Temperature Converter – Celsius, Fahrenheit & Kelvin
Temperature Converter
Convert Celsius, Fahrenheit, Kelvin and more, instantly with a clear formula breakdown.
Conversion Formula & Steps
Thermal State
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Temperature Converter – Convert Celsius, Fahrenheit & Kelvin
Temperature shows up everywhere — cooking, weather, medicine, engineering — and the scale it's reported in depends entirely on where you are and what field you're in. This converter handles the translation between scales so you don't have to do the arithmetic by hand, whether you're adjusting an oven temperature from a US recipe or checking a lab reading in Kelvin.
How to Use the Temperature Converter
- Enter the temperature you want to convert in the input field.
- Select the original unit from the "From Unit" dropdown.
- Select the unit you want to convert to from the "To Unit" dropdown.
- View the result — it updates instantly, along with the formula used and every other scale at once.
Use the swap button to reverse the two units in one tap, or one of the quick presets to jump straight to a common reference point like freezing water or body temperature.
Temperature Conversion Formulas
Every conversion in this tool is built from the standard definitions below. Celsius is used as the common base: any input is first converted to Celsius, then from Celsius to the target scale.
| Scale | Symbol | To Celsius | From Celsius |
|---|---|---|---|
| Fahrenheit | °F | °C = (°F − 32) × 5/9 | °F = (°C × 9/5) + 32 |
| Kelvin | K | °C = K − 273.15 | K = °C + 273.15 |
| Rankine | °R | °C = (°R − 491.67) × 5/9 | °R = (°C + 273.15) × 9/5 |
| Réaumur | °Ré | °C = °Ré × 1.25 | °Ré = °C × 0.8 |
| Delisle | °De | °C = 100 − (°De × 2/3) | °De = (100 − °C) × 1.5 |
| Newton | °N | °C = °N × 100/33 | °N = °C × 33/100 |
| Rømer | °Rø | °C = (°Rø − 7.5) × 40/21 | °Rø = (°C × 21/40) + 7.5 |
These definitions are consistent with those used by measurement authorities such as the National Institute of Standards and Technology, and the underlying algebra is explained well by Khan Academy's thermodynamics material if you want the reasoning behind the numbers.
Understanding the Temperature Scales
Celsius (°C)
Devised in 1742 by Anders Celsius, this scale sets water's freezing point at 0°C and its boiling point at 100°C at standard atmospheric pressure, with 100 equal divisions between them. It's the scale used by most of the world.
Fahrenheit (°F)
Proposed by Daniel Gabriel Fahrenheit in 1724, this scale puts water's freezing point at 32°F and its boiling point at 212°F — a 180-degree span. It remains the everyday scale in the United States.
Kelvin (K)
The SI base unit for temperature, with 0 K set at absolute zero — the point where molecular motion is at its theoretical minimum. Because it's an absolute scale, it's written without a degree symbol, and it's the scale used throughout physics and chemistry. Further background is available on Wikipedia's Kelvin entry.
Rankine, Réaumur, Delisle, Newton and Rømer
These are less common today but still appear in specific contexts: Rankine in US engineering thermodynamics, and the other four mainly in historical texts. Delisle is unusual in that it runs in reverse — higher numbers mean colder temperatures.
Common Temperature Conversions
| Reference point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20–25 | 68–77 | 293.15–298.15 |
| Average body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
When to Use a Temperature Converter
- Adjusting an oven temperature from a recipe written in a different scale.
- Reading a weather forecast or thermostat set to a scale you're not used to.
- Checking a Kelvin or Rankine value from a lab, textbook, or engineering spec.
- Comparing a body-temperature reading against a Fahrenheit reference.
Common Mistakes
Converting a specific reading (like "20°C outside") uses the full formula, offset included. But a temperature change of 20°C is not 68°F — it's a 36°F change (20 × 1.8), because a change in temperature has no fixed starting point to offset from. Mixing these two up is the most common error in manual temperature math.
Frequently Asked Questions
How do I convert Celsius to Fahrenheit quickly in my head?
Double the Celsius value and add 30. For example, 20°C × 2 = 40, plus 30 = 70°F. The exact answer (68°F) is close enough for everyday use.
At what point do Celsius and Fahrenheit read the exact same number?
Celsius and Fahrenheit intersect at exactly -40 degrees. So -40°C equals -40°F.
Why is Kelvin preferred in scientific research?
Kelvin is an absolute scale starting at absolute zero. Formulas that depend on true thermal energy, such as the ideal gas law, need an absolute-zero-based scale to avoid producing invalid negative-energy results.
What is absolute zero, and can it be reached?
Absolute zero is 0 K, equal to -273.15°C or -459.67°F. It's the theoretical point where molecular motion is at its minimum. It can be approached extremely closely but never fully reached.
Why doesn't Kelvin use a degree symbol (°)?
Kelvin is an SI base unit of absolute measurement, similar to meters or kilograms, rather than a relative degree scale, so it's written simply as K.
What is standard room temperature in °C and °F?
Standard room temperature is generally defined as 20°C to 25°C, which equals 68°F to 77°F, or 293.15 K to 298.15 K.
How is the Rankine scale used in engineering?
Rankine is an absolute scale using Fahrenheit-sized degrees. It's used in thermodynamic and heat-transfer calculations in industries that work in Fahrenheit but need an absolute-zero reference point.
What is normal human body temperature in Celsius, Fahrenheit, and Kelvin?
Average human body temperature is approximately 37°C, 98.6°F, and 310.15 K.
Is this temperature converter free to use?
Yes, it's completely free with no sign-up, and there's no limit on how many conversions you can run.
How does the Delisle scale differ from other systems?
Delisle runs in the opposite direction from most scales: boiling water is 0°De and freezing water is 150°De, so higher numbers mean colder temperatures.
Can a temperature fall below 0 Kelvin?
In classical thermodynamics, 0 K is the lower bound, and this converter treats it as such. Certain quantum systems can technically show "negative Kelvin" states, but these describe unusual population-inversion conditions, not everyday temperatures colder than absolute zero.
Can I use this tool for scientific or engineering reference?
The formulas match standard definitions used by organizations like NIST, so results are accurate for everyday and educational use. For safety-critical or published scientific work, verify important values against an authoritative source directly.
Conclusion
Whether you're converting a recipe's oven temperature, checking a weather report, or working through a physics problem, accurate conversion comes down to using the right formula and being careful with points versus changes. This tool handles the calculation; the formulas above are there so you can always see exactly how it got there.
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