Temperature Conversion
Convert units accurately, with the formula shown.
What is the Temperature Conversion?
This temperature conversion tool switches between Celsius, Fahrenheit, Kelvin and Rankine instantly, using the exact defined relationships between the scales — not rounded approximations. Type a value in any one of the four and the other three update immediately, along with the formula used to get there. It is built for the situations where "close enough" causes real problems: converting a recipe or oven setting, reading a weather forecast from another country, checking a scientific measurement, or working out whether a fever reading in one scale is actually high in another.
Temperature conversion trips people up in a way length or weight conversion does not, because the scales do not share a zero point. Doubling a Celsius reading does not double how hot something feels in Fahrenheit — the widget and the explanation below both exist to make that concrete rather than abstract.
Why use a free temperature conversion?
All four scales at once
Celsius, Fahrenheit, Kelvin and Rankine convert together from a single input, so you never have to chain two conversions to get from Fahrenheit to Kelvin.
Exact defined constants, not rounded ones
The Celsius–Kelvin offset (273.15) and the Fahrenheit–Rankine offset (459.67) are the internationally defined values, applied in full double-precision arithmetic before the result is rounded for display.
Built on the same conversion engine as every other converter
Temperature shares its underlying code with the unit converter and byte converter, so the same formatting and precision rules apply everywhere on the site.
No signup, no upload, instant result
The calculation runs in your browser as you type. Nothing is sent to a server.
How do you use the Temperature Conversion?
- 1
Enter a temperature
Type a number into any of the scale fields — Celsius, Fahrenheit, Kelvin or Rankine.
- 2
Read the converted values
The other three scales update immediately, calculated directly from what you typed rather than from an already-rounded intermediate value.
- 3
Switch which field you edit
You can type into any field, not just Celsius — the conversion works the same way in every direction.
How is temperature conversion calculated?
°F = °C × 9/5 + 32 K = °C + 273.15 °R = °F + 459.67Where:
- °C
- Temperature in degrees Celsius
- °F
- Temperature in degrees Fahrenheit
- K
- Temperature in kelvin (no degree symbol — kelvin is an absolute unit)
- °R
- Temperature in degrees Rankine, an absolute scale sized in Fahrenheit-degree steps
The simple version
Celsius and Fahrenheit do not share a zero point, so you cannot just multiply — you have to scale the number and then shift it. To go from Celsius to Fahrenheit, multiply by 9/5 (because a Fahrenheit degree is smaller than a Celsius degree) and then add 32 (because 0°C is not 0°F, it is 32°F). Kelvin is Celsius shifted so that 0 K sits at absolute zero, the coldest physically possible temperature — that shift is exactly 273.15. Rankine does the same thing to Fahrenheit that Kelvin does to Celsius.
What does it look like in practice?
Water boiling at sea level
Input
100°C
Result
212°F · 373.15 K · 671.67 °R
100 × 9/5 + 32 = 212. Adding 273.15 gives kelvin directly from Celsius; Rankine is Fahrenheit + 459.67.
A mild room temperature
Input
20°C
Result
68°F · 293.15 K
20 × 9/5 = 36, plus 32 = 68. This is the temperature most often used as a "room temperature" baseline in recipes and product specs.
Human body temperature
Input
98.6°F
Result
37°C · 310.15 K
Working backwards: (98.6 − 32) × 5/9 = 37. This is the traditional average oral temperature — modern studies put the average closer to 36.6°C (97.9°F), so treat 37°C as a round reference point, not a strict cutoff.
The point where the scales agree
Input
−40°C
Result
−40°F
−40 × 9/5 + 32 = −72 + 32 = −40. Celsius and Fahrenheit only ever produce the same number at this one point — everywhere else they diverge, which is exactly why you cannot treat the conversion as a simple ratio.
How accurate is it?
Every conversion uses the internationally defined exact relationships between the four scales — there is no approximation in the formula itself. The Celsius-to-Kelvin offset of 273.15 and the Fahrenheit-to-Rankine offset of 459.67 are exact by definition, and the 9/5 scale factor between Celsius and Fahrenheit degrees is an exact fraction, not a decimal approximation like 1.8000001.
- The calculation happens in full double-precision floating point; only the displayed result is rounded, so chaining conversions (e.g. °F → K → °C) does not accumulate visible error.
- Kelvin and Rankine are absolute scales — 0 K and 0 °R represent absolute zero, the theoretical lower limit of temperature. Negative kelvin or negative Rankine values are not physically meaningful, though the calculator will still compute them if you enter one.
- "Room temperature," "body temperature" and similar everyday reference points are conventions, not physical constants — different sources round them slightly differently.
What should you know before using it?
Key reference points across all four scales:
| Reference point | Celsius / Fahrenheit / Kelvin / Rankine |
|---|---|
| Absolute zero | −273.15°C / −459.67°F / 0 K / 0 °R |
| Scales cross (equal reading) | −40°C / −40°F / 233.15 K / 419.67 °R |
| Water freezes | 0°C / 32°F / 273.15 K / 491.67 °R |
| Average human body temperature | 37°C / 98.6°F / 310.15 K / 558.27 °R |
| Water boils (sea level) | 100°C / 212°F / 373.15 K / 671.67 °R |
Which tools relate to the temperature conversion?
The temperature conversion is one of 14 unit converters on this site. These are the ones most often used alongside it — either because they handle the next step in the same job, or because they answer a question this tool raises.
Unit Converter
The general-purpose converter behind length, weight, area and more — temperature is the one quantity in that set that needs an offset, not just a multiplier.
Length Converter
A more typical conversion: pure multiplication by a fixed factor, with no zero-point shift to account for.
Weight Converter
Same pattern as length — kilograms and pounds scale by a constant factor with no offset, unlike temperature.
When should you use a temperature conversion?
Because the temperature conversion runs entirely in your browser, it suits work you would not want to hand to a third-party server — client files, unpublished drafts, anything under an NDA. These are the situations people reach for it in most often.
Cooking and oven temperatures
US recipes list Fahrenheit oven settings; most of the world uses Celsius. Converting the wrong way by even a small margin can under- or over-bake something.
Weather and travel
A forecast of "25" means something completely different in Celsius than in Fahrenheit — useful when checking conditions abroad.
Science and engineering
Kelvin is the SI base unit for thermodynamic calculations; Rankine still shows up in US engineering contexts that otherwise use imperial units.
Health readings
Thermometers sold in different countries default to different scales, so converting a fever reading correctly matters for deciding whether it is actually high.
Frequently asked questions
How do you convert Celsius to Fahrenheit?
Why is temperature conversion not just a ratio like other units?
What is the difference between Kelvin and Celsius?
What is Rankine used for?
At what temperature do Celsius and Fahrenheit show the same number?
Can this calculator produce a negative Kelvin value?
Is normal human body temperature exactly 37°C?
Is the temperature conversion safe to use?
This tool runs entirely inside your browser. Your text, files and settings are processed on your own device and are never uploaded to our servers — there is nothing for us to store, log or leak. You can confirm it yourself: open your browser DevTools, switch to the Network tab, and use the tool. You will not see an upload request. It also means the tool keeps working if your connection drops mid-task.
Where do these figures come from?
Every method, threshold and standard this page relies on, with a link to the document that defines it. Check them — a tool that will not show its sources is asking you to take its word for it.
- 1The International System of Units (SI Brochure)
Bureau International des Poids et Mesures (BIPM), 9th edition · 2019
Supports: The exact 273.15 offset defining the kelvin scale relative to Celsius, and kelvin as an SI base unit with no degree symbol.
- 2NIST Guide for the Use of the International System of Units (SI)
National Institute of Standards and Technology, SP 811
Supports: The exact Fahrenheit conversion formula (°F = °C × 9/5 + 32) and the Rankine scale's relationship to Fahrenheit.
Need something this tool can’t do?
If you need a feature added, spotted something wrong, or want a custom tool or website built for your business, tell us. We read every message and we build what people actually ask for.
More tools
Tools that pair well with this one
Browse every category
- SEO Tools31
- Keyword Tools7
- Domain & IP Tools18
- Text Tools20
- Writing Tools12
- Image Tools31
- PDF Tools17
- Unit Converters14
- Calculators21
- Developer Tools13
- Generators16
- Design & CSS Tools6
- Games8
Looking for something else? See all 214 tools, browse the full site index or read the guides on our blog.