clothing, metabolism, and heat transfer all affect comfort. The air temperature is only one part of the story.
That lesson reached thermal equilibrium
Class is over. It is officially cheese fries time.
You now know that temperature conversion changes both the starting point and the degree size, that Celsius makes a useful middle scale, and that 68 °F equals 20 °C and 293.15 K.
You also know why boiling points move, why metal can feel colder than wood at the same temperature, why Delisle runs backward, and why converting ten decimal places does not create ten decimal places of accuracy. That seems like enough thermal excitement for one lesson, and Sage appears ready to celebrate with her favorite food.
Quick answers
Temperature conversion questions
How do I convert Celsius to Fahrenheit?
Multiply Celsius by 9/5, then add 32. For example, (20 × 9/5) + 32 = 68 °F.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For example, (68 − 32) × 5/9 = 20 °C.
At what temperature are Celsius and Fahrenheit equal?
They are equal at −40. In other words, −40 °C and −40 °F describe the same temperature.
Why does kelvin not use a degree symbol?
The kelvin is an SI base unit, not a degree scale. Write 293.15 K, not 293.15 °K. A temperature difference of 1 K is the same size as a difference of 1 °C.
What is absolute zero?
Absolute zero is the lowest possible thermodynamic temperature: 0 K, −273.15 °C, −459.67 °F, or 0 °R.
Does water always boil at 100 °C?
No. Water boils at about 100 °C at standard atmospheric pressure. At lower pressure, such as high altitude, it boils at a lower temperature. At higher pressure, such as inside a pressure cooker, it boils at a higher temperature.
Can liquid water be colder than 0 °C?
Yes. Very pure, undisturbed water can remain liquid below 0 °C. This is called supercooling. A disturbance or seed crystal can trigger rapid freezing.
Why can an infrared thermometer disagree with a probe?
An infrared thermometer usually reads surface radiation, while a probe measures at the point where it is touching or inserted. Surface emissivity, shiny reflections, sensor placement, response time, and internal temperature gradients can all produce different readings.