Temperature Conversion: Celsius, Fahrenheit and Kelvin
A recipe from London bakes at 180, one from New York at 350, and a physics textbook puts the same oven in the 400s: Celsius, Fahrenheit and Kelvin describing similar heat. The conversions between them are exact and simple. This guide lays out the six formulas the Temperature Converter uses, works an example for each pairing, and points out where people most often go wrong.
What the Three Scales Are and Why They Differ
Each scale was anchored to different reference points, which is why their numbers look so unalike.
Celsius sets 0 at the freezing point of water and 100 at its boiling point (at standard pressure). Its 100-degree span makes it intuitive and it is the everyday standard almost everywhere outside the United States.
Fahrenheit places water's freezing point at 32 and its boiling point at 212, a 180-degree span. The finer degrees give more granularity without decimals, and it remains the day-to-day scale in the US.
Kelvin starts at absolute zero and uses steps the same size as Celsius degrees. There are no negative kelvins, which is why science uses it: thermodynamic equations need a scale with a true zero. Since 2019 the kelvin has been defined through the Boltzmann constant rather than the properties of water, but the offset from Celsius is unchanged: 0 K = −273.15 °C, and water freezes at 273.15 K.
Two facts explain every conversion. First, Celsius and Fahrenheit have different zero points and different degree sizes (a Fahrenheit degree is 5/9 the size of a Celsius degree). Second, Celsius and Kelvin share a degree size but are offset by exactly 273.15.
How to Convert Between the Scales
Here are the six exact conversions:
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K − 273.15
- Fahrenheit to Kelvin: K = ((°F − 32) × 5/9) + 273.15
- Kelvin to Fahrenheit: °F = ((K − 273.15) × 9/5) + 32
The Celsius–Fahrenheit conversions need both a scaling step (the 9/5 or 5/9 factor for the different degree sizes) and a shift step (the 32 for the different zero points). The Celsius–Kelvin conversions need only the shift, because the degrees are identical. The Fahrenheit–Kelvin pair simply chains the two together: convert to Celsius in the middle, then on to the target.
Worked example — a few everyday conversions.
Convert 25 °C (a pleasant room) to Fahrenheit:
1. 25 × 9/5 = 45
2. 45 + 32 = 77 °F
Convert 98.6 °F (body temperature) to Celsius:
1. 98.6 − 32 = 66.6
2. 66.6 × 5/9 = 37 °C
Convert 25 °C to Kelvin:
1. 25 + 273.15 = 298.15 K
Convert 350 °F (a hot oven) to Kelvin, chaining through Celsius:
1. 350 − 32 = 318
2. 318 × 5/9 = 176.67 °C
3. 176.67 + 273.15 = 449.82 K
The Temperature Converter takes a value plus a from-unit and a to-unit and applies the matching formula from the list above.
Practical Use and Common Mistakes
These conversions show up constantly. Travelers read a 30 °C forecast and want to know it is a warm 86 °F. Cooks adapt a 200 °C European recipe to a 392 °F American oven. Students convert lab readings into Kelvin before plugging them into a gas-law equation, where only an absolute scale gives correct results. Weather indices work in one scale or the other: the Heat Index Calculator and Wind Chill Calculator use the US formulas, so a Celsius reading has to be converted first.
A handful of errors recur.
Forgetting the offset before scaling. When going from Fahrenheit to Celsius, subtract 32 first, then multiply by 5/9. Doing it in the wrong order — multiplying before subtracting — gives a badly wrong answer. The reverse direction must add 32 last.
Using 273 instead of 273.15. For rough mental math the rounded offset is fine, but precise scientific work needs the full 273.15. The 0.15 matters when accuracy does.
Mixing up the 9/5 and 5/9 factors. Heating up the number (Celsius to Fahrenheit) multiplies by the larger 9/5; cooling the number down (Fahrenheit to Celsius) multiplies by 5/9. A quick sanity check: 100 °C should land at 212 °F, not 32.
Expecting negative Kelvin. If a conversion produces a temperature below 0 K, something is wrong — absolute zero is the floor, and nothing colder exists.
Confusing a temperature with a temperature difference. A change of 5 °C equals a change of 9 °F and a change of 5 K. When converting an interval rather than a reading, apply only the scaling factor, never the 32 or 273.15 offset.
Key takeaways
• Two scales, two adjustments: Celsius–Fahrenheit conversions need both a 9/5 (or 5/9) scaling and a 32-degree shift; Celsius–Kelvin needs only the 273.15 shift
• Mind the order: When converting Fahrenheit to Celsius, subtract 32 before multiplying by 5/9; reverse the order going the other way
• Use the full offset: Round to 273 for mental math, but use 273.15 for any precise scientific conversion
• Differences are not readings: To convert a temperature change rather than a temperature, apply only the scaling factor and drop the 32 or 273.15 offset